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Why does spiral or annular thickening occur in water conducting plant vessels?

Why does spiral or annular thickening occur in water conducting plant vessels?


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From what I behold, spiral and annular thickening of xylem and trachied cell walls leaves a lot of not thickened regions of cell wall. Lignin is the material which prevents water from escaping these water conduits. If lignin is absent from certain regions, does not that mean that water flows out of the xylem vessels and trachieds through their cell walls? If so, is not this disadvantageous to a plant? Why can't a plant simply thicken the cell walls with uniform lignin deposition, only sparing pits to allow lateral flow into the required tissue? I would appreciate your kind response.


Non uniform lignification of xylem vessels is beneficial to a plant in that it enables growth. It is at the unlignified parts of plants that growth occurs since fully lignified tissues are dead and so can not grow. Spiral and annular thickening allows for vessels to elongate and thus allows tissues to expand without breaking the vessel walls.

Water cannot just go on flowing from plant cells because they are not lignified, plant cells have cellulose cell walls and cell membranes which prevent water loss. In addition, water in the xylem is not under pressure so I do not think water would be able to flow out of the xylem vessels except in cases where water is forced to move to adjacent phloem tissues by osmosis during phloem translocation.


The Structure and Function of Lignin in Plant Cells

LIgnin is a substance found in vascular plants, usually within the cell walls and also between cells themselves. It is largely a supportive structure and is part of the secondary thickening of tall plants. It is a major constituent of ‘woody’ material. Most trees would topple over without lignin supporting their tissues, in addition to the long, dead cells of the xylem vessels. When secondary thickening is formed it is called lignification.

Surprisingly, although a dead tissue, lignin is incredibly strong and can help support tall trees of several hundered tonnes whilst maintaining its integrity. Lignin also is not simply a passive material. It often has tiny pores called lenticels through it through which the plant cells beneath in the epidermis can take in gas for gaseous exchange in respriation and also it may have specialised links called tracheids which link xylem vessels to other cells.

It plays a part, along with the xylem vessels, along whose walls it is usually deposited, in controlling the transportation of liquid in plants. It prevent the walls of the xylem vessels collapsingunder pressure and adds strength to the woody material of an older plant.

Lignin is not simple instructure and in fact, different plants have different kinds of lignin. It is usually composed of amino compounds and bonded in complex ways to carbohydrate molecules in the wood. Older plants with woody material which we eat like cabbage become ‘stringy’ and taste woody becuse the layers of lignin have got too thick over time.

Lignins are of different types including those which are in different plant genera of soft, hard and grasses. the structure of lignin varies from plant species to plant species and even from one tissue to the next in plants. Their precise function is not yet clear but they effectively strengthen bonds to increase stability of water transport vessels and also keep them open to allow the passage of water, dissolved nutrients and waste products from the plant.

Though useful to plants as they increase in size and girth, lignin does have its drawbacks. Beans, for example become enedible as they age due in part to lignin deposits and as paper is made, used and recycled, lignin tends to remain as it is hard and difficult to dissolve so each time you recycle paper, the content of lignin increases and this impairs the quality of the paper. Finding bio-friendly ways of removing it without resorting to chemicals like dioxin is a challenge for the future.


Permanent Tissue in Plants | Botany

Permanent tissues are simple or complex, according to the nature of component cells. A simple permanent tissue is composed of only one type of cells and is thus homogeneous whereas the complex permanent tissue is heterogeneous, consisting of more than one type of cell elements.

a. Simple Tissue:

Simple permanent tissues are of following types:

Parenchymatous tissue (Fig. 136) is usually made of isodiametric cells with intercellular spaces. The cells are living and contain vacuolated protoplast. The cell wall is thin, homogeneous and made up of cellulose. Only the parenchyma cells of epidermis or outermost skin of aerial organs have cuticularised outer walls.

Parenchyma is by far the most abundant type of tissue distributed in all the organs of the plants, under- ground and aerial. Their main function is the manufacture and storage of food matters, which is referred to as vital function.

Besides, the paren­chyma cells may serve as store­house of water and various waste products. They also help in the conduction of water and food matters in solution. The parenchyma cells of the epi­dermis are protective in func­tion.

In the aqua tic plants the parenchyma cells often assume stellate (star-like) or armed appear­ance due to presence of abundant air chambers. These cells, also called aerenchyma, give buoyancy to the plants.

Collenchymatous tissue (Fig. 137) is composed of somewhat elongate cells with peculiar thickenings, confined to the corners of the cells. They often remain interlocked. The cells have protoplast. Chloroplasts are, normally absent, but may be present in some cells. They usually do not manufacture food.

Collenchyma cells look pentagonal or hexagonal in cross-section. The soft and plastic walls are made up of cellulose and pecdn. Extra-deposition of cellulose on walls abutting on intercellular spaces gives them characteristic thickened corners.

Collenchyma occurs chiefly in the superficial regions of stems (beneath the epi­dermis) and midribs of leaves. They are abundant in the rapidly elongating organs like leaf-stalk and floral stalk. Though soft, it is a mechanical or strength-giving tissue, particularly meant for the sup­port of growing organs. It is called a temporary supporting tissue.

Sclerenchymatous tissue (Fig. 138) consists of long needle-like cells with pointed ends. As the cells are much longer than their breadth they are also called fibres. They gradually lose protoplasm and become dead. In cross-section they look typically angular. The walls are hard and lignified. Simple pits are present.

Sometimes lignification becomes so much pronounced that the central cavity (lumen) is almost blocked. They usually occur in groups or patches with peculiarly interlocked or dovetailed ends. They are present in both aerial and under­ground organs. Sclerenchyma is the most important mechanical or strength-giving tissue of the plant.

Sclereids or stone cells (Fig. 138) are modified scleren­chyma. They are not elongated but isodiametric or irregular in shape. The gritty texture of fruits like pears and guava, is due to the presence of sclereids. There are some peculiar types of sclereids occurring in many leaves. Ones projecting into the inter­cellular cavities in the stem and leaf of water-lily are hair-like in appearance and so are also called ‘internal hairs’.

b. Complex Tissue:

A complex tissue is hetero­geneous, as it consists of more than one type of cell elements. The two main complex tissues are xylem and phloem, present in the vascular bundles or conducting strands of higher plants. Vascular system is continuous in the plant body, and is instrumental in the conduction of water and soluble food materials.

Xylem is the hard woody part of the vascular bundle which is concerned with the conduction of water. It also gives sufficient mechanical strength to the plant members.

The following elements form this complex tissue:

(iii) Some parenchyma cells called xylem parenchyma or wood parenchyma,

(iv) Sclerenchymatous fibres called xylem fibres.

Tracheids (Fig. 139) are elongated cells with fairly large cavities and chisel-like tapering ends. They extend parallel to the long axis of the organs. Tracheids are typically angular in cross-sections. Protoplasts disappear so tracheids are dead cells. The cell wall is hard, lignified with different types of loca­lised thickenings, of which bordered pits are more common.

They are the primitive conducting elements, forming very important elements of xylem of the gymnosperms, but may also occur in the angiosperms. Tracheids are pri­marily meant for the conduction of water. Secondarily, they can give mechanical support with the aid of hard lignified walls.

Tracheae or vessels are long tube-like bodies occurring parallel to the long axis of the organs. A trachea is formed from a row of cylindrical cells, attached end to end, by the total dissolution of the partition walls (Fig. 140). So they are long tube-like bodies with fairly large cavities. Tracheae are more or less circular in cross-section.

The protoplast vanishes with the formation of the tube. The cell wall is hard and lignified with different types of localised thickenings from annular to pitted. Tracheae are the most important elements of xylem in the angiosperms, and are thus the advanced conducting elements. Their main function is conduction of water, as they are quite well-suited for the purpose. They can give mechanical strength as well.

It should be noted that a tracheid is a single cell, but a trachea or vessel is a tube-like body formed from a row of cells. The first formed xylem vessels have smaller cavities and usually annular and spiral thickenings. They are known as protoxylem vessels but lately formed ones, called metaxylem vessels, have wider cavities and reticulate, scalariform or pitted thickenings.

The complex tissue phloem is composed of some thin-walled elements. It is res­ponsible for the conduction of elaborated food materials.

The elements of phloem are mainly:

Often some fibres, called phloem fibres, may remain asso­ciated with them.

(Figs. 141 & 142) is a long tube-like body formed by a series of cells where the partition walls are partially absorbed in a sieve-like manner. The perforated partition walls are called the sieve plates through which cytoplasm of adjoining cells are in communication.

Sieve plates may be horizontal or obliquely inclined. Unlike vessel, a sieve tube is living, having lining cytoplasm with a large central vacuole. The nucleus disintegrates with the maturity of the tube. The cell sap of the vacuoles is rich in nitrogenous matters. The wall is thin and made of cellulose. Sieve tubes are very important elements of phloem, and perform the function of conduction of prepared food materials.

A highly glistening carbohydrate, called callose, is often deposited on the sieve plate in the form of a pad known as callous pad. As a results, the cell to cell communication is temporarily or permanently cut down. The callous pad formed with the approach of winter usually dissolves in spring to re –establish the communication. This is the seasonal callus may also be formed in the functional sieves tubes.

(Figs. 141 & 142) are closely associated with sieve tubes and are connected with them by pores. They are elongated cells with dense cytoplasm and prominent nuclei. Elongate cells with sieve-like perforate walls are present in gymnosperms. They are called sieve cells. These are primitive, whereas the sieve-tubes are advanced elements.

Besides simple and complex tissues, there are special ones, called secretory tissues, concerned with the secretory or excretory materials of plants. Glands are the common secretory tissues which store up essential oils, etc.

Faint spots on the leaf of lemon and petals of sweetly scented flowers are the glands. Similarly ducts with resin are present in some plants. Laticiferous ducts are long tube-like bodies containing the milky fluid latex. They occur abundantly in Plumeria (B. Katgolap), Calotropis (B. Akanda).


Secondary Growth in Dicot Stem (With Diagram)

Primary growth produces growth in length and development of lateral appendages. Secondary growth is the formation of secondary tissues from lateral meristems. It increases the diameter of the stem. In woody plants, secondary tissues constitute the bulk of the plant. They take part in providing protection, support and conduction of water and nutrients.

Secondary tissues are formed by two types of lateral meristems, vascular cambium and cork cambium or phellogen. Vascular cambium produces secondary vascular tissues while phellogen forms periderm.

Secondary growth occurs in perennial gymnosperms and dicots such as trees and shrubs. It is also found in the woody stems of some herbs. In such cases, the secondary growth is equivalent to one annual ring, e.g., Sunflower.

A. Formation of Secondary Vascular Tissues:

They are formed by the vascular cambium. Vascular cambium is produced by two types of meristems, fascicular or intra-fascicular and inter-fascicular cambium. Intra-fascicular cambium is a primary meristem which occurs as strips in vascular bundles. Inter-fascicular cambium arises secondarily from the cells of medullary rays which occur at the level of intra-fascicular strips.

These two types of meristematic tissues get connected to form a ring of vascular cambium. Vascular cam­bium is truly single layered but appears to be a few layers (2-5) in thickness due to presence of its immediate derivatives. Cells of vascular cambium divide periclinally both on the outer and inner sides (bipolar divisions) to form secondary permanent tissues.

The cells of vascular cambium are of two types, elongated spindle-shaped fusiform initials and shorter isodiametric ray initials (Fig. 6.29). Both appear rectangular in T.S. Ray initials give rise to vascular rays.

Fusiform initials divide to form secondary phloem on the outer side and secondary xylem on the inner side (Fig. 6.28 B). With the formation of secondary xylem on the inner side, the vascular cambium moves gradually to the outside by adding new cells.

The phenomenon is called dilation. New ray cells are also added. They form additional rays every year (Fig. 6.28 D). The vascular cambium undergoes two types of divisions— additive (periclinal divisions for formation of secondary tissues) and multi­plicative (anticlinal divisions for dilation).

Ray initials produce radial system (= horizontal or transverse system) while fusiform initials form axial system (= vertical system) of sec­ondary vascular tissues.

The vascular rays or secondary medullary rays are rows of radially arranged cells which are formed in the secondary vascular tissues. They are a few cells in height.

Depending upon their breadth, the vascular rays are uniseriate (one cell in breadth) or multiseriate (two or more cells in breadth). Vascular rays may be homo-cellular (having one type of cells) or hetero-cellular (with more than one type of cells). The cells of the vascular rays enclose intercellular spaces.

The part of the vascular ray present in the secondary xylem is called wood or xylem ray while the part present in the secondary phloem is known as phloem ray. The vascular rays conduct water and or­ganic food and permit diffusion of gases in the radial direction. Besides, their cells store food.

2. Secondary Phloem (Bast):

It forms a narrow circle on the outer side of vascular cam­bium. Secondary phloem does not grow in thickness because the primary and the older sec­ondary phloem present on the outer side gets crushed with the development of new functional phloem (Fig. 6.28 D). Therefore, rings (annual rings) are not produced in secondary phloem. The crushed or non-functioning phloem may, however, have fibres and sclereids.

Secondary phloem is made up of the same type of cells as are found in the primary phloem (metaphloem)— sieve tubes, companion cells, phloem fibres and phloem paren­chyma.

Phloem pairenchyma is of two types— axial phloem parenchyma made up of longitudinally arranged cells and phloem ray parenchyma formed of radially arranged parenchyma cells that constitute the part of the vascular ray present in the phloem.

Elements of secondary phloem show a more regular arrangement. Sieve tubes are comparatively more numerous but are shorter and broader. Sclerenchyma fibres occur either in patches or bands. Sclereids are found in many cases. In such cases secondary phloem is differentiated into soft bast (secondary phloem without fibres) and hard bast (part of phloem with abundant fibres).

It forms the bulk of the stem and is commonly called wood. The secondary xylem consists of vessels, tracheids (both tracheary elements), wood fibres and wood parenchyma.

Wood parenchyma may contain tannins and crystals besides storing food. It is of two types— axial parenchyma cells arranged longitudinally and radial ray parenchyma cells arranged in radial or horizontal fashion. The latter is part of vascular ray present in secondary xylem.

Secondary xylem does not show distinction into protoxylem and meta-xylem elements. Therefore, vessels and tracheids with annular and spiral thicken­ings are absent. The tracheary elements of secondary xylem are similar to those of meta-xylem of the primary xylem with minor differences. They are comparatively shorter and more thick-walled. Pitted thickenings are more common. Fibres are abundant.

Width of secondary xylem grows with the age of the plant. The primary xylem persists as conical projection on its inner side. Pith may become narrow and ultimately get crushed. The yearly growth of secondary xylem is distinct in the areas which expe­rience two seasons, one favourable spring or rainy season) and the other un-favourable (autumn, winter or dry summer).

In favourable season the temperature is optimum. There is a good sunshine and humidity. At this time the newly formed leaves produce hormones which stimulate cambial activity. The activity decreases and stops towards the approach of un-favourable sea­son. Hence the annual or yearly growth appears in the form of distinct rings which are called annual rings (Fig. 6.30).

Annual rings are formed due to sequence of rapid growth (favourable season, e.g., spring), slow growth (before the onset of un-favourable period, e.g., autumn) and no growth (un-favourable season, e.g., winter). Annual rings are not distinct in tropical areas which do not have long dry periods.

Annual Rings (Growth Rings). It is the wood formed in a single year. It consists of two types of wood, spring wood and autumn wood (Fig. 6.31). The spring or early wood is much wider than the autumn or late wood. It is lighter in colour and of lower density. Spring wood consists of larger and wider xylem elements.

The autumn or late wood is dark coloured and of higher density. It contains compactly arranged smaller and narrower elements which have comparatively thicker walls. In autumn wood, tracheids and fibres are more abundant than those found in the spring wood.

The transition from spring to autumn wood in an annual ring is gradual but the transition from autumn wood to the spring wood of the next year is sudden. Therefore, each year’s growth is quite distinct. The number of annual rings corresponds to the age of that part of the stem. (They can be counted by increment borer).

Besides giving the age of the plant, the annual rings also give some clue about the climatic conditions of the past through which the plant has passed. Dendrochronology is the science of counting and analysing annual growth rings of trees.

Softwood and Hardwood:

Softwood is the technical name of gymnosperm wood be­cause it is devoid of vessels. Several of the softwoods are very easy to work with (e.g., Cedrus, Pinus species). However, all of them are not ‘soft’. The softness depends upon the content of fibres and vascular rays. 90-95% of wood is made of tracheids and fibres. Vascular rays constitute 5-10% of the wood.

Hardwood is the name of dicot wood which possesses abundant vessels. Due to the presence of vessels, the hardwoods are also called porous woods. In Cassia fistula and Dalbergia sisso the vessels are comparatively very broad in the spring wood while they are quite narrow in the autumn wood. Such a secondary xylem or wood is called ring porous.

In others (e.g., Syzygium cumini) larger sized vessels are distributed throughout spring wood and autumn wood. This type of secondary xylem or wood is known as diffuse porous. Ring porous wood is more advanced than diffuse porous wood as it provides for better translocation when the requirement of the plant is high.

Sapwood and Heartwood:

The wood of the older stems (dalbergia, Acacia) gets differentiated into two zones, the outer light coloured and functional sapwood or alburnum and the inner darker and nonfunctional heartwood or duramen (Fig. 6.33). The tracheids and vessels of the heart wood get plugged by the in growth of the adjacent parenchyma cells into their cavities through the pits. These ingrowths are called tyloses (Fig. 6.32).

Ultimately, the parenchyma cells become lignified and dead. Various types of plant products like oils, resins, gums, aromatic substances, essential oils and tannins are deposited in the cells of the heartwood. These substances are collectively called extractives. They provide colour to the heartwood. They are also antiseptic. The heartwood is, therefore, stronger and more durable than the sapwood.

It is resistant to attack of insects and microbes. Heart wood is commercial source of Cutch (Acacia catechu), Haematoxylin (Haematoxylon campechianum), Brasilin (Caesalpinia sappan) and Santalin (Pterocarpus santalinus). Heart­wood is, however, liable to be attacked by wood rotting fungi. Hollow tree trunks are due to their activity.

B. Formation of Periderm:

In order to provide for increase in girth and prevent harm on the rupturing of the outer ground tissues due to the formation of secondary vascular tissues, dicot stems produce a cork cambium or phellogen in the outer cortical cells. Rarely it may arise from the epidermis (e.g., Teak, Oleander), hypodermis (e.g., Pear) or phloem parenchyma.

Phellogen cells divide on both the outer side as well as the inner side (bipolar) to form secondary tissues. The secondary tissue produced on the inner side of the phellogen is parenchymatous or collenchymatous. It is called secondary cortex or phelloderm. Its cells show radial arrangement.

Phellogen produces cork or phellem on the outer side. It consists of dead and com­pactly arranged rectangular cells that possess suberised cell walls. The cork cells contain tannins. Hence, they appear brown or dark brown in colour. The cork cells of some plants are filled with air e.g., Quercus suber (Cork Oak or Bottle Cork). The phelloderm, phellogen and phellem together constitute the periderm (Fig. 6.34).

Cork prevents the loss of water by evaporation. It also protects the interior against entry of harmful micro-organisms, mechanical injury and extremes of temperature. Cork is light, compressible, nonreactive and sufficiently resistant to fire.

It is used as stopper for bottles, shock absorption and insulation. At places phellogen produces aerating pores instead of cork. These pores are called lenticels. Each lenticel is filled by a mass of somewhat loosely arranged suberised cells called complementary cells.

Lenticels are aerating pores in the bark of plants. They appear on the surface of the bark as raised scars containing oval, rounded or oblong depressions (Fig. 6.34 A). They occur in woody trees but not in climbers. Normally they are formed in areas with underlying rays for facilitating gas exchange. Lenticels may occur scattered or form longi­tudinal rows.

A lenticel is commonly produced beneath a former stomate or stoma of the epidermis. Its margin is raised and is formed by surrounding cork cells. The lenticel is filled up by loosely arranged thin walled rounded and suberised (e.g., Prunus) or un-suberised cells called comple­mentary cells (Fig. 6.34 B).

They enclose intercellular spaces for gaseous exchange. The complementary cells are formed from loosely arranged phellogen cells and division of sub-stomatal parenchyma cells. The suberised nature of complementary cells checks excessive evaporation of water.

In temperate plants the lenticels get closed during the winter by the formation of com­pactly arranged closing cells over the complementary cells.

In common language and economic botany, all the dead cells lying outside phello­gen are collectively called bark. The outer layers of the bark are being constantly peeled off on account of the formation of new secondary vascular tissues in the interior. The peeling of the bark may occur in sheets (sheets or ring bark, e.g., Eucalyptus) or in irregular strips (scaly bark).

The scaly bark is formed when the phellogen arises in strips instead of rings, e.g., Acacia (vem. Kikar). Bark formed in early growing season is early or soft bark. The one formed towards end of growing season is late or hard bark.

Bark is insect repellent, decay proof, fire-proof and acts as a heat screen. Commercially it is employed in tanning (e.g., Acacia), drugs (e.g., Cinchona— quinine) or as spice (e.g., Cannamon, vem. Dalchini). The cork of Quercus suber is employed in the manufacture of bottle stoppers, insulators, floats, sound proofing and linoleum.

Significance of Secondary Growth:

1. Secondary growth adds to the girth of the plant. It provides support to increasing weight of the aerial growth.

2. Secondary growth produces a corky bark around the tree trunk that protects the interior from abrasion, heat, cold and infection.

3. It adds new conducting tissues for replacing old non-functioning ones as well as for meeting increased demand for long distance transport of sap and organic nutrients.

Anomalous Secondary Growth:

It is abnormal type of secondary growth that occurs in some arborescent monocots (e.g., Dracaena, Yucca, Agave) and storage roots (e.g., Beet, Sweet Potato). In arborescent monocot stems, a secondary cambium grows in hypodermal region. The latter forms con­junctive tissue and patches of meristematic cells. The meristematic patches grow into sec­ondary vascular bundles.

Anomalous vascular bundles also occur in cortex (cortical bundles, e.g., Nyctanthes) and pith (e.g., Boerhaavia). In storage roots (e.g., Beet), accessory cambial rings appear on the outside of endodermis. They produce less secondary xylem but more secondary phloem. The secondary phloem contains abundant storage parenchyma.

Importance of Secondary Growth:

1. It is a means of replacement of old non-functional tissues with new active tissues.

2. The plants showing secondary growth can grow and live longer as compared to other plants.

3. It provides a fire proof, insect proof and insulating cover around the older plant parts.

4. Commercial cork is a product of secondary growth. It is obtained from Quercussuber (Cork Oak).

5. Wood is a very important product of secondary growth. It represents secondary xylem.


DESCRIPTIONS

General introduction

Most of the data illustrated here derive from the lateral walls of elongate centally placed cells. Their geological antiquity points to tracheids as opposed to vessels, but there is little evidence of end walls in the record, and, at best, tracheidal status can be inferred from overlapping ends of cells in Rhynie Chert plants (e.g. Ventarura Powell, Edwards & Trewin 2000 ) where there are no indications of perforations (Fig. 2h), with supporting evidence from minor variations in cell diameter consistent with tapering ends.

In the absence of developmental information, the use of the terms primary (1°) and secondary (2°) wall must be treated with caution. This particularly applies to forms (e.g. S-type tracheids) which have little or no resemblance to extant examples (Fig. 3a–d).

The absence of developmental information also demands caution relating to identification of protoxylem, and is usually based on concentrations of the elements of smallest diameter, which are usually poorly preserved. The size distribution criterion works well in strands which are centrarch and large (e.g. Psilophyton Fig. 2f), but exarchy, particularly in pyrite permineralizations, is more difficult to demonstrate convincingly (Fig. 2e, but see the silicified zosterophyll axis in Fig. 2d). The vast majority of cells show no examples of distortion relating to extension growth and from their usually very regular thickenings are considered metaxylem. Isolated ‘spirals of secondary thickening’ have been illustrated in Leclercqia complexa ( Grierson 1976 ) whereas in another lycophyte, Drepanophycus qujingensis, the vertical stretching of pits in the wall between secondary thickenings (Fig. 2i) is also suggestive of elongation experienced by protoxylem ( Li & Edwards 1995 ). Secondary xylem has not yet been demonstrated in pre-Middle Devonian plants.

Although it is assumed from comparative biochemistry and phylogenetic relationships that the ‘secondary’ walls are lignified, this polymer has not yet been demonstrated in fossils of early land plants (see Ewbank et al. 1997 and above). This point is worthy of emphasis because the walls of bryophyte hydroids are reinforced by other polyphenols, and some of the cells described here have novel architecture which is difficult to match with conducting cells of embryophytes. However, the resistance to decay exhibited by these cells suggests that their walls were impregnated by lignin or a precursor.

The detailed descriptions and extensive illustrations presented here are necessary because reference cannot be made directly to tracheids of extant plants. In the case of the earliest xylem, the present is certainly not the key to the past, although some general similarities assist in its interpretation. The oldest examples of pitting which have been investigated in depth and which do not require such extensive exposition in view of their similarities to modern forms include the circular and scalariform bordered pits in the metaxylem of the Middle Devonian herbaceous lycophyte Leclercqia. ‘Conventional’ protoxylem, represented by annular and helical secondary thickenings and remnants of primary wall, was also illustrated ( Grierson 1976 ).

The letter prefixing tracheid types usually refers to the genus in which cells were first described ( Kenrick & Edwards 1988 Kenrick, Edwards & Dales 1991a Kenrick & Crane 1991 ).

G-type [ Fig. 5 , based on Gosslingia breconensis, a Lower Devonian zosterophyll ( Kenrick & Edwards 1988 )]

Schematic diagrams of areas of tracheid wall. Stippling indicates predominantly cellulose. (a) S-type. (b) P-type. (c) G-type. (d) I-type. (e) Minarodendron.

Secondary thickenings are annular, helical to occasionally approaching reticulate and are connected by a sheet of resilient material which is fused to the presumed primary wall. The sheet is perforated by holes of varying size and usually rounded shape (Fig. 2l. m. o & p). Some are laterally fused. Sizes may vary in a single sheet, and between tracheids in a strand, although appear generally consistent within a single tracheid. The surface of the thickenings are smooth and they sometimes have a presumed cellulose core (Fig. 2n). Inferences on the chemistry of the wall layers are based on the distribution of coalified material and mineral in pyrite permineralizations ( Kenrick & Edwards 1988 ), and the assumption that spaces filled with pyrite within wall systems (e.g. Fig. 2j) were areas where more readily metabolizable insoluble material (i.e. cellulose) had been removed by bacteria whereas coalified layers in the fossil were less easily biodegraded (i.e. lignified). The overall distribution of these perforations does not exactly match in adjacent tracheids (cf. conventional pits in tracheids, sclereids), although occasionally they coincide such that the cells appear directly connected (Fig. 2k). Whether or not the equivalent of a pit-closing membrane was originally present in these regions remains uncertain, particularly as in degradation of extant tracheary elements, pit closing membranes are the first wall structures to be metabolized and disappear ( Boutelje & Bravery 1968 ). However, such a layer persists in the P-type tracheids (below). Such uncertainty is frustrating in considering implications of the pit-closing membrane for the functioning of the elements.

Recent experimental studies on pyritization of plants indicate that pyrite can be precipitated and subsequently grow within cellulose cell walls and middle lamellae, thus possibly accentuating the thickness of the compound middle lamella in the fossils. Extant pteridophyte tracheids frequently show a core of cellulose and Cook & Friedman (1998 ) have recently demonstrated a partially lignified ‘degradation prone’ region within the secondary thickenings of the lycopsid Huperzia selago and of Equisetum ( Friedman & Cook 2000 ). Thus layering of walls occurs in extant forms. However, the frequently perforated connecting sheet has not been seen in extant metaxylem. The closest is in the protoxylem of the Psilotaceae ( Bierhorst 1960 ), where, following the development of typical annular and helical thickenings, an additional wall is deposited over the primary wall and covers all or part of the area between the thickenings. Such cells are frequently distorted and show no regular perforations of the sheet extended between the thickenings. This is developed to a varying extent and thus ‘outlined [one or two] simple pit-like areas.’ Bierhorst called the interconnecting layer the ‘secondary secondary wall’ with lignification complete or tapering away on either side of the secondary thickening.

Distribution

G-type tracheids characterize certain early members of the Lycophytina (sensu Kenrick & Crane 1997 ) including the Zosterophyllopsida, and a few early Lycopsida (e.g. Asteroxylon, Drepanophycus). They are normally recorded in silicified and pyritized xylem, and rarely in conventional coalified compressions (but see Hueber 1983 Baragwanathia). Thus, for example, Zosterophyllum myretonianum is described as possessing annular tracheids ( Lang 1927 ). These taxa, where three-dimensionally preserved, have exarch (Fig. 2d & e) or weakly mesarch strands. However, G-type tracheids also occur in the centrarch xylem of the Barinophytales (e.g. Barinophyton citrulliforme) ( Brauer 1980 ) and Hsuaceae ( Li 1992 ), whose gross morphology (strobili with or without bracts borne on naked axes and lateral trusses of terminal sporangia on naked axes, respectively) does not fit comfortably with either zosterophylls or lycopsids, and in Eophyllophyton, considered a basal euphyllophyte by Kenrick & Crane (1997 ).

P-type [ Fig. 5 , based mainly on Psilophyton dawsonii a Lower Devonian trimerophyte ( Hartman & Banks 1980 )]

Metaxylem tracheids are long (< 3·0 mm) and markedly faceted (5–7 sides) compared with G-type. Pitting is essentially scalariform and, in specimens superbly preserved in calcium carbonate-rich nodules, pit-closing membranes are present. SEMs of the thickenings (Fig. 3h), when the latter and compound middle lamella have been stripped away show the attachments of the thickenings as ridges (crassulae) emphasizing the narrowness of the attachment sites. Thus in section the vertically adjoining and adjacent thickenings produce a butterfly-like appearance, and the pitting may be described as scalariform bordered. Rare examples show almost circular bordered pits. However in addition, the transversely elongate pit aperture is transversed by a sheet of material which is perforated by rough circular holes arranged in one or two transverse rows or which comprises less regular strands or a reticulum (Fig. 3i). As in G-type tracheids the cores of the coalified thickenings are hollow, again suggestive that this area was originally occupied by cellulose, but in contrast to the G-type, the perforated sheet between thickenings is attached at the pit aperture rather than in the position of the pit-closing membrane. Protoxylem is described as having narrow spiral and scalariform elements ( Banks, Leclercq & Hueber 1975 ), but was not examined ultrastructually.

Distribution

Gensel (1979 ) recorded similar pitting in Psilophyton forbesii and P. charientos[and possibly in the trimerophyte Gothanophyton] and is currently researching this type in Pertica-like fossils (Gensel pers. comm. 2001). P-type thickenings might well therefore characterize tracheid architecture in early members of Euphyllophyta (sensu Kenrick & Crane 1997 ). However, broadly similar tracheids with perforated sheets, strands or a reticulum traversing pit apertures also occur in later Palaeozoic lycophytes. The best described is Minarodendron, a late Mid Devonian herbaceous lycophyte (Fig. 5 Li 1990 ), which differs in that the free surfaces of the scalariform thickenings extend prominently into the lumen, forming ridges and the resulting slightly depressed connecting sheets bear one or two transversely orientated rows of circular perforations (pitlets). A single row characterizes the axial xylem of Barsostrobus, an Upper Devonian lycophyte cone ( Fairon-Demaret 1977 ). Perforations also occur in Carboniferous herbaceous forms Selaginellites and Eskdalia. Vertical strands or a reticulum replace the perforated sheet in certain Carboniferous lepidodendrids and, in some taxa, are described as ‘Williamson's striations’. See Li (1990 ) for more detailed consideration.

S-type tracheids [ Fig. 5 , first identified in Sennicaulis hippocrepiformis, a taxon of sterile axes from the Lower Devonian ( Kenrick et al. 1991a) ]

Prominent helical (possibly annular) thickenings (Fig. 3a & b) show a spongy texture which is also present in the underlying and intervening lateral walls of the elements (Fig. 3c & d). Both thickenings and intervening lateral walls are covered by a very thin layer with numerous closely spaced holes. This construction (Fig. 5a) was originally elucidated from pyritized axes of Sennicaulis ( Kenrick et al. 1991a ) and to a lesser extent from limonite/goethite permineralizations of plants assigned to the Rhyniaceae ( Kenrick & Crane 1991 Kenrick, Remy & Crane 1991b ). The skimpy lining layer and spongy skeleton were thought to be composed of a lignin-like polymer because of their persistence in the fossils. The voids in the ‘sponge’ may have been filled with fluid or easily biodegraded polysaccharides (e.g. cellulose). Kenrick & Crane (1991 ) concluded that a similar construction occurred in silicified Rhynia gwynne-vaughanii and illustrated helical secondary thickenings which comprised large globular structures (approximately 4·5 µm diameter) intermingled with smaller examples (Fig. 3e & f). Because of the limitations of the chert for ultrastructural studies they could not provide details of the walls between elements, nor unequivocal evidence for a perforated lumen-lining layer. However, they did illustrate an extremely thin opaque layer (100–300 nm thick) in this region. Comparisons of the density of staining within cellulose walls in other tissues preserved in the chert with that in the interiors of the globule structures do not support a cellulosic composition.

Considering the perforations, although they are similar in size to plasmodesmata, their presence on the presumably last deposited layer of the wall covering the presumed sponge-filled secondary thickenings, make it unlikely that they had a similar developmental origin.

Distribution

This xylem architecture has been unequivocally demonstrated in Sennicaulis, Stockmansella, Huvenia, the probable gametophyte, Sciadophyton, and may occur in Rhynia gwynne-vaughanii ( Kenrick & Crane 1997 ). In all cases the xylem was probably centrarch, although distribution of cell size is difficult to distinguish in strands as small as in Rhynia and there appears to be a central zone of ?smaller cells in the strand of Sennicaulis. These plants are further united because their sporangia, where known, may have been abscissed or isolated from the parent axes at maturity, and they were placed in the Rhyniopsida by Kenrick & Crane (1997 ). Perhaps relevant to this account are the centrarch strands of tubular cells of Taeniocrada dubia described by Hueber 1982 ) in abstract, but not illustrated, as possessing ‘sponge-textured helical thickenings within the walls of the tubes and not the final innermost layer in the lumen as characteristic in the formation of walls in the tracheids’. A microporate layer lined the lumen and the limiting layer was described as thin and fibrillar. The latter has not been seen nor described in detail in the S-type tracheid. S-types are not illustrated in plants younger than the Middle Devonian. Figure 3g shows a possible coalified example from the Lochkovian (basal Devonian) in which central cells are two-layered, the inner perforated with pores, the smallest of plasmodesmata size (approximately 60 nm), the outer homogeneous and fused with that of adjacent cells. The lateral walls may be internally smooth (i.e. no helical/annular thickenings) or the perforated wall may form irregular ± horizontal folds or extend as hollow strands across the lumen. The latter may branch and partially occlude the lumen. These fragments of cells have the perforated layer in common with S-type tracheids, although it is much thicker than in the latter. (Cell diameter is much smaller, < 20 µm). A very small number show a spongy or granular texture in the outer wall, but this never extends into the folds.

Uniformly thick-walled cells

Examples are confined to plants with branching sporophytes and stomata. They have not been assigned a letter, because it seems likely that the plants in which they occur are not closely related, and the chemistry of the walls cannot be assumed to be similar

Nothia aphylla (Lower Devonian Rhynie Chert El-Saadawy & Lacey 1979 )

Kerp, Hass & Mossbruger (2001 ) recently described the water-conducting cells in this Rhynie Chert plant as elongate (< 700 µm long) fusiform cells with strongly thickened walls (2–3 µm thick) and tapering ends (Fig. 4k). They thus lack additional secondary thickenings or pitting. The walls appear dark and thus suggestive of lignification and were thought more similar to fibres or hydroids than tracheids by the authors. The existence of possible bilayering is evidenced by examples where the walls are split but remain attached at the corners. In the rhizome these cells occur in a central core with three other types including possible sieve cells. Xylem in erect axes is similar with smaller elements (approximately 10 µm diameter) in the centre and larger (< 30 µm) to the outside.

Aglaophyton major (Lower Devonian Rhynie Chert Edwards D.S. 1986 )

The central strand (Fig. 4d) of Aglaophyton has a core of thin-walled cells, which are angular in cross section and, although of variable shape and diameter (18–44 µm), show no regular gradation of size. It is surrounded by a few layers of cells with uniformly thick walls (1·5–2·0 µm), which are 22–50 µm in diameter. A further encircling zone is interpreted as phloem ( Edwards, D.S. 1986 ). The two inner types have dark-coloured walls, which Edwards attributed to the presence of lignin-like polyphenols. He found no evidence of conventional secondary thickenings, but variations in the appearance of the walls may hold clues to a more elaborate construction. In some examples he found evidence of bi-layering, a narrow outer layer and an inner thicker one, parts of which had separated or broken away, with fragments preserved in the matrix of the lumen. In others the walls exhibited a reticulate or more regular hexagonal appearance which Edwards attributed to partial degradation of the coalified wall (see also Lemoigne & Zdebska 1980 ). He considered the regularity in patterns indicative of crystallization and suggested that this had occurred in association with bacterial attack. Vesicles and their fusion were also observed in the elongate cells of the main central strand (Fig. 4h). A similar patterning was observed in the less robust central cells, and in both types he noted small spheres of remarkably regular size, termed vesicles (Figs 4g & i). These have been superbly illustrated in small ‘transition’ cells (Fig. 4e) in sporophyte and gametophyte ( Remy & Hass 1996 ). Ongoing research at Münster (Kerp and Hass, pers. comm.) suggests that vesicles lining the thinner wall form a continuous layer by lateral fusion which is of similar dimensions to Edwards’ thicker inner wall layer.

Distribution

Neither Nothia nor Aglaophyton have been found outside the Rhynie Chert. Kerp & Hass (pers. comm.) point to similarities in the construction of the secondary thickenings of Rhynia gwynne-vaughanii, where the spongy structure appears composed of vesicles of varying size (see also p. 65 and 66 (cf. Figs 3e & 4h) Kenrick & Crane 1991 ). The larger ones are of similar dimensions to those in Aglaophyton. Vesicles have not been observed in Nothia. Smooth cells have recently been demonstrated in the central strand of a small naked, stomatous fragment from the basal Devonian, which is impossible to name ( Edwards & Axe 2000 ).

The lack of conventional tracheidal thickenings in the central cells of the terete strand of Aglaophyton was pivotal to the removal of this genus from Rhynia and its isolation from the Tracheophyta ( Edwards, D.S 1986 Edwards & Edwards 1986 ). Kenrick & Crane (1997 ) placed it in the protracheophytes, a group of nonvascular polysporangiates. Other authors have commented on the similarities with a bryophytic conducting strand, particularly polytrichaceous examples ( Edwards, D. S. 1986 ). On the basis of all other anatomical attributes, Aglaophyton sporophytes and gametophytes (called Lyonophyton) would seem to have functioned as homoiohydric plants. The smooth walls of Nothia were considered to have been secondarily derived from G-type pitting by Kenrick & Crane (1997 ), who placed it in the zosterophylls, despite major differences in sporangial organization and dehiscence.

C-type tracheids (as seen in Cooksonia pertoni Edwards et al. 1992 )

These resemble conventional annular and spiral tracheids except that the imperforate lateral walls are thick (Fig. 3j & k) when compared with the primary wall in protoxylem. TEM observations show voids in the centre of the tracheids (Fig. 3k). Similar cells in a smooth unidentifable branching axis (Fig. 3b) show homogeneous thickenings which range from annular to sparcely reticulate (Fig. 3g & q). The composition of the lateral walls is unknown: all the data come from mesofossils.

Distribution

To date such organization has been described from only three Cooksonia pertoni specimens (Fig. 3j & k Edwards et al. 1992 ) and in one sterile branching axis with intact naked tip ( Edwards, Axe & Duckett in press ). It may also be the type present in the earliest illustrated tracheids (Upper Silurian: Edwards & Davies 1976 ) which were recovered on a film pull, and show similarities in diameter between transverse thickenings and vertical walls (Fig. 3l).

I-type [ Fig. 5 , known from indeterminate smooth stomatous axes Edwards & Axe 2000 Edwards et al. in press ]

Information derives from mesofossils. Central cells (approximately 12 µm diameter) have bilayered walls (Fig. 4a–c). The outer is imperforate and fused with that of adjacent cells (on homogenization 2 µm thick). The inner, sometimes detached layer, has rounded perforations, approximately 100–300 nm in diameter, with bevilled edges. In section, these holes widen slightly to the base of a cavity, thus superficially resembling bordered pits, but with much smaller dimensions. They do not appear to coincide on adjacent cells, but suitably fractioned cell wall complexes are rare (Fig. 4c). The distribution of pits led Edwards and Axe to divide the elements into two types α (Fig. 4a) and β (Fig. 4b). In retrospect this terminology was unfortunate and should be replaced by forma α and β. In forma α, the pits are scattered (< 4 µm −2 ) in forma β some are aligned, others fused. Fracture between aligned examples and separations of the inner layer, results in the production of partially detached squarish flakes. Some cells are characterized by solid fringes or rod-like projections with smooth surfaces.

Distribution

Three specimens are known, one of which shows branching ( Edwards et al. in press ). Occasional stomata are present. The absence of any sporangia precludes identification. A further impediment is the lack of information on the chemistry of the walls. This type of wall structure is superficially similar to simply pitted tracheary elements, although the shapes and distribution of the pits lack their regularity. Edwards & Axe (2000 ) noted similarities in sizes of pits with the perforations in hydroids of gametophytes of liverworts (Calobryales and Pallaviciniinae: Ligrone, Duckett & Renzaglia 2000 ), but stomata are absent in these lower plants.

Tubular structures with internal thickenings ‘Banded tubes’ (Infraturma Endomurali Burgess & Edwards 1991 )

Tubular aseptate structures with internal regular annular or helical thickenings (Fig. 3n–q) that are continuous (homogenized) with lateral walls (Fig. 3m) occur in organic residues produced when Upper Llandovery (basal Silurian) through Lower Devonian rocks are dissolved in hydrofluoric acid ( Edwards & Wellman 1996 ). Helical thickenings may be single or arranged in up to four spiralling bands in parallel that produce a diamond lattice type appearance in transmitted light (Porcatitubulus spiralis Burgess & Edwards 1991 ). More complex examples show close-set transverse ridging (P. microspiralis, P. microannulatus) and these can reach 750 µm long ( Wellman 1995 ). The vast majority are parallel-sided, quite wide (approximately 30 µm) and incomplete at both ends. One example is reported with an imperforate papillate tip ( Pratt, Phillips & Dennison 1978 ) another shows abrupt narrowing, but here the tube is broken ( Burgess & Edwards 1991 ). Although most are recovered as isolated tubes, they also occur in clusters with parallel alignment or less regular organization. Some are associated with meshes of smooth-walled tubes.

Distribution and affinity

When recovered on maceration of rocks, banded tubes are associated with phytodebris (cuticle and spores) indicative of a terrestrial origin. They occur in limited numbers in the Upper Llandovery, but increase in abundance and diversity throughout the Silurian. Burgess & Edwards (1991 ) named them using an artificial classification system devised for dispersed spores to facilitate their use in biostratigraphy. Their derivation remains controversial. Their similarities with tracheary elements are clear, and relatively thick lateral walls allow favourable comparison with the C-type, although the latter do not show such regularity, frequency and complexity in the thickenings. Niklas & Smocovitis (1983 ) isolated a strand of uniformly thick-walled and banded tubes from an irregularly shaped compression which they concluded was an indeterminate non-vascular land plant. In dispersed assemblages banded tubes are consistently associated with smooth tubes and cuticles of the Nematophytales, erected for land plants neither algal nor vascular ( Lang 1937 ), and have been recovered from nematophytalean Prototaxites-type plants (Nematosketum Burgess & Edwards 1988 ). Prototaxites itself has recently been assigned to the fungi ( Hueber 2001 ), and the presence of isolated tubes on and within a variety of organs has also led to the suggestion that they belonged to a saprotroph (e.g. Edwards & Richardson 2000 ). Further possible sources are the walls of bryophyte sporangia ( Kroken, Graham & Cook 1996 ). From these reports it is obvious that the presence of banded tubes in Llandovery rocks cannot be accepted as evidence for vascular plants and indeed even their functioning as water-conducting cells remains conjectural.


Complex Tissues: Xylem and Phloem (With Diagram)

The complex tissues are heterogeneous in nature, being com­posed of different types of cell elements. The latter remain contiguous and form a struc­tural part of the plant, adapted to carry on a specialised function.

Xylem and phloem are the complex tissues which constitute the component parts of the vascular bundle. They are also called vascular tissues.

The vascular system occupies a unique position in the plant body, both from the point of view of prominence and physiological importance. The term ‘vascular plants’ has been in use since a long time.

In recent years a new phylum Tracheophyta has been introduced to include all vascular plants it covers pteridophyta and spermatophyta of old classifications. Vascular bundles form a continuous and inter­connected system in the different organs of the plants.

They are primarily responsible for transport of water and solutes and elaborated food matters.

Xylem:

Xylem is a complex tissue forming a part of the vascular bundle. It is primarily ins­trumental for conduction of water and solutes, and also for mechanical support. Primary xylem originates from the procambium of apical meristem, and secondary xylem from the vascular cambium. As a complex tissue it consists of different types of cells and elements, living and non-living.

The tissues composing xylem are tracheids, tracheae or vessels, fibres, called xylem fibres or wood fibres, and parenchyma, referred to as xylem or wood parenchyma. Of the above mentioned elements only the parenchyma cells are living and the rest are dead. A term hadrome was once used for xylem. It included the elements excepting the fibres.

Tracheids:

A tracheid is a very much elongate cell (Fig. 538) occurring along the long axis of the organ. The cells are devoid of protoplast, and hence dead. A tracheid has a fairly large cavity or lumen without any contents and tapering blunt or chisel-like ends.

The end walls usually do not uniformly taper in all planes. Tracheids are round or polyhedral in cross-section. They are really the most primitive and fundamental cell- types in xylem from phylogenetic point of view. The wood of ancient vascular plants was exclusively made of tracheids. This is the only type of element found in the fossils of seed-plants.

In modern plants they practi­cally occur in all groups including the an­giosperms, though they predominate in lower vascular plants, the pteridophytes and gym­nosperms. More effective conducting ele­ments, tracheae or vessels, have evolved from the tracheids.

The wall is hard, moderately thick and usually lignified. Secondary walls are depo­sited in different manners, so that the tra­cheids may be annular, spiral, reticulate, scalariform or pitted. But pits of the bordered type are most abundant. Through these pits they establish communication with ad­joining tracheids and also with other cells, living or non-living.

The nature of the pits on the walls of the tracheids is variable in lower vascular plants the pits are elongated giving them scalariform appearance (Fig. 538 C & D), those of gymnosperms and angios­perms have round pits with well-developed borders (Fig. 538 A & B). Tracheids occur both in primary and secondary xylem.

Due to the presence of central lumen and hard lignified wall tracheids are nicely adapted for transport of water and solutes. They also serve as supporting tissue.

A typical fibre differs from a tracheid in more pronounced thickening of the wall and correspondingly much smaller lumen, as well as in reduction of the size of the pits. An intermediate type of cell element, called fibre-tracheid, is found in some plants.

They have smaller pits with reduced or vestigial borders. In some cases protoplast persists up to the mature stage, and may even divide, so that transverse partition walls are noticed within the original wall. These are called septate fibre-tracheids.

These are long tube-like bodies ideally suited for the con­duction of water and solutes. A trachea or vessel is formed from a row of cylindrical cells arranged in longitudinal series where the partition walls become perforated, so that the whole thing serves like a tube.

In tracheids the only openings are the pit-pairs, whereas the vessels are distinct ‘perforate’ bodies. Thus translocation of solutes becomes more easy in a vessel, as it proceeds more or less in a straight line but the line of conduc­tion is rather indirect in a group of tracheids.

Perforations are commonly confined to the end-walls, but they may occur on the lateral walls as well.

The walls undergoing perforations are referred to as perforation plates, which are mainly of two types multiple plates and simple ones. In primitive plants it has been found that the end-walls between the cells do not completely dissolve, but the openings or perforations remain either in more or less parallel series like bars called scalariform perforation (Fig. 539A) or in form of a network known as reticulate perforation, or even may form a group of circular holes (foraminate perforation).

In advanced types of plants the dissolution of the end-wall is more or less complete, and the perforation occurs in form of a single large circle. This is referred to as simple perforation (Fig. 539B).

There is ana­tomical evidence in support of the fact that the single large circular or oval perforation has been formed by gradual disappearance of the trans­verse bars of scalariform and other types. The vessels are considerably long bodies in ash plant, Fraxinus excelsior of family Oleaceae vessels has been reported to be as long as 10 ft.

Like tra­cheids these elements are devoid of protoplast and have hard and lignified cell-wall with different types of localised thickenings. Some forms inter­mediate between typical tracheids and vessels have been noticed. These elements, analogous to fibre-tracheids, are called vessel-tracheids.

Ontogeny of a Vessel:

A vessel or a tra­chea originates from a row of meristematic cells of procambium or vascular cambium which remain attached end on end in longitudinal series (Fig. 540).

As usual the cells grow and secondary walls are laid down, only the primary walls where perforations will take place remain uncovered. The secondary walls undergo lignification and other changes.

The protoplast in the mean time becomes progressively more and more vacuolated and ultimately dies and disappears. The primary walls swell due to increase of pectic inter­cellular substance and break down, thus forming the continuous vessel.

It should be noted that a vessel or trachea arises from a group of cells, unlike a tracheid, which is an elongate ‘imperforate’ single cell. The individual cells taking part in the formation of the vessel are called vessel elements.

The walls of the vessels are thick, hard and lignified. The secondary walls are depo­sited in different patterns, so that the thickenings may be ring-like, spiral, scalariform, reticulate or pitted. The pits are mostly of bordered types.

Tracheids are more primitive than the vessels. In fact, in the primitive types of ves­sels the form of a tracheid is maintained, but with advance in evolutionary line the dia­meter of a vessel may so much increase that it may become drum-shaped (Fig. 539 C & D) in appearance.

Vessels have originated phylogenetically from the tracheids and occur in the pteridophytes Pteridium and Selaginella, in the highest gymnosperms, Gnetales, and in the dicotyledons and monocotyledons.

Comparative-studies on the dicotyledons have revealed that evolution of vessel members have proceeded from the long narrow elements with tapering ends to short ones with wider cavities having transverse or inclined end-walls which ultimately dissolved.

Suggestions about independent development of vessels by parallel evolution has also been put forward (Cheadle, 1953). Vessels first appeared in the secondary xylem and then proceeded towards primary xylem.

In some dicotyledons belonging to the families Winteraceae, Trochodendraceae and Tetracentraceae and others of the lowest taxonomic group, curiously the vessels are absent (Bailey and others).

They do not occur in some xerophytes, parasites and aquatic plants. These have been interpreted as cases of reduction of xylem tissues involving evolutionary loss. In monocotyledons vessels are not present in secondary xylem (which tissue is lacking in many monocotyledons). Here vessels first appeared in the roots and then extended to the aerial organs (Cheadle, 󈧹 Fann. 󈧺).

These are the most important elements of xylem. They are primarily adapted for easy transport of water and solutes, and, secondarily, for mechanical support.

Xylem Fibres:

Some fibres remain associated with other elements in the complex tissue, xylem, and they mainly give mechanical support. As previously stated, fibres are very much elongated, usually dead cells with lignified walls.

Xylem fibres or wood fibres are mainly of two types: fibre-traeheids (Fig. 536 D & E) and libiriform fibres (Fig. 536 A & B) which usually intergrade, so much so that it is difficult to draw a line of de­marcation between them.

Fibre-tracheids, as already reported, are intermediate forms between typical fibres and tracheids they possess bordered pits, though the borders are not well-developed. Libiriform fibres ate narrow ones with highly thickened secondary wall.

The central lumen is almost obliterated and pits are simple. They resemble the phloem fibres, and hence the name. They occur abundantly in many woody dicotyledons.

Phylogeny of Tracheary Elements:

The tracheary elements have developed during the evolution of land plants (Bailey, 󈧹). In the lower vascular plants the func­tion of conduction and support were combined in the tracheids.

With increasing specialisa­tion woods evolved with conducting elements—the vessel members being more efficient in conduction that in providing mechanical support. On the other hand fibres evolved as principal supporting tissue.

Thus from the primitive tracheids two lines of specialisa­tion diverged—one toward the vessel and the other toward the fibre.

The following structural features may be taken as the basis in support of the evolu­tion of the tracheary elements from primitive tracheids which are usually long imper­forate cells with small diameter, angular in cross-section, having lignified scalariformly pitted walls.

(i) The primitive vessels are also elongate bodies like the tracheids with rather small diameter and tapering ends. Similar condition is still noticed in lower dicotyle­dons. With evolutionary advance they gradually become shorter and wider, often be­coming drum-shaped in appearance.

(ii) The wall of the primitive tracheid is rather thin, more or less of equal thickness, and it is angular in cross-section. Same condition prevails in primitive vessels. With progressive advance considerable thickening appeared and the vessels became circular or nearly so in cross-section.

(iii) In the primitive vessels the perforation plates are multiple, usually scalariform with numerous bars, and oblique end-walls. Progressive increase in specialisation led to gradual decrease in the number of bars and their ultimate disappearance, so that the perforation plates become simple with transverse end-walls. These are positively advanced characters.

(iv) The pitting of the vessel wall also changed from early scalariform arrangement, characteristic of tracheids, to small bordered pit pairs, first in opposite (arranged in transverse rows) and ultimately in alternate (arranged spirally or irregularly) pattern. Moreover the pit pairs between vessels and parenchyma changed from bordered to half-bordered and then to simple.

In the specialisation of the xylem fibres adapted for more efficient support there has been steady increase in thickness of the wall leading to decrease in cell-lumen. The pits changed from elongate to circular, the borders becoming reduced and functionless, and ultimately disappeared. Thus the evolutionary sequence was from tracheids, through fibre-tracheids to libiriform fibres.

Xylem Parenchyma:

Living parenchyma is a constituent of xylem of most plants. In primary xylem they remain associated with other elements and derive their origin from the same meristem. In secondary xylem parenchyma occurs in two forms: xylem parenchyma (Fig. 541 A) is somewhat elongate cells and lie in vertical series attached end on end ray parenchyma (Fig. 541 B) cells occur in radial transverse series in many woody plants.

Parenchyma is abundant in the secondary xylem of most of the plants, excepting a few conifers like Pinus, Taxus and Araucaria. These are the only living cells in xylem.

The cells may be thin-walled or thick-walled. If lignified secondary wall is present, the pit-pairs between the cells and the adjacent xylem element may be bordered, half-bordered or simple. Between two parenchyma cells the pit is obviously simple.

These cells are particularly meant for storage of starch and fatty food other matters like tannins, crystals, etc., may also be present. As a constituent part of xylem they are possibly involved in conduction of water and solutes and mechanical support.

Phloem:

The other specialised complex tissue forming a part of the vascular bundle is phloem It is composed of sieve elements, companion cells, parenchyma and some fibres. Sclerotic cells may also be present.

Phloem is chiefly instrumental for transloca­tion of organic solutes—the elaborated food materials in solution. The elements of phloem originate from the procambium of apical meristem or the vascular cambium.

Two terms, bast and leptome, have been used for phloem, though they are not exactly synonymous with it. Bast, derived from the word ‘bind’, was introduced before the
discovery of sieve elements it mainly meant the fibres. The soft-walled parts of phloem, obviously excluding the fibres, were referred to as leptome.

Sieve Elements:

The most important constituents of phloem are the sieve elements, the sieve tubes and sieve cells. From onto­genetic point of view a sieve tube resembles a vessel and a sieve cell a tracheid.

Sieve tubes (Fig. 542) are long tube-like bodies formed from a row of cells arranged in longitudinal series where the end-walls are perforated in a sieve-like manner. The perfo­rated end-walls are called the sieve plates, through which cytoplasmic connections are established between adjacent cells.

The perforations or sieve areas, as they are called, may be compared to the pit fields of the primary wall with plasmodesmata connec­tions. But the sieve areas are more promi­nent than pit fields and the connecting strands are more wide and conspicuous.

It may be that a number of plasmodesmata fuse to form a connecting strand. Moreover, an insoluble substance, called callose, pro­bably a carbohydrate of unknown chemical composition, is impregnated into cellulose or replaces cellulose forming a case round each connecting strand which passes through the sieve area (Fig. 543A).

A sieve area in surface view looks like a depression on the wall having a pretty good number of dots. Each dot represents a connecting strand in cross-section and remains surrounded by a case of callose (Fig. 543).

In sectional view sieve areas appear like thin places on the wall through which the connecting strands pass from one cell to another (Fig. 543). The sieve plate or the per­forated end-wall is really the primary walls of two cells with the middle lamella in between them. The end-walls may be obliquely inclined or transverse.

A sieve plate is called simple (Figs. 542 & 543), if it has only one sieve area, whereas the plate may be compound (Fig. 544) with several sieve areas arranged in scalariform, reticulate or other manners. Though rare, the sieve areas may occur on the side walls as well. From evolutionary point of view simple sieve plates on transverse end-walls are more advanced charac­ters than compound plates on oblique walls.

The cylindri­cal cells which take part in the formation of the sieve tube are called sieve tube elements. Like vessel elements the sieve tubes have also undergone decrease in length with evolutionary advance.

Sieve cells (Fig. 542C), which may be compared to the tracheids, are narrow elongated cells without conspicuous sieve areas. They usually have greatly inclined walls, which overlap in the tissue, sieve areas being more numerous in the ends.

Sieve cells are more primitive than the sieve tubes. They occur in lower vascular plants and

gymnosperms. In fact, sieve tubes have evolved from the sieve cells, as vessels have evolved from the tracheids, and so sieve tubes occur in all angiosperms. In mono­cotyledons, unlike the xylem elements, sieve tubes first appeared in the aerial organs, the course being from the leaves to the stem and, lastly, to the roots.

Ontogeny of the Sieve Elements:

In spite of close ontogenetic resemblance between tracheary elements of xylem and sieve elements of phloem, the latter unlike the former, are living. They originate from the mother cells (Fig 545) which are usually short cylindrical or elongate ones.

The mother cell divides longitudinally into two daughter cells, one of which serves as the sieve element and the other one becomes the companion cell, of course in those cases where companion, cells occur. The sieve element undergoes gradual differentiation. It grows in length, cytoplasm gets more and more vacuolated, so that it may have a lining layer of cytoplasm round a large central vacuole.

The most outstanding character is the disintegration of the nucleus with the maturity of the sieve elements. In fact, a distinct nucleus is present in every cell at the meristematic stage. During differentiation the nucleus disorganises (Fig. 545F).

It is the only living functioning element without a nucleus. Small colourless plastids are also present in the protoplast. They contain carbohydrates which give wine-red reaction with iodine and are interpreted as starch grains. Slimy proteinaceous bodies abundantly occur in the sieve tubes, what is commonly called slime. It is said that slime originates in the cytoplasm as small discrete bodies, which eventually fuse and get dispersed in the vacuoles.

In fixed preparations funnel-shaped slime bodies may be distinctly seen in form of plates referred to as slime plugs (Fig. 545H), on the sieve plates. In this connection a very interesting statement has come from a well-known authority, Prof. K. Esau, to the effect that in some plants the nucleolus is extruded from the nucleus before it finally disorganises and that the nucleolus persists in the tube.

Slime bodies have not been observed in pteridophytes, gymnosperms and monocotyledons. Sieve areas develop from the primary pit fields and the connecting strands originating from one or a group of plasmodesmata become more conspicuous which remain surrounded by callose cylinders.

Another theory demands that pores are formed by dissolution of cell wall and no plasmodesmata occur at the pore sites. The connecting strands were thought to be entirely cytoplasmic in nature but it is argued that may contain vacuolar substances and thus establish connections between vacuoles of neighbouring elements.

The wall of sieve elements is primary and chiefly composed of cellulose. Thick walls are found only in exceptional cases. The tubes often cannot withstand the pressure from adjoining cells and ultimately get crushed.

It has been stated that protoplasmic strands pass through the pores of the sieve areas and that the strands remain surrounded by callose. With the differentiation of the tube the amount of callose increases and finally forms something like a pad on the sieve plate.

This pad is referred to as callus pad. Due to its formation the cell to cell communica­tion is considerably cut down or entirely prevented. The callus pad is usually formed with the approach of resting or inactive season and it disappears when the active season (spring) sets in. This type is Known as seasonal or dormancy callus.

In old functionless sieve tubes callus becomes permanent, what is called definitive callus. Though the term defini­tive callus is often used to designate the former type, it is desirable to confine it to perma­nent callus of old and functionless tubes.

Companion Cells:

Companion cells (Figs. 542 & 545) remain associated with the sieve tubes of angiosperms, both ontogenetically and physiologically. These are smaller elongate cells, having dense cytoplasm and prominent nuclei. Starch grains are never present.

They occur along the lateral walls of the sieve tubes. A companion cell may be equal in length to the accompanying sieve tube element or the mother cell may be divid­ed transversely forming a series of companion cells (Fig. 545).

A sieve tube element and a companion cell originate from the same mother cell. Their functional association is evi­dent from the fact that companion cells continue so long the sieve tubes function, and die when the tubes are disorganised.

The companion cells are so firmly attached to the sieve tubes that they cannot be normally separated by maceration. In transverse section it appears as a small triangular, rectangular or polyhedral cell with dense protoplast (Figs. 542 & 545).

In pteridophytes and gymnosperms some small parenchymatous cells remain asso­ciated with sieve cells, which are known as albuminous cells. They die in natural course when the sieve cells become functionless. Thus the relation between sieve Cells and albu­minous Cells is similar to that existing between sieve tubes and companion cells, except­ing that they have no common origin.

Companion cells occur abundantly in angiosperms, particularly in the monocotyledons. They are absent in some primitive dicotyledons and also in the primary phloem of some angiosperms. The wall between the sieve tube and companion cell is thin and provided with primary pit fields.

Parenchyma:

Besides companion cells and albuminous cells, a good number of parenchyma cells remain associated with sieve elements. These are living cells with cellu­lose walls having primary pit fields. They are mainly concerned with storage of organic food matters. Tannins, crystals and other materials may also be present.

The parenchyma cells of primary phloem are somewhat elongate and occur with the sieve elements along the long axis (Fig. 542). In secondary phloem they may be of two types.

Those which occur in vertical series are called phloem parenchyma and others occur­ring in horizontal planes are known as ray cells, the position being just like the parenchyma and ray cells of secondary xylem. The cell wall is primary, composed of cellulose. Parenchyma is absent in the phloem of monocotyledons.

Fibres:

Sclerenchymatous fibres constitute a part of phloem in a large number of seed plants, though they are rare in pteridophytes and some spermatophytes. They occur both in primary and secondary phloem. These are typical elongated cells having inter­locked ends, lignified walls with simple pits. The fibres of primary phloem are essentially similar to those occurring in cortex and secondary phloem.

They are of considerable commercial importance, as these fibres are abundantly used for the manufacture of ropes and cords. The flax fibres, unlike others, have non-lignified walls. Sclerotic cells are often present in primary phloem. They probably develop from parenchyma with the age of the tissue. So it is a case of ‘secondary sclerosis’.


DESCRIPTIONS

General introduction

Most of the data illustrated here derive from the lateral walls of elongate centally placed cells. Their geological antiquity points to tracheids as opposed to vessels, but there is little evidence of end walls in the record, and, at best, tracheidal status can be inferred from overlapping ends of cells in Rhynie Chert plants (e.g. Ventarura Powell, Edwards & Trewin 2000 ) where there are no indications of perforations (Fig. 2h), with supporting evidence from minor variations in cell diameter consistent with tapering ends.

In the absence of developmental information, the use of the terms primary (1°) and secondary (2°) wall must be treated with caution. This particularly applies to forms (e.g. S-type tracheids) which have little or no resemblance to extant examples (Fig. 3a–d).

The absence of developmental information also demands caution relating to identification of protoxylem, and is usually based on concentrations of the elements of smallest diameter, which are usually poorly preserved. The size distribution criterion works well in strands which are centrarch and large (e.g. Psilophyton Fig. 2f), but exarchy, particularly in pyrite permineralizations, is more difficult to demonstrate convincingly (Fig. 2e, but see the silicified zosterophyll axis in Fig. 2d). The vast majority of cells show no examples of distortion relating to extension growth and from their usually very regular thickenings are considered metaxylem. Isolated ‘spirals of secondary thickening’ have been illustrated in Leclercqia complexa ( Grierson 1976 ) whereas in another lycophyte, Drepanophycus qujingensis, the vertical stretching of pits in the wall between secondary thickenings (Fig. 2i) is also suggestive of elongation experienced by protoxylem ( Li & Edwards 1995 ). Secondary xylem has not yet been demonstrated in pre-Middle Devonian plants.

Although it is assumed from comparative biochemistry and phylogenetic relationships that the ‘secondary’ walls are lignified, this polymer has not yet been demonstrated in fossils of early land plants (see Ewbank et al. 1997 and above). This point is worthy of emphasis because the walls of bryophyte hydroids are reinforced by other polyphenols, and some of the cells described here have novel architecture which is difficult to match with conducting cells of embryophytes. However, the resistance to decay exhibited by these cells suggests that their walls were impregnated by lignin or a precursor.

The detailed descriptions and extensive illustrations presented here are necessary because reference cannot be made directly to tracheids of extant plants. In the case of the earliest xylem, the present is certainly not the key to the past, although some general similarities assist in its interpretation. The oldest examples of pitting which have been investigated in depth and which do not require such extensive exposition in view of their similarities to modern forms include the circular and scalariform bordered pits in the metaxylem of the Middle Devonian herbaceous lycophyte Leclercqia. ‘Conventional’ protoxylem, represented by annular and helical secondary thickenings and remnants of primary wall, was also illustrated ( Grierson 1976 ).

The letter prefixing tracheid types usually refers to the genus in which cells were first described ( Kenrick & Edwards 1988 Kenrick, Edwards & Dales 1991a Kenrick & Crane 1991 ).

G-type [ Fig. 5 , based on Gosslingia breconensis, a Lower Devonian zosterophyll ( Kenrick & Edwards 1988 )]

Schematic diagrams of areas of tracheid wall. Stippling indicates predominantly cellulose. (a) S-type. (b) P-type. (c) G-type. (d) I-type. (e) Minarodendron.

Secondary thickenings are annular, helical to occasionally approaching reticulate and are connected by a sheet of resilient material which is fused to the presumed primary wall. The sheet is perforated by holes of varying size and usually rounded shape (Fig. 2l. m. o & p). Some are laterally fused. Sizes may vary in a single sheet, and between tracheids in a strand, although appear generally consistent within a single tracheid. The surface of the thickenings are smooth and they sometimes have a presumed cellulose core (Fig. 2n). Inferences on the chemistry of the wall layers are based on the distribution of coalified material and mineral in pyrite permineralizations ( Kenrick & Edwards 1988 ), and the assumption that spaces filled with pyrite within wall systems (e.g. Fig. 2j) were areas where more readily metabolizable insoluble material (i.e. cellulose) had been removed by bacteria whereas coalified layers in the fossil were less easily biodegraded (i.e. lignified). The overall distribution of these perforations does not exactly match in adjacent tracheids (cf. conventional pits in tracheids, sclereids), although occasionally they coincide such that the cells appear directly connected (Fig. 2k). Whether or not the equivalent of a pit-closing membrane was originally present in these regions remains uncertain, particularly as in degradation of extant tracheary elements, pit closing membranes are the first wall structures to be metabolized and disappear ( Boutelje & Bravery 1968 ). However, such a layer persists in the P-type tracheids (below). Such uncertainty is frustrating in considering implications of the pit-closing membrane for the functioning of the elements.

Recent experimental studies on pyritization of plants indicate that pyrite can be precipitated and subsequently grow within cellulose cell walls and middle lamellae, thus possibly accentuating the thickness of the compound middle lamella in the fossils. Extant pteridophyte tracheids frequently show a core of cellulose and Cook & Friedman (1998 ) have recently demonstrated a partially lignified ‘degradation prone’ region within the secondary thickenings of the lycopsid Huperzia selago and of Equisetum ( Friedman & Cook 2000 ). Thus layering of walls occurs in extant forms. However, the frequently perforated connecting sheet has not been seen in extant metaxylem. The closest is in the protoxylem of the Psilotaceae ( Bierhorst 1960 ), where, following the development of typical annular and helical thickenings, an additional wall is deposited over the primary wall and covers all or part of the area between the thickenings. Such cells are frequently distorted and show no regular perforations of the sheet extended between the thickenings. This is developed to a varying extent and thus ‘outlined [one or two] simple pit-like areas.’ Bierhorst called the interconnecting layer the ‘secondary secondary wall’ with lignification complete or tapering away on either side of the secondary thickening.

Distribution

G-type tracheids characterize certain early members of the Lycophytina (sensu Kenrick & Crane 1997 ) including the Zosterophyllopsida, and a few early Lycopsida (e.g. Asteroxylon, Drepanophycus). They are normally recorded in silicified and pyritized xylem, and rarely in conventional coalified compressions (but see Hueber 1983 Baragwanathia). Thus, for example, Zosterophyllum myretonianum is described as possessing annular tracheids ( Lang 1927 ). These taxa, where three-dimensionally preserved, have exarch (Fig. 2d & e) or weakly mesarch strands. However, G-type tracheids also occur in the centrarch xylem of the Barinophytales (e.g. Barinophyton citrulliforme) ( Brauer 1980 ) and Hsuaceae ( Li 1992 ), whose gross morphology (strobili with or without bracts borne on naked axes and lateral trusses of terminal sporangia on naked axes, respectively) does not fit comfortably with either zosterophylls or lycopsids, and in Eophyllophyton, considered a basal euphyllophyte by Kenrick & Crane (1997 ).

P-type [ Fig. 5 , based mainly on Psilophyton dawsonii a Lower Devonian trimerophyte ( Hartman & Banks 1980 )]

Metaxylem tracheids are long (< 3·0 mm) and markedly faceted (5–7 sides) compared with G-type. Pitting is essentially scalariform and, in specimens superbly preserved in calcium carbonate-rich nodules, pit-closing membranes are present. SEMs of the thickenings (Fig. 3h), when the latter and compound middle lamella have been stripped away show the attachments of the thickenings as ridges (crassulae) emphasizing the narrowness of the attachment sites. Thus in section the vertically adjoining and adjacent thickenings produce a butterfly-like appearance, and the pitting may be described as scalariform bordered. Rare examples show almost circular bordered pits. However in addition, the transversely elongate pit aperture is transversed by a sheet of material which is perforated by rough circular holes arranged in one or two transverse rows or which comprises less regular strands or a reticulum (Fig. 3i). As in G-type tracheids the cores of the coalified thickenings are hollow, again suggestive that this area was originally occupied by cellulose, but in contrast to the G-type, the perforated sheet between thickenings is attached at the pit aperture rather than in the position of the pit-closing membrane. Protoxylem is described as having narrow spiral and scalariform elements ( Banks, Leclercq & Hueber 1975 ), but was not examined ultrastructually.

Distribution

Gensel (1979 ) recorded similar pitting in Psilophyton forbesii and P. charientos[and possibly in the trimerophyte Gothanophyton] and is currently researching this type in Pertica-like fossils (Gensel pers. comm. 2001). P-type thickenings might well therefore characterize tracheid architecture in early members of Euphyllophyta (sensu Kenrick & Crane 1997 ). However, broadly similar tracheids with perforated sheets, strands or a reticulum traversing pit apertures also occur in later Palaeozoic lycophytes. The best described is Minarodendron, a late Mid Devonian herbaceous lycophyte (Fig. 5 Li 1990 ), which differs in that the free surfaces of the scalariform thickenings extend prominently into the lumen, forming ridges and the resulting slightly depressed connecting sheets bear one or two transversely orientated rows of circular perforations (pitlets). A single row characterizes the axial xylem of Barsostrobus, an Upper Devonian lycophyte cone ( Fairon-Demaret 1977 ). Perforations also occur in Carboniferous herbaceous forms Selaginellites and Eskdalia. Vertical strands or a reticulum replace the perforated sheet in certain Carboniferous lepidodendrids and, in some taxa, are described as ‘Williamson's striations’. See Li (1990 ) for more detailed consideration.

S-type tracheids [ Fig. 5 , first identified in Sennicaulis hippocrepiformis, a taxon of sterile axes from the Lower Devonian ( Kenrick et al. 1991a) ]

Prominent helical (possibly annular) thickenings (Fig. 3a & b) show a spongy texture which is also present in the underlying and intervening lateral walls of the elements (Fig. 3c & d). Both thickenings and intervening lateral walls are covered by a very thin layer with numerous closely spaced holes. This construction (Fig. 5a) was originally elucidated from pyritized axes of Sennicaulis ( Kenrick et al. 1991a ) and to a lesser extent from limonite/goethite permineralizations of plants assigned to the Rhyniaceae ( Kenrick & Crane 1991 Kenrick, Remy & Crane 1991b ). The skimpy lining layer and spongy skeleton were thought to be composed of a lignin-like polymer because of their persistence in the fossils. The voids in the ‘sponge’ may have been filled with fluid or easily biodegraded polysaccharides (e.g. cellulose). Kenrick & Crane (1991 ) concluded that a similar construction occurred in silicified Rhynia gwynne-vaughanii and illustrated helical secondary thickenings which comprised large globular structures (approximately 4·5 µm diameter) intermingled with smaller examples (Fig. 3e & f). Because of the limitations of the chert for ultrastructural studies they could not provide details of the walls between elements, nor unequivocal evidence for a perforated lumen-lining layer. However, they did illustrate an extremely thin opaque layer (100–300 nm thick) in this region. Comparisons of the density of staining within cellulose walls in other tissues preserved in the chert with that in the interiors of the globule structures do not support a cellulosic composition.

Considering the perforations, although they are similar in size to plasmodesmata, their presence on the presumably last deposited layer of the wall covering the presumed sponge-filled secondary thickenings, make it unlikely that they had a similar developmental origin.

Distribution

This xylem architecture has been unequivocally demonstrated in Sennicaulis, Stockmansella, Huvenia, the probable gametophyte, Sciadophyton, and may occur in Rhynia gwynne-vaughanii ( Kenrick & Crane 1997 ). In all cases the xylem was probably centrarch, although distribution of cell size is difficult to distinguish in strands as small as in Rhynia and there appears to be a central zone of ?smaller cells in the strand of Sennicaulis. These plants are further united because their sporangia, where known, may have been abscissed or isolated from the parent axes at maturity, and they were placed in the Rhyniopsida by Kenrick & Crane (1997 ). Perhaps relevant to this account are the centrarch strands of tubular cells of Taeniocrada dubia described by Hueber 1982 ) in abstract, but not illustrated, as possessing ‘sponge-textured helical thickenings within the walls of the tubes and not the final innermost layer in the lumen as characteristic in the formation of walls in the tracheids’. A microporate layer lined the lumen and the limiting layer was described as thin and fibrillar. The latter has not been seen nor described in detail in the S-type tracheid. S-types are not illustrated in plants younger than the Middle Devonian. Figure 3g shows a possible coalified example from the Lochkovian (basal Devonian) in which central cells are two-layered, the inner perforated with pores, the smallest of plasmodesmata size (approximately 60 nm), the outer homogeneous and fused with that of adjacent cells. The lateral walls may be internally smooth (i.e. no helical/annular thickenings) or the perforated wall may form irregular ± horizontal folds or extend as hollow strands across the lumen. The latter may branch and partially occlude the lumen. These fragments of cells have the perforated layer in common with S-type tracheids, although it is much thicker than in the latter. (Cell diameter is much smaller, < 20 µm). A very small number show a spongy or granular texture in the outer wall, but this never extends into the folds.

Uniformly thick-walled cells

Examples are confined to plants with branching sporophytes and stomata. They have not been assigned a letter, because it seems likely that the plants in which they occur are not closely related, and the chemistry of the walls cannot be assumed to be similar

Nothia aphylla (Lower Devonian Rhynie Chert El-Saadawy & Lacey 1979 )

Kerp, Hass & Mossbruger (2001 ) recently described the water-conducting cells in this Rhynie Chert plant as elongate (< 700 µm long) fusiform cells with strongly thickened walls (2–3 µm thick) and tapering ends (Fig. 4k). They thus lack additional secondary thickenings or pitting. The walls appear dark and thus suggestive of lignification and were thought more similar to fibres or hydroids than tracheids by the authors. The existence of possible bilayering is evidenced by examples where the walls are split but remain attached at the corners. In the rhizome these cells occur in a central core with three other types including possible sieve cells. Xylem in erect axes is similar with smaller elements (approximately 10 µm diameter) in the centre and larger (< 30 µm) to the outside.

Aglaophyton major (Lower Devonian Rhynie Chert Edwards D.S. 1986 )

The central strand (Fig. 4d) of Aglaophyton has a core of thin-walled cells, which are angular in cross section and, although of variable shape and diameter (18–44 µm), show no regular gradation of size. It is surrounded by a few layers of cells with uniformly thick walls (1·5–2·0 µm), which are 22–50 µm in diameter. A further encircling zone is interpreted as phloem ( Edwards, D.S. 1986 ). The two inner types have dark-coloured walls, which Edwards attributed to the presence of lignin-like polyphenols. He found no evidence of conventional secondary thickenings, but variations in the appearance of the walls may hold clues to a more elaborate construction. In some examples he found evidence of bi-layering, a narrow outer layer and an inner thicker one, parts of which had separated or broken away, with fragments preserved in the matrix of the lumen. In others the walls exhibited a reticulate or more regular hexagonal appearance which Edwards attributed to partial degradation of the coalified wall (see also Lemoigne & Zdebska 1980 ). He considered the regularity in patterns indicative of crystallization and suggested that this had occurred in association with bacterial attack. Vesicles and their fusion were also observed in the elongate cells of the main central strand (Fig. 4h). A similar patterning was observed in the less robust central cells, and in both types he noted small spheres of remarkably regular size, termed vesicles (Figs 4g & i). These have been superbly illustrated in small ‘transition’ cells (Fig. 4e) in sporophyte and gametophyte ( Remy & Hass 1996 ). Ongoing research at Münster (Kerp and Hass, pers. comm.) suggests that vesicles lining the thinner wall form a continuous layer by lateral fusion which is of similar dimensions to Edwards’ thicker inner wall layer.

Distribution

Neither Nothia nor Aglaophyton have been found outside the Rhynie Chert. Kerp & Hass (pers. comm.) point to similarities in the construction of the secondary thickenings of Rhynia gwynne-vaughanii, where the spongy structure appears composed of vesicles of varying size (see also p. 65 and 66 (cf. Figs 3e & 4h) Kenrick & Crane 1991 ). The larger ones are of similar dimensions to those in Aglaophyton. Vesicles have not been observed in Nothia. Smooth cells have recently been demonstrated in the central strand of a small naked, stomatous fragment from the basal Devonian, which is impossible to name ( Edwards & Axe 2000 ).

The lack of conventional tracheidal thickenings in the central cells of the terete strand of Aglaophyton was pivotal to the removal of this genus from Rhynia and its isolation from the Tracheophyta ( Edwards, D.S 1986 Edwards & Edwards 1986 ). Kenrick & Crane (1997 ) placed it in the protracheophytes, a group of nonvascular polysporangiates. Other authors have commented on the similarities with a bryophytic conducting strand, particularly polytrichaceous examples ( Edwards, D. S. 1986 ). On the basis of all other anatomical attributes, Aglaophyton sporophytes and gametophytes (called Lyonophyton) would seem to have functioned as homoiohydric plants. The smooth walls of Nothia were considered to have been secondarily derived from G-type pitting by Kenrick & Crane (1997 ), who placed it in the zosterophylls, despite major differences in sporangial organization and dehiscence.

C-type tracheids (as seen in Cooksonia pertoni Edwards et al. 1992 )

These resemble conventional annular and spiral tracheids except that the imperforate lateral walls are thick (Fig. 3j & k) when compared with the primary wall in protoxylem. TEM observations show voids in the centre of the tracheids (Fig. 3k). Similar cells in a smooth unidentifable branching axis (Fig. 3b) show homogeneous thickenings which range from annular to sparcely reticulate (Fig. 3g & q). The composition of the lateral walls is unknown: all the data come from mesofossils.

Distribution

To date such organization has been described from only three Cooksonia pertoni specimens (Fig. 3j & k Edwards et al. 1992 ) and in one sterile branching axis with intact naked tip ( Edwards, Axe & Duckett in press ). It may also be the type present in the earliest illustrated tracheids (Upper Silurian: Edwards & Davies 1976 ) which were recovered on a film pull, and show similarities in diameter between transverse thickenings and vertical walls (Fig. 3l).

I-type [ Fig. 5 , known from indeterminate smooth stomatous axes Edwards & Axe 2000 Edwards et al. in press ]

Information derives from mesofossils. Central cells (approximately 12 µm diameter) have bilayered walls (Fig. 4a–c). The outer is imperforate and fused with that of adjacent cells (on homogenization 2 µm thick). The inner, sometimes detached layer, has rounded perforations, approximately 100–300 nm in diameter, with bevilled edges. In section, these holes widen slightly to the base of a cavity, thus superficially resembling bordered pits, but with much smaller dimensions. They do not appear to coincide on adjacent cells, but suitably fractioned cell wall complexes are rare (Fig. 4c). The distribution of pits led Edwards and Axe to divide the elements into two types α (Fig. 4a) and β (Fig. 4b). In retrospect this terminology was unfortunate and should be replaced by forma α and β. In forma α, the pits are scattered (< 4 µm −2 ) in forma β some are aligned, others fused. Fracture between aligned examples and separations of the inner layer, results in the production of partially detached squarish flakes. Some cells are characterized by solid fringes or rod-like projections with smooth surfaces.

Distribution

Three specimens are known, one of which shows branching ( Edwards et al. in press ). Occasional stomata are present. The absence of any sporangia precludes identification. A further impediment is the lack of information on the chemistry of the walls. This type of wall structure is superficially similar to simply pitted tracheary elements, although the shapes and distribution of the pits lack their regularity. Edwards & Axe (2000 ) noted similarities in sizes of pits with the perforations in hydroids of gametophytes of liverworts (Calobryales and Pallaviciniinae: Ligrone, Duckett & Renzaglia 2000 ), but stomata are absent in these lower plants.

Tubular structures with internal thickenings ‘Banded tubes’ (Infraturma Endomurali Burgess & Edwards 1991 )

Tubular aseptate structures with internal regular annular or helical thickenings (Fig. 3n–q) that are continuous (homogenized) with lateral walls (Fig. 3m) occur in organic residues produced when Upper Llandovery (basal Silurian) through Lower Devonian rocks are dissolved in hydrofluoric acid ( Edwards & Wellman 1996 ). Helical thickenings may be single or arranged in up to four spiralling bands in parallel that produce a diamond lattice type appearance in transmitted light (Porcatitubulus spiralis Burgess & Edwards 1991 ). More complex examples show close-set transverse ridging (P. microspiralis, P. microannulatus) and these can reach 750 µm long ( Wellman 1995 ). The vast majority are parallel-sided, quite wide (approximately 30 µm) and incomplete at both ends. One example is reported with an imperforate papillate tip ( Pratt, Phillips & Dennison 1978 ) another shows abrupt narrowing, but here the tube is broken ( Burgess & Edwards 1991 ). Although most are recovered as isolated tubes, they also occur in clusters with parallel alignment or less regular organization. Some are associated with meshes of smooth-walled tubes.

Distribution and affinity

When recovered on maceration of rocks, banded tubes are associated with phytodebris (cuticle and spores) indicative of a terrestrial origin. They occur in limited numbers in the Upper Llandovery, but increase in abundance and diversity throughout the Silurian. Burgess & Edwards (1991 ) named them using an artificial classification system devised for dispersed spores to facilitate their use in biostratigraphy. Their derivation remains controversial. Their similarities with tracheary elements are clear, and relatively thick lateral walls allow favourable comparison with the C-type, although the latter do not show such regularity, frequency and complexity in the thickenings. Niklas & Smocovitis (1983 ) isolated a strand of uniformly thick-walled and banded tubes from an irregularly shaped compression which they concluded was an indeterminate non-vascular land plant. In dispersed assemblages banded tubes are consistently associated with smooth tubes and cuticles of the Nematophytales, erected for land plants neither algal nor vascular ( Lang 1937 ), and have been recovered from nematophytalean Prototaxites-type plants (Nematosketum Burgess & Edwards 1988 ). Prototaxites itself has recently been assigned to the fungi ( Hueber 2001 ), and the presence of isolated tubes on and within a variety of organs has also led to the suggestion that they belonged to a saprotroph (e.g. Edwards & Richardson 2000 ). Further possible sources are the walls of bryophyte sporangia ( Kroken, Graham & Cook 1996 ). From these reports it is obvious that the presence of banded tubes in Llandovery rocks cannot be accepted as evidence for vascular plants and indeed even their functioning as water-conducting cells remains conjectural.


Xylem cell death-related signalling

The signals related to initiation and execution of xylem cell death are poorly understood. This is partly due to difficulties in identifying signalling that is specifically related to cell death and not secondary cell wall formation. Most pharmacological agents that block xylem cell death also block secondary cell wall formation ( Yamamoto et al., 1997 Groover and Jones, 1999 Yu et al., 2002 Twumasi et al., 2010), suggesting that the two processes are co-regulated. Co-regulation of xylem maturation has indeed recently been demonstrated to occur via the action of NAC transcription factors, as described later in this review. However, it is clear that even though the different phases of xylem maturation are jointly regulated by a few master switches, it is likely that the individual processes have separate controls as well. For instance, bursting of the vacuole must involve unique regulatory aspects to allow the correct timing of cellular autolysis in response to endogenous and exogenous stimuli.

Plant growth regulators

Auxins and cytokinins are prerequisites for TE differentiation in vitro ( Fukuda and Komamine 1980), but it seems that their only function is the early reprogramming of mesophyll cells into the TE differentiation programme ( Milioni et al., 2001). Brassinosteroids, on the other hand, are believed to play a role during late xylem maturation based on experiments with Zinnia TEs in vitro. Brassinosteroid precursors have been shown to accumulate during TE differentiation, whereas inhibition of brassinosteroid synthesis in TE cultures undergoing differentiation prevented cells from maturing and undergoing cell death ( Yamamoto et al., 1997). Ethylene is another hormone that deserves special attention based on its crucial function in other cell death processes ( He et al., 1996 Tuominen et al., 2004). It has been shown that maturing Zinnia TEs accumulate ethylene ( Pesquet and Tuominen, 2011), whereas blocking ethylene signalling using silver thiosulphate (STS) appears to block TE maturation (E. Pesquet and H. Tuominen, unpublished results). STS-induced changes in TE maturation are unique in the sense that TEs develop cellulosic secondary walls but do not lignify or die. It has been shown recently that cell death precedes bulk lignification in TEs in vitro ( Pesquet et al., 2010), which means that the STS-mediated arrest of TE maturation is most probably due to blocking of cell death, which in turn blocks lignification. Therefore, it can be concluded on the basis of these experiments in the Zinnia in vitro system that ethylene seems to interfere with the cell death programme also in TEs. This conclusion is, however, not supported by Arabidopsis mutant analyses because no developmental defects have been reported for any of the dominant ethylene receptor or downstream signalling mutants, even though complete removal of ethylene biosynthesis is reportedly lethal ( Tsuchisaka et al., 2009). It has to be emphasized that inhibitors like STS are never specific to one pathway ( Strader et al., 2009). Nevertheless, it is possible that the in vitro system actually reveals some basic regulatory aspects of xylem differentiation that are masked or compensated for by the cellular context in intact vascular tissues.

Polyamines are implicated in several different processes during xylem differentiation, including cell wall formation, lignin biosynthesis, and auxin–cytokinin signalling ( Ge et al., 2006 Cui et al., 2010 Vera-Sirera et al., 2010). Interestingly, ACAULIS5 (ACL5), which encodes the biosynthetic enzyme for the synthesis of a recently identified tetra-amine, thermospermine, is specifically expressed in Arabidopsis vessel elements prior to secondary wall deposition ( Muñiz et al., 2008). acl5 loss-of-function mutants exhibit incorrect or incomplete secondary cell wall formation as well as early expression of xylem cell death markers, and consequently early vessel cell death compared with the wild type, suggesting that thermospermine has a protective role against premature xylem maturation and cell death ( Muñiz et al., 2008). Exogenous thermospermine has been shown to inhibit Zinnia TE differentiation almost completely ( Kakehi et al., 2010), which could be due to accentuated protection against premature TE maturation, resulting in complete arrest of TE differentiation. Genetic analyses have further indicated a basic helix–loop–helix (bHLH) transcription factor SUPPRESSOR OF ACAULIS51 (SAC51) as a target of ACL5 function. ACL5 or thermospermine is believed to activate translation of SAC51 by inhibiting one of the negatively acting upstream open reading frames of SAC51 ( Imai et al., 2006, 2008). SAC51 has been shown to be a direct target of one of the NAC transcription factors (VND7) ( Zhong et al., 2010b), and it is possible that SAC51 coordinates signals coming from the NAC transcription factors and ACL5 to control the rate of differentiation specifically in differentiating TEs ( Fig. 3).

Transcriptional regulation of tracheary element cell death. Cell death is regulated as an integral part of the xylem maturation programme by the activity of the NAC transcription factors VND6 and VND7 that induce expression of both cell death- and secondary wall-related genes. Thermospermine synthase ACL5 is proposed to impede the rate of xylem maturation by activating translation of SAC51 ( Vera-Sirera et al., 2010) even though it is not clear how SAC51 mediates inhibition of xylem maturation. Expression of SAC51 as well as XND1, that is another rate-inhibitory factor, is induced by VND7 ( Zhong et al., 2010b). Lignification requires activity of second level transcription factors ( Zhong et al., 2010a) that are activated by the NAC master switches. TE cell death is required for bulk lignification ( Pesquet et al., 2010).

Transcriptional regulation of tracheary element cell death. Cell death is regulated as an integral part of the xylem maturation programme by the activity of the NAC transcription factors VND6 and VND7 that induce expression of both cell death- and secondary wall-related genes. Thermospermine synthase ACL5 is proposed to impede the rate of xylem maturation by activating translation of SAC51 ( Vera-Sirera et al., 2010) even though it is not clear how SAC51 mediates inhibition of xylem maturation. Expression of SAC51 as well as XND1, that is another rate-inhibitory factor, is induced by VND7 ( Zhong et al., 2010b). Lignification requires activity of second level transcription factors ( Zhong et al., 2010a) that are activated by the NAC master switches. TE cell death is required for bulk lignification ( Pesquet et al., 2010).

Other signalling components

It has been suggested that calcium ions regulate vacuolar integrity during TE maturation. An increase in Ca 2+ influx appears to accompany cell death of differentiating TEs. Both chelation of extracellular Ca 2+ and blocking of Ca 2+ influx channels have been shown to suppress vacuolar rupture and DNA degradation in differentiating Zinnia TEs ( Groover and Jones, 1999). It has also been proposed that Ca 2+ influx is controlled by the activity of a secreted 40 kDa serine protease, which continuously accumulates in the extracellular space, inducing a massive Ca 2+ influx and TE cell death above a certain critical level of the protease ( Groover and Jones, 1999). Reactive oxygen species (ROS) are important signalling compounds in various cell death processes both in animals and in plants. In Zinnia in vitro cultures, it has been found that differentiating TEs are constantly exposed to a highly oxidative environment, but no bursts of rapidly increasing ROS levels seem to occur ( Groover et al., 1997 Gómez Ros et al., 2006). Inhibition of ROS production by diphenyleneiodonium does not influence TE cell death either ( Groover et al., 1997). However, ROS levels have been correlated with the extent of xylem lignification in planta ( Srivastava et al., 2007), and ROS production has been shown to be required for lignification in Zinnia TEs ( Karlsson et al., 2005). It seems, therefore, that ROS are involved in regulation of TE lignification but not cell death.

During apoptotic cell death in animals, changes in Ca 2+ , pH, and ROS production can trigger formation of the mitochondrial permeability transition pore (PTP), leading to release of proteins, such as cytochrome c, from the intermembrane space ( Danial and Korsmeyer, 2004). Mitochondrial depolarization and morphological changes have also been observed as fast responses to various cell death-inducing conditions in plants ( Logan, 2008). Also in differentiating Zinnia TEs, it has been reported that mitochondrial membranes are depolarized and cytochrome c is released into the cytosol prior to vacuolar rupture ( Yu et al., 2002). However, cyclosporin A, which blocks apoptosis in animals probably by disrupting the PTP, has been shown to inhibit Zinnia TE formation and to block DNA degradation and cell death induced by the anticancer drug BetA without blocking cytochrome c release ( Yu et al., 2002). It can therefore be concluded that cytochrome c alone is not sufficient to induce DNA degradation and that the mitochondrial changes, observed prior to vacuolar bursting, appear to be a side effect rather than an evolutionarily conserved trigger of TE cell death.


Insect Structure and Function



The arthropods are a large group of invertebrate animals which include insects, spiders, millipedes, centipedes and crustacea such as lobsters and crabs. All arthropods have a hard exoskeleton or cuticle, segmented bodies and jointed legs. The crustacea and insects also have antennae, compound eyes and, often, three distinct regions to their bodies: head, thorax and abdomen.

General Characteristics of Insects

The insects differ from the rest of the arthropods in having only three pairs of jointed legs on the thorax and, typically, two pairs of wings. There are a great many different species of insects and some, during evolution, have lost one pair of wings, as in the houseflies, crane flies and mosquitoes. Other parasitic species like the fleas have lost both pairs of wings. In beetles, grasshoppers and cockroaches, the first pair of wings has become modified to form a hard outer covering over the second pair.

Cuticle and ecdysis. The value of the external cuticle is thought to lie mainly in reducing the loss from the body of water vapour through evaporation, but it also protects the animal from damage and bacterial invasion, maintains its shape and allows rapid locomotion. The cuticle imposes certain limitations in size, however, for if arthropods were to exceed the size of some of the larger crabs, the cuticle would become too heavy for the muscles to move the limbs.

Between the segments of the body and at the joints of the limbs and other appendages, the cuticle is flexible and allows movement. For the most part, however, the cuticle is rigid and prevents any increase in the size of the insect except during certain periods of its development when the insect sheds its cuticle (ecdysis) and increases its volume before the new cuticle has time to harden. Only the outermost layer of the cuticle is shed, the inner layers are digested by enzymes secreted from the epidermis and the fluid so produced is absorbed back into the body. Muscular contractions force the blood into the thorax, causing it to swell and so split the old cuticle along a predetermined line of weakness. The swallowing of air often accompanies ecdysis assisting the splitting of the cuticle and keeping the body expanded while the new cuticle hardens. In insects, this moulting, or ecdysis, takes place only in the larval and pupal form and not in adults. In other words, mature insects do not grow.

Breathing. Running through the bodies of all insects is a branching system of tubes, tracheae which contain air. They open to the outside by pores called spiracles and they conduct air from the atmosphere to all living regions of the body The tracheae are lined with cuticle which is thickened in spiral bands This thickening keeps the tracheae open against the internal pressure of body fluids. The spiracles, typically, open on the flanks of each segment of the body, but in some insects there are only one or two openings. The entrance to the spiracle is usually supplied with muscles which control its opening or closure. Since the spiracles are one of the few areas of the body from which evaporation of water can occur, the closure of the spiracles when the insect is not active and therefore needs less oxygen, helps to conserve moisture. The tracheae branch repeatedly until they terminate in very fine tracheoles which invest or penetrate the tissues and organs inside the body. The walls of tracheae and tracheoles are permeable to gases, and oxygen is able to diffuse through them to reach the living cells. As might be expected the supply of tracheoles is most dense in the region of very active muscle, e.g. the flight muscles in the thorax.

The movement of oxygen from the atmosphere, through the spiracles, up the tracheae and tracheoles to the tissues, and the passage of carbon dioxide in the opposite direction, can be accounted for by simple diffusion but in active adult insects there is often a ventilation process which exchanges up to 60 per cent of the air in the tracheal system. In many beetles, locusts, grasshoppers and cockroaches, the abdomen is slightly compressed vertically (dorso-ventrally) by contraction of internal muscles. In bees and wasps the abdomen is compressed rhythmically along its length, slightly telescoping the segments. In both cases, the consequent rise of blood pressure in the body cavity compresses the tracheae along their length (like a concertina) and expels air from them. When the muscles relax, the abdomen springs back into shape, the tracheae expand and draw in air. Thus, unlike mammals, the positive muscular action in breathing is that which results in expiration.

This tracheal respiratory system is very different from the respiratory systems of the vertebrates, in which oxygen is absorbed by gills or lungs and conveyed in the blood stream to the tissues. In the insects, the oxygen diffuses through the trachea and tracheoles directly to the organ concerned. The carbon dioxide escapes through the same path although a proportion may diffuse from the body surface.

Blood system. The tracheal supply carrying oxygen to the organs gives the circulatory system a rather different role in insects from that in vertebrates. Except where the tracheoles terminate at some distance from a cell, the blood has little need to carry dissolved oxygen and, with a few exceptions, it contains no haemoglobin or cells corresponding to red blood cells. There is a single dorsal vessel which propels blood forward and releases it into the body cavity, thus maintaining a sluggish circulation. Apart from this vessel, the blood is not confined in blood vessels but occupies the free space between the cuticle and the organs in the body cavity. The blood therefore serves mainly to distribute digested food, collect excretory products and, in addition, has important hydraulic functions in expanding certain regions of the body to split the old cuticle and in pumping up the crumpled wings of the newly emerged adult insect.

Sensory system

Touch. From the body surface of the insect there arises a profusion of fine bristles most of which have a sensory function, responding principally to touch, vibration, or chemicals. The tactile (touch-sensitive) bristles are jointed at their bases and when a bristle is displaced to one side, it stimulates a sensory cell which fires impulses to the central nervous system.

The tactile bristles are numerous on the tarsal segments, the head, wing margins, or antennae according to the species and as well as informing the insect about contact stimuli, they probably respond to air currents and vibrations in the ground or in the air.

Proprioceptors. Small oval or circular areas of cuticle are differentially thickened and supplied with sensory fibres. They probably respond to distortions in the cuticle resulting from pressure, and so feed back information to the central nervous system about the position of the limbs. Organs of this kind respond to deflections of the antennae during flight and are thought to "measure" the air speed and help to adjust the wing movements accordingly. In some insects there are stretch receptors associated with muscle fibres, apparently similar to those in vertebrates.

Sound. The tactile bristles on the cuticle and on the antennae respond to low-frequency vibrations but many insects have more specialized sound detectors in the form of a thin area of cuticle overlying a distended trachea or air sac and invested with sensory fibres. Such tympanal organs appear on the thorax or abdomen or tibia according to species and are sensitive to sounds of high frequency. They can be used to locate the source of sounds as in the case of the male cricket "homing" on the sound of the female's "chirp", and in some cases can distinguish between sounds of different frequency.

Smell and taste. Experiments show that different insects can distinguish between chemicals which we describe as sweet, sour, salt and bitter, and in some cases more specific substances. The organs of taste are most abundant on the mouthparts, in the mouth, and on the tarsal segments but the nature of the sense organs concerned is not always clear.

Smell is principally the function of the antennae. Here there are bristles, pegs or plates with a very thin cuticle and fine perforations through which project nerve endings sensitive to chemicals. Sometimes these sense organs are grouped together and sunk into olfactory pits. In certain moths the sense of smell is very highly developed. The male Emperor moth will fly to an unmated female from a distance of a mile, attracted by the "scent" which she exudes. A male moth's antennae may carry many thousand chemo-receptors.

Sight. The compound eyes of insects consist of thousands of identical units called ommatidia packed closely together on each side of the head. Each ommatidium consists of a lens system formed partly from a thickening of the transparent cuticle and partly from a special crystalline cone. This lens system concentrates light from within a cone of 20°, on to a transparent rod, the rhabdom. The light, passing down this rhabdom, stimulates the eight or so retinal cells grouped round it to fire nervous impulses to the brain. Each ommatidium can therefore record the presence or absence of light, its intensity, in some cases its colour and, according to the position of the ommatidium in the compound eye, its direction. Although there may be from 2000 to 10,000 or more ommatidia in the compound eye of an actively flying insect, this number cannot reconstruct a very accurate picture of the outside world. Nevertheless, the "mosaic image" so formed, probably produces a crude impression of the form of well-defined objects enabling bees, for example, to seek out flowers and to use landmarks for finding their way to and from the hive. It is likely that the construction of compound eyes makes them particularly sensitive to moving objects, e.g. bees are more readily attracted to flowers which are being blown by the wind.

Flower-visiting insects, at least, can distinguish certain colours from shades of grey of equal brightness. Bees are particularly sensitive to blue, violet and ultra-violet but cannot distinguish red and green from black and grey unless the flower petals are reflecting ultra-violet light as well. Some butterflies can distinguish yellow, green and red. The simple eyes of, for example, caterpillars, consist of a cuticular lens with a group of light-sensitive cells beneath, rather like a single ommatidium. They show some colour sensitivity and, when grouped together, some ability to discriminate form. The ocelli which occur in the heads of many flying insects probably respond only to changes in light intensity.

Locomotion

Movement in insects depends, as it does in vertebrates, on muscles contracting and pulling on jointed limbs or other appendages. The muscles are within the body and limbs, however, and are attached to the inside of the cuticle. A pair of antagonistic muscles is attached across a joint in a way which could bend and straighten the limb. Many of the joints in the -insect are of the "peg and socket" type. They permit movement in one plane only, like a hinge joint, but since there are several such joints in a limb, each operating in a different direction, the limb as a whole can describe fairly free directional movement.

Walking. The characteristic walking pattern of an insect involves moving three legs at a time. The body is supported by a "tripod" of three legs while the other three are swinging forward to a new position. On the last tarsal joint are claws and, depending on the species, adhesive pads which enable the insect to climb very smooth surfaces. The precise mechanism of adhesion is uncertain. Modification of the limbs and their musculature enables insects to leap, e.g. grasshopper, or swim, e.g. water beetles.

Flying. In insects with relatively light bodies and large wings such as butterflies and dragonflies, the wing muscles in the thorax pull directly on the wing where it is articulated to the thorax, levering it up and down. Insects such as bees, wasps and flies, with compact bodies and a smaller wing area have indirect flight muscles which elevate and depress the wings very rapidly by pulling on the walls of the thorax and changing its shape. In both cases there are direct flight muscles which, by acting on the wing insertion, can alter its angle in the air. During the downstroke the wing is held horizontally, so thrusting downwards on the air and producing a lifting force. During the upstroke the wing is rotated vertically and offers little resistance during its upward movement through the air.

Feeding methods

It is not possible to make very useful generalizations about the feeding methods of insects because they are so varied. However, insects do have in common three pairs of appendages called mouth parts, hinged to the head below the mouth and these extract or manipulate food in one way or another. The basic pattern of these mouth parts is the same in most insects but in the course of evolution they have become modified and adapted to exploit different kinds of food source. The least modified are probably those of insects such as caterpillars, grasshoppers, locusts and cockroaches in which the first pair of appendages, mandibles, form sturdy jaws, working sideways across the mouth and cutting off pieces of vegetation which are manipulated into the mouth by the other mouth parts, the maxillae and labium.

Aphids are small insects (e.g. greenfly) which feed on plant juices that they suck from leaves and stems. Their mouthparts are greatly elongated to form a piercing and sucking proboscis. The maxillae fit together to form a tube which can be pushed into plant tissues to reach the food-conducting vessels of the phloem and so extract nutrients.

The mosquito has mandibles and maxillae in the form of slender, sharp stylets which can cut through the skin of a mammal as well as penetrating plant tissues. To obtain a blood meal the mosquito inserts its mouth parts through the skin to reach a capillary and then sucks blood through a tube formed from the labrum or "front lip" which precedes the mouth parts.

Another tubular structure, the hypopharynx, serves to inject into the wound a substance which prevents the blood from clotting and so blocking the tubular labrum. In both aphid and mosquito the labium is rolled round the other mouth parts, enclosing them in a sheath when they are not being used.

In the butterfly, only the maxillae contribute to the feeding apparatus. The maxillae are greatly elongated and in the form of half tubes, i.e. like a drinking straw split down its length. They can be fitted together to form a tube through which nectar is sucked from the flowers.

The housefly also sucks liquid but its mouthparts cannot penetrate tissue. Instead the labium is enlarged to form a proboscis which terminates in two pads whose surface is channelled by grooves called pseudotracheae. The fly applies its proboscis to the food and pumps saliva along the channels and over the food. The saliva dissolves soluble parts of the food and may contain enzymes which digest some of the insoluble matter. The nutrient liquid is then drawn back along the pseudotracheae and pumped into the alimentary canal.

For illustrations to accompany this article see Insect Structure and Function


The most distinctive xylem cells are the long tracheary elements that transport water. Tracheids and vessel elements are distinguished by their shape vessel elements are shorter, and are connected together into long tubes that are called vessels. [6]

Xylem also contains two other cell types: parenchyma and fibers. [7]

  • in vascular bundles, present in non-woody plants and non-woody parts of woody plants
  • in secondary xylem, laid down by a meristem called the vascular cambium in woody plants
  • as part of a stelar arrangement not divided into bundles, as in many ferns.

In transitional stages of plants with secondary growth, the first two categories are not mutually exclusive, although usually a vascular bundle will contain primary xylem only.

The branching pattern exhibited by xylem follows Murray's law. [8]

Primary xylem is formed during primary growth from procambium. It includes protoxylem and metaxylem. Metaxylem develops after the protoxylem but before secondary xylem. Metaxylem has wider vessels and tracheids than protoxylem.

Secondary xylem is formed during secondary growth from vascular cambium. Although secondary xylem is also found in members of the gymnosperm groups Gnetophyta and Ginkgophyta and to a lesser extent in members of the Cycadophyta, the two main groups in which secondary xylem can be found are:

    (Coniferae): there are approximately 600 known species of conifers. [9] All species have secondary xylem, which is relatively uniform in structure throughout this group. Many conifers become tall trees: the secondary xylem of such trees is used and marketed as softwood. (Angiospermae): there are approximately 250,000 [9] known species of angiosperms. Within this group secondary xylem is rare in the monocots. [10] Many non-monocot angiosperms become trees, and the secondary xylem of these is used and marketed as hardwood.

The xylem, vessels and tracheids of the roots, stems and leaves are interconnected to form a continuous system of water-conducting channels reaching all parts of the plants. The system transports water and soluble mineral nutrients from the roots throughout the plant. It is also used to replace water lost during transpiration and photosynthesis. Xylem sap consists mainly of water and inorganic ions, although it can also contain a number of organic chemicals as well. The transport is passive, not powered by energy spent by the tracheary elements themselves, which are dead by maturity and no longer have living contents. Transporting sap upwards becomes more difficult as the height of a plant increases and upwards transport of water by xylem is considered to limit the maximum height of trees. [11] Three phenomena cause xylem sap to flow:

    : Sugars produced in the leaves and other green tissues are kept in the phloem system, creating a solute pressure differential versus the xylem system carrying a far lower load of solutes- water and minerals. The phloem pressure can rise to several MPa, [12] far higher than atmospheric pressure. Selective inter-connection between these systems allows this high solute concentration in the phloem to draw xylem fluid upwards by negative pressure.
  • Transpirational pull: Similarly, the evaporation of water from the surfaces of mesophyll cells to the atmosphere also creates a negative pressure at the top of a plant. This causes millions of minute menisci to form in the mesophyll cell wall. The resulting surface tension causes a negative pressure or tension in the xylem that pulls the water from the roots and soil.
  • Root pressure: If the water potential of the root cells is more negative than that of the soil, usually due to high concentrations of solute, water can move by osmosis into the root from the soil. This causes a positive pressure that forces sap up the xylem towards the leaves. In some circumstances, the sap will be forced from the leaf through a hydathode in a phenomenon known as guttation. Root pressure is highest in the morning before the stomata open and allow transpiration to begin. Different plant species can have different root pressures even in a similar environment examples include up to 145 kPa in Vitis riparia but around zero in Celastrus orbiculatus. [13]

The primary force that creates the capillary action movement of water upwards in plants is the adhesion between the water and the surface of the xylem conduits. [14] [15] Capillary action provides the force that establishes an equilibrium configuration, balancing gravity. When transpiration removes water at the top, the flow is needed to return to the equilibrium.

Transpirational pull results from the evaporation of water from the surfaces of cells in the leaves. This evaporation causes the surface of the water to recess into the pores of the cell wall. By capillary action, the water forms concave menisci inside the pores. The high surface tension of water pulls the concavity outwards, generating enough force to lift water as high as a hundred meters from ground level to a tree's highest branches.

Transpirational pull requires that the vessels transporting the water be very small in diameter otherwise, cavitation would break the water column. And as water evaporates from leaves, more is drawn up through the plant to replace it. When the water pressure within the xylem reaches extreme levels due to low water input from the roots (if, for example, the soil is dry), then the gases come out of solution and form a bubble – an embolism forms, which will spread quickly to other adjacent cells, unless bordered pits are present (these have a plug-like structure called a torus, that seals off the opening between adjacent cells and stops the embolism from spreading). Even after an embolism has occurred, plants are able to refill the xylem and restore the functionality. [16]

Cohesion-tension theory

The cohesion-tension theory is a theory of intermolecular attraction that explains the process of water flow upwards (against the force of gravity) through the xylem of plants. It was proposed in 1894 by John Joly and Henry Horatio Dixon. [17] [18] Despite numerous objections, [19] [20] this is the most widely accepted theory for the transport of water through a plant's vascular system based on the classical research of Dixon-Joly (1894), Eugen Askenasy (1845–1903) (1895), [21] [22] and Dixon (1914,1924). [23] [24]

Water is a polar molecule. When two water molecules approach one another, the slightly negatively charged oxygen atom of one forms a hydrogen bond with a slightly positively charged hydrogen atom in the other. This attractive force, along with other intermolecular forces, is one of the principal factors responsible for the occurrence of surface tension in liquid water. It also allows plants to draw water from the root through the xylem to the leaf.

Water is constantly lost through transpiration from the leaf. When one water molecule is lost another is pulled along by the processes of cohesion and tension. Transpiration pull, utilizing capillary action and the inherent surface tension of water, is the primary mechanism of water movement in plants. However, it is not the only mechanism involved. Any use of water in leaves forces water to move into them.

Transpiration in leaves creates tension (differential pressure) in the cell walls of mesophyll cells. Because of this tension, water is being pulled up from the roots into the leaves, helped by cohesion (the pull between individual water molecules, due to hydrogen bonds) and adhesion (the stickiness between water molecules and the hydrophilic cell walls of plants). This mechanism of water flow works because of water potential (water flows from high to low potential), and the rules of simple diffusion. [25]

Over the past century, there has been a great deal of research regarding the mechanism of xylem sap transport today, most plant scientists continue to agree that the cohesion-tension theory best explains this process, but multiforce theories that hypothesize several alternative mechanisms have been suggested, including longitudinal cellular and xylem osmotic pressure gradients, axial potential gradients in the vessels, and gel- and gas-bubble-supported interfacial gradients. [26] [27]

Measurement of pressure

Until recently, the differential pressure (suction) of transpirational pull could only be measured indirectly, by applying external pressure with a pressure bomb to counteract it. [28] When the technology to perform direct measurements with a pressure probe was developed, there was initially some doubt about whether the classic theory was correct, because some workers were unable to demonstrate negative pressures. More recent measurements do tend to validate the classic theory, for the most part. Xylem transport is driven by a combination [29] of transpirational pull from above and root pressure from below, which makes the interpretation of measurements more complicated.

Xylem appeared early in the history of terrestrial plant life. Fossil plants with anatomically preserved xylem are known from the Silurian (more than 400 million years ago), and trace fossils resembling individual xylem cells may be found in earlier Ordovician rocks. [ citation needed ] The earliest true and recognizable xylem consists of tracheids with a helical-annular reinforcing layer added to the cell wall. This is the only type of xylem found in the earliest vascular plants, and this type of cell continues to be found in the protoxylem (first-formed xylem) of all living groups of vascular plants. Several groups of plants later developed pitted tracheid cells independently through convergent evolution. In living plants, pitted tracheids do not appear in development until the maturation of the metaxylem (following the protoxylem).

In most plants, pitted tracheids function as the primary transport cells. The other type of vascular element, found in angiosperms, is the vessel element. Vessel elements are joined end to end to form vessels in which water flows unimpeded, as in a pipe. The presence of xylem vessels (also called trachea [30] ) is considered to be one of the key innovations that led to the success of the angiosperms. [31] However, the occurrence of vessel elements is not restricted to angiosperms, and they are absent in some archaic or "basal" lineages of the angiosperms: (e.g., Amborellaceae, Tetracentraceae, Trochodendraceae, and Winteraceae), and their secondary xylem is described by Arthur Cronquist as "primitively vesselless". Cronquist considered the vessels of Gnetum to be convergent with those of angiosperms. [32] Whether the absence of vessels in basal angiosperms is a primitive condition is contested, the alternative hypothesis states that vessel elements originated in a precursor to the angiosperms and were subsequently lost.

To photosynthesize, plants must absorb CO
2 from the atmosphere. However, this comes at a price: while stomata are open to allow CO
2 to enter, water can evaporate. [33] Water is lost much faster than CO
2 is absorbed, so plants need to replace it, and have developed systems to transport water from the moist soil to the site of photosynthesis. [33] Early plants sucked water between the walls of their cells, then evolved the ability to control water loss (and CO
2 acquisition) through the use of stomata. Specialized water transport tissues soon evolved in the form of hydroids, tracheids, then secondary xylem, followed by an endodermis and ultimately vessels. [33]

The high CO
2 levels of Silurian-Devonian times, when plants were first colonizing land, meant that the need for water was relatively low. As CO
2 was withdrawn from the atmosphere by plants, more water was lost in its capture, and more elegant transport mechanisms evolved. [33] As water transport mechanisms, and waterproof cuticles, evolved, plants could survive without being continually covered by a film of water. This transition from poikilohydry to homoiohydry opened up new potential for colonization. [33] Plants then needed a robust internal structure that held long narrow channels for transporting water from the soil to all the different parts of the above-soil plant, especially to the parts where photosynthesis occurred.

During the Silurian, CO
2 was readily available, so little water needed expending to acquire it. By the end of the Carboniferous, when CO
2 levels had lowered to something approaching today's, around 17 times more water was lost per unit of CO
2 uptake. [33] However, even in these "easy" early days, water was at a premium, and had to be transported to parts of the plant from the wet soil to avoid desiccation. This early water transport took advantage of the cohesion-tension mechanism inherent in water. Water has a tendency to diffuse to areas that are drier, and this process is accelerated when water can be wicked along a fabric with small spaces. In small passages, such as that between the plant cell walls (or in tracheids), a column of water behaves like rubber – when molecules evaporate from one end, they pull the molecules behind them along the channels. Therefore, transpiration alone provided the driving force for water transport in early plants. [33] However, without dedicated transport vessels, the cohesion-tension mechanism cannot transport water more than about 2 cm, severely limiting the size of the earliest plants. [33] This process demands a steady supply of water from one end, to maintain the chains to avoid exhausting it, plants developed a waterproof cuticle. Early cuticle may not have had pores but did not cover the entire plant surface, so that gas exchange could continue. [33] However, dehydration at times was inevitable early plants cope with this by having a lot of water stored between their cell walls, and when it comes to it sticking out the tough times by putting life "on hold" until more water is supplied. [33]

To be free from the constraints of small size and constant moisture that the parenchymatic transport system inflicted, plants needed a more efficient water transport system. During the early Silurian, they developed specialized cells, which were lignified (or bore similar chemical compounds) [33] to avoid implosion this process coincided with cell death, allowing their innards to be emptied and water to be passed through them. [33] These wider, dead, empty cells were a million times more conductive than the inter-cell method, giving the potential for transport over longer distances, and higher CO
2 diffusion rates.

The earliest macrofossils to bear water-transport tubes are Silurian plants placed in the genus Cooksonia. [34] The early Devonian pretracheophytes Aglaophyton and Horneophyton have structures very similar to the hydroids of modern mosses. Plants continued to innovate new ways of reducing the resistance to flow within their cells, thereby increasing the efficiency of their water transport. Bands on the walls of tubes, in fact apparent from the early Silurian onwards, [35] are an early improvisation to aid the easy flow of water. [36] Banded tubes, as well as tubes with pitted ornamentation on their walls, were lignified [37] and, when they form single celled conduits, are considered to be tracheids. These, the "next generation" of transport cell design, have a more rigid structure than hydroids, allowing them to cope with higher levels of water pressure. [33] Tracheids may have a single evolutionary origin, possibly within the hornworts, [38] uniting all tracheophytes (but they may have evolved more than once). [33]

Water transport requires regulation, and dynamic control is provided by stomata. [39] By adjusting the amount of gas exchange, they can restrict the amount of water lost through transpiration. This is an important role where water supply is not constant, and indeed stomata appear to have evolved before tracheids, being present in the non-vascular hornworts. [33]

An endodermis probably evolved during the Silu-Devonian, but the first fossil evidence for such a structure is Carboniferous. [33] This structure in the roots covers the water transport tissue and regulates ion exchange (and prevents unwanted pathogens etc. from entering the water transport system). The endodermis can also provide an upwards pressure, forcing water out of the roots when transpiration is not enough of a driver.

Once plants had evolved this level of controlled water transport, they were truly homoiohydric, able to extract water from their environment through root-like organs rather than relying on a film of surface moisture, enabling them to grow to much greater size. [33] As a result of their independence from their surroundings, they lost their ability to survive desiccation – a costly trait to retain. [33]

During the Devonian, maximum xylem diameter increased with time, with the minimum diameter remaining pretty constant. [36] By the middle Devonian, the tracheid diameter of some plant lineages (Zosterophyllophytes) had plateaued. [36] Wider tracheids allow water to be transported faster, but the overall transport rate depends also on the overall cross-sectional area of the xylem bundle itself. [36] The increase in vascular bundle thickness further seems to correlate with the width of plant axes, and plant height it is also closely related to the appearance of leaves [36] and increased stomatal density, both of which would increase the demand for water. [33]

While wider tracheids with robust walls make it possible to achieve higher water transport pressures, this increases the problem of cavitation. [33] Cavitation occurs when a bubble of air forms within a vessel, breaking the bonds between chains of water molecules and preventing them from pulling more water up with their cohesive tension. A tracheid, once cavitated, cannot have its embolism removed and return to service (except in a few advanced angiosperms [40] [41] which have developed a mechanism of doing so). Therefore, it is well worth plants' while to avoid cavitation occurring. For this reason, pits in tracheid walls have very small diameters, to prevent air entering and allowing bubbles to nucleate. Freeze-thaw cycles are a major cause of cavitation. Damage to a tracheid's wall almost inevitably leads to air leaking in and cavitation, hence the importance of many tracheids working in parallel. [33]

Cavitation is hard to avoid, but once it has occurred plants have a range of mechanisms to contain the damage. [33] Small pits link adjacent conduits to allow fluid to flow between them, but not air – although ironically these pits, which prevent the spread of embolisms, are also a major cause of them. [33] These pitted surfaces further reduce the flow of water through the xylem by as much as 30%. [33] Conifers, by the Jurassic, developed an ingenious improvement, using valve-like structures to isolate cavitated elements. These torus-margo structures have a blob floating in the middle of a donut when one side depressurizes the blob is sucked into the torus and blocks further flow. [33] Other plants simply accept cavitation for instance, oaks grow a ring of wide vessels at the start of each spring, none of which survive the winter frosts. Maples use root pressure each spring to force sap upwards from the roots, squeezing out any air bubbles.

Growing to height also employed another trait of tracheids – the support offered by their lignified walls. Defunct tracheids were retained to form a strong, woody stem, produced in most instances by a secondary xylem. However, in early plants, tracheids were too mechanically vulnerable, and retained a central position, with a layer of tough sclerenchyma on the outer rim of the stems. [33] Even when tracheids do take a structural role, they are supported by sclerenchymatic tissue.

Tracheids end with walls, which impose a great deal of resistance on flow [36] vessel members have perforated end walls, and are arranged in series to operate as if they were one continuous vessel. [36] The function of end walls, which were the default state in the Devonian, was probably to avoid embolisms. An embolism is where an air bubble is created in a tracheid. This may happen as a result of freezing, or by gases dissolving out of solution. Once an embolism is formed, it usually cannot be removed (but see later) the affected cell cannot pull water up, and is rendered useless.

End walls excluded, the tracheids of prevascular plants were able to operate under the same hydraulic conductivity as those of the first vascular plant, Cooksonia. [36]

The size of tracheids is limited as they comprise a single cell this limits their length, which in turn limits their maximum useful diameter to 80 μm. [33] Conductivity grows with the fourth power of diameter, so increased diameter has huge rewards vessel elements, consisting of a number of cells, joined at their ends, overcame this limit and allowed larger tubes to form, reaching diameters of up to 500 μm, and lengths of up to 10 m. [33]

Vessels first evolved during the dry, low CO
2 periods of the late Permian, in the horsetails, ferns and Selaginellales independently, and later appeared in the mid Cretaceous in angiosperms and gnetophytes. [33] Vessels allow the same cross-sectional area of wood to transport around a hundred times more water than tracheids! [33] This allowed plants to fill more of their stems with structural fibers, and also opened a new niche to vines, which could transport water without being as thick as the tree they grew on. [33] Despite these advantages, tracheid-based wood is a lot lighter, thus cheaper to make, as vessels need to be much more reinforced to avoid cavitation. [33]

Xylem development can be described by four terms: centrarch, exarch, endarch and mesarch. As it develops in young plants, its nature changes from protoxylem to metaxylem (i.e. from first xylem to after xylem). The patterns in which protoxylem and metaxylem are arranged is important in the study of plant morphology.

Protoxylem and metaxylem

As a young vascular plant grows, one or more strands of primary xylem form in its stems and roots. The first xylem to develop is called 'protoxylem'. In appearance protoxylem is usually distinguished by narrower vessels formed of smaller cells. Some of these cells have walls which contain thickenings in the form of rings or helices. Functionally, protoxylem can extend: the cells are able to grow in size and develop while a stem or root is elongating. Later, 'metaxylem' develops in the strands of xylem. Metaxylem vessels and cells are usually larger the cells have thickenings which are typically either in the form of ladderlike transverse bars (scalariform) or continuous sheets except for holes or pits (pitted). Functionally, metaxylem completes its development after elongation ceases when the cells no longer need to grow in size. [42] [43]

Patterns of protoxylem and metaxylem

There are four main patterns to the arrangement of protoxylem and metaxylem in stems and roots.

  • Centrarch refers to the case in which the primary xylem forms a single cylinder in the center of the stem and develops from the center outwards. The protoxylem is thus found in the central core and the metaxylem in a cylinder around it. [44] This pattern was common in early land plants, such as "rhyniophytes", but is not present in any living plants. [citation needed]

The other three terms are used where there is more than one strand of primary xylem.

  • Exarch is used when there is more than one strand of primary xylem in a stem or root, and the xylem develops from the outside inwards towards the center, i.e. centripetally. The metaxylem is thus closest to the center of the stem or root and the protoxylem closest to the periphery. The roots of vascular plants are normally considered to have exarch development. [42]
  • Endarch is used when there is more than one strand of primary xylem in a stem or root, and the xylem develops from the inside outwards towards the periphery, i.e. centrifugally. The protoxylem is thus closest to the center of the stem or root and the metaxylem closest to the periphery. The stems of seed plants typically have endarch development. [42]
  • Mesarch is used when there is more than one strand of primary xylem in a stem or root, and the xylem develops from the middle of a strand in both directions. The metaxylem is thus on both the peripheral and central sides of the strand with the protoxylem between the metaxylem (possibly surrounded by it). The leaves and stems of many ferns have mesarch development. [42]

In his book De plantis libri XVI (On Plants, in 16 books) (1583), the Italian physician and botanist Andrea Cesalpino proposed that plants draw water from soil not by magnetism (ut magnes ferrum trahit, as magnetic iron attracts) nor by suction (vacuum), but by absorption, as occurs in the case of linen, sponges, or powders. [45] The Italian biologist Marcello Malpighi was the first person to describe and illustrate xylem vessels, which he did in his book Anatome plantarum . (1675). [46] [note 1] Although Malpighi believed that xylem contained only air, the British physician and botanist Nehemiah Grew, who was Malpighi's contemporary, believed that sap ascended both through the bark and through the xylem. [47] However, according to Grew, capillary action in the xylem would raise the sap by only a few inches in order to raise the sap to the top of a tree, Grew proposed that the parenchymal cells become turgid and thereby not only squeeze the sap in the tracheids but force some sap from the parenchyma into the tracheids. [48] In 1727, English clergyman and botanist Stephen Hales showed that transpiration by a plant's leaves causes water to move through its xylem. [49] [note 2] By 1891, the Polish-German botanist Eduard Strasburger had shown that the transport of water in plants did not require the xylem cells to be alive. [50]


Secondary Growth in Dicotyledonous Stems (With Diagram) | Botany

Let us make an in-depth study of Secondary growth in Dicotyledonous Stems. After reading this article you will learn about: 1. Introduction to Secondary Growth 2. Secondary Growth in Various Parts of Dicotyledonous Stems.

Introduction to Secondary Growth:

The primary body of the plant is developed from the apical meristem. Sometimes as in monocotyledons and pteridophytes, the primary plant body is complete in itself and does not grow in thickness by cambial activity.

However, in dicotyledons, the primary permanent tissues make the fundamental parts of the plant, and the further growth in thickness is completed by cambial activity, called secondary growth in thickness.

The tissues, formed during secondary growth are called secondary tissues. Secondary tissues may be two types—the vascular tissues that are developed by the true cambium, and cork and phelloderm, which are formed by phellogen or cork-cambium.

In a typical dicotyledonous stem, the secondary growth starts in the intra- and extrastelar regions.

Secondary Growth in Various Parts of Dicotyledonous Stems:

Cambium:

The vascular bundles of dicotyledonous stems are collateral and open, and arranged in a ring. They contain a single layer of cambium cells, which separate the xylem from the phloem, called fascicular cambium, i.e., the cambium of the vascular bundle, (fascicle = bundle).

When the primary xylem and primary phloem are first differentiated there is no cambium across the pith rays or medullary rays to connect the edges of the cambium within vascular bundles.

As soon as the differentiation of the first xylem and phloem of the bundles takes place, the cells of the pith or medullary rays which lie in between the edges of the cambium within the bundles, divide accordingly and form a layer of cambium across the medullary rays.

The newly formed cambium connects the fascicular cambium found within the vascular bundles, and thus a complete cambium ring is formed. The newly formed cambium strip which occurs in the gaps between the bundles is called inter-fascicular cambium, i.e., the cambium in between two vascular bundles. Thus a complete cambium ring is formed.

The cambium layer consists essentially of a single layer of cells. These cells divide in a direction parallel with the epidermis. Each time a cambial cell divides into two one of the daughter cells remains meristematic, while the other is differentiated into a permanent tissue.

If the cell that is differentiated is next to the xylem it forms xylem, while if it is next to phloem it becomes phloem towards the outer side of the cambium. The cambium cells divide continuously in this manner producing secondary tissues on both sides of it. In this way, new cells are added to the xylem and the phloem, and the vascular bundles increase in size.

While there is more or less alternation in the production of xylem and phloem cells from a cambium cell, more cells are formed on the xylem side than on the phloem side. The cells formed from the cambium in the region of the pith rays become pith-ray cells. The activity of the cambium thus increases the length of the pith rays grow equally.

The formation of new cells from the cambium result in an enlargement of the stem that is known as the secondary thickening. The formation of new cells in secondary thickening continues throughout the life of the plant. It is in this way that the trunks of trees continue to grow in diameter. The cambium perpetuates and remains active for a considerable long period of time.

The thin-walled cells of the vascular cambium are highly vacuolate and in this respect are unlike most other meristematic cells. The electron microscopic structure reveals their highly vacuolate nature. Many ribosomes and dictyosomes, and well developed endoplasmic reticulum, are present (Srivastava, L.M., 1966).

Secondary Xylem:

The cambium ring cuts off new cells on its inner side are gradually modified into xylary elements, called the secondary xylem. This tissue serves many important functions, such as conduction of water and nutrients, mechanical support, etc. The secondary xylem of tree trunks is of great economic value, since it constitutes the timber and wood of commerce.

The secondary xylem consists of a compact mass of thick-walled cells so arranged as to form two systems—a longitudinal (vertical) and a transverse radiating system. The longitudinal system consists of elongate, overlapping and interlocked cells—tracheids, fibres and vessel elements—and longitudinal rows of parenchyma cells.

All these cells possess their long axes parallel with the long axis of the organ of which they are a part.

The secondary xylem consists of scalariform and pitted vessels, tracheids, wood fibres and wood parenchyma. These elements of secondary xylem are more or less similar to those occur in primary xylem. Vessels or tracheae are most abundant and are usually shorter than those of primary xylem. Mostly the vessels are pitted. Annular and spiral tracheids and vessels are altogether absent.

Xylem parenchyma cells may be long and fusiform, but sometimes they are short. They are living cells and usually meant for storage of food material (starch and fat) in them. Tannins and crystals are frequently found in these cells. Xylem parenchyma may occur either in the association of the vessels or quite independently. The fibres of secondary xylem possess thick walls and bordered pits.

Distribution of Wood (Xylem) Parenchyma:

Wood parenchyma is distributed in three ways:

(i) Terminal wood parenchyma

(ii) Diffuse or metatracheal wood parenchyma and

(iii) Vasicentric or paratracheal wood parenchyma.

Terminal Wood Parenchyma:

In some gymnosperm woods, wood parenchyma is absent in other (e.g., Larix and Pseudotsuga), and in some angiosperm woods (e.g., Magnolia and Salix), wood parenchyma cells occur only in the last-formed tissue of the annual ring. Such woods have terminal wood parenchyma.

Diffuse or Metatracheal Wood Parenchyma:

Where parenchyma occurs not only in this region, but also remains scattered throughout the annual ring, some of the cells lying among the tracheids, and fibre-tracheids the plant has diffuse or metatracheal wood parenchyma (e.g., in Malus, Quercus, Diospyros, etc.).

Vasicentric or Paratracheal Wood Parenchyma:

Where parenchyma occurs at the edge of the annual ring and elsewhere only about vessels and does not occur isolated among tracheids and fibres, the plant possesses vasicentric or paratracheal wood parenchyma (e.g., in Acer, Fraxinus, etc.).

The xylem rays or wood rays extend radially in the secondary xylem. They are strap or ribbon like. They originate from the ray initials. The xylem rays run as a continuous band to the secondary phloem through the cambium, thus forming a continuous conducting system. All vascular rays are initiated by the cambium and, once formed, are increased in length indefinitely by the cambium.

Commonly these rays are known as medullary rays, or pith rays, on the basis of their similarity and parenchymatous nature with the pith rays of herbaceous dicotyledonous stems. These radial rays may be best called vascular rays, as these rays are of vascular tissue partly of xylem and partly of phloem.

The xylem rays traverse in the secondary xylem and establish communication with the living cells of the vascular tissue. In gymnosperm wood where no wood parenchyma is present, every tracheid is in direct contact with at least one ray. Vessels also in their longitudinal extent come into contact with many rays.

In herbaceous stems, such as of Ranunculus, where vascular bundles are separated by projecting parenchymatous wedges, and in vines, such as Clematis, where the bundles are separated by bands of secondary parenchyma, vascular rays are not found. The xylem rays help in the exchange of gases. They also aid in the conduction of water and food from phloem to the cambium and xylem parenchyma.

Annual Rings or Growth Rings:

The secondary xylem in the stems of perennial plants commonly consists of concentric layers, each one of which represents a seasonal increment. In transverse section of the axis, these layers appear as rings, and are called annual rings or growth rings.

They are commonly termed as annual rings because in the woody plants of temperate regions and in those of tropical regions where there is an annual alternation of growing and dormant period, each layer represents the growth of one year.

The width of growth rings varies greatly and depends upon the rate of the growth of tree. Unfavourable growing seasons produce narrow rings, and favourable seasons wide ones. Annual or growth rings are characteristic of woody plants of temperate climates.

Such rings are weakly developed in tropical forms except where there are marked climate changes such as distinct moist and dry seasons. Annuals and herbaceous stems show, naturally, but one layer.

In regions with a pronounced cold season, the activity of the cambium takes place only during the spring and summer seasons thus giving rise the growth in diameter of woody plants. The wood of one season is sharply distinct from that of the next season. In spring or summer the cambium is more active and forms a greater number of vessels with wider cavities.

As the number of leaves increases in the spring season, additional vessels are needed for the transport of sap at that time to supply the increased leaves. In winter or autumn season, however, there is less need of vessels for sap transport, the cambium is less active and gives rise to narrow pitted vessels, tracheids and wood fibres.

The wood developed in the summer or spring season is called spring wood or early wood, and the wood formed in winter or autumn season is known as autumn wood or late wood. However, the line of demarcation is quite conspicuous between the late wood of one year and the early wood of next year. An annual ring, therefore, consists of two parts—an inner layer, early wood, and an outer layer late wood.

Each annual ring corresponds to one year’s growth, and on the basis of these rings the age of a particular plant can easily be calculated. The determination of age of a tree by counting the annual rings is known as dendrochronology.

Sometimes two annual rings are formed in a single year, and in such cases the counting of the annual rings does not show the correct age of the tree. This happens perhaps because of the drought conditions prevailed in the middle of a growing season.

Tyloses:

In many plants, the walls of the xylem vessels produce balloon like outgrowths into the lumen of the vessels are called tyloses. Usually these structures are formed in secondary xylem but they may also develop in primary xylem vessels. Tyloses are formed by the enlargement of the pit membranes of the half-bordered pits present in between a parenchyma cell and a vessel or a tracheid.

Usually they are sufficiently large and the lumen of the vessel is almost blocked. The nucleus of the xylem parenchyma cells along with cytoplasm passes into this balloon like outgrowth. The delicate pit membrane forms the balloon like tylosis inside the lumen cavity. In fully developed tyloses, starch crystals, resin gums and other substances are found, but they are not found very frequently.

The wall of tylosis may remain thin and membranous or very rarely it becomes thick and even lignified. The tylosis may remain very small or sufficiently large in size as the case may be. They may be one or few in number (e.g., in Populus) in a single cell or many (e.g., in white oak) and may fill the complete cell.

They are commonly found in many angiospermic families. Normally they develop in the heart wood of angiosperms and block the lumen of the vessels, and thus add to the durability of the wood. Tyloses also occur in the vessels of Coleus, Cucurbita, Rumex, Asarum and Convolvulus. Tyloses prevent rapid entrance of water, air and fungus by blocking the lumen of the vessel.

Tyloses are said to undergo division in some plants and form multicellular tissue, which fills the lumen compactly, as in Robinia and Madura. The tyloses are characteristic of certain species, and always absent in others.

In many plants the development of tylosis takes place by means of wounding. They may be present in the inner part of leaf traces after the leaf has fallen. Such tyloses occur rarely they are irregular in shape and size.

In the wood of conifers there is also found a closing of the cavity of resin canals by the enlargement of the epithelial cells. These enlarged cells are commonly known as tylosoids.

Sapwood and Heartwood:

The outer region of the old trees consisting of recently formed xylem elements is sapwood or alburnum. This is of light colour and contains some living cells also in the association of vessels and fibres. This part of the stem performs the physiological activities, such as conduction of water and nutrients, storage of food, etc.

The central region of the old trees, which was formed earlier, is filled up with tannins, resins, gums and other substances which make it hard and durable, is called heartwood or duramen. It looks black due to the presence of various substances in it. Usually the vessels remain plugged with tyloses. The function of heartwood is no longer of conduction it gives only mechanical support to the stem.

The sapwood changes into heartwood very gradually. During the transformation a number of changes occur—all living cells lose protoplasts water contents of cell walls are reduced food materials are withdrawn from the living cells tyloses are frequently formed which block the vessels the parenchyma walls become lignified oils, gums, tannins, resins and other substances develop in the cells.

In certain plants—for example, Ulmus and Malus pumila, the heartwood remains saturated with water in other plants, for example, in Fraxinus the heartwood may become very dry. The oils, resins and colouring materials infiltrate the walls, and gums and resins may fill the lumina of the cells.

In Diospyros and Swietenia, the cell cavities are filled with a dark-coloured gummy substance. The colour of heartwood, in general, is the result of the presence of these substances. Generally the heartwood is darker in colour than sapwood. However, in some genera, such as Betula, Populus, Picea, Agathis the heartwood is hardly darker in colour than the sapwood.

The proportion of sapwood and heartwood is highly variable in different species. Some trees do not have clearly differentiated heartwood (e.g., Populus, Salix, Picea, Abies), others possess thin sapwood (e.g., Robinia, Moras, Taxus), the still others possess a thick sapwood (e.g., Acer, Fraxinus, Fagus).

From economic point of view, heartwood is more useful than sapwood. Heartwood, as timber, is more durable than sapwood, because the reduction of food materials available for pathogens by the absence of protoplasm and starch.

The formation of resins, oils and tannins, and the blocking of the vessels by tyloses and gums, render the wood less susceptible to attack by the organisms of decay. The haemotoxylin is obtained from the heartwood of Haematoxylon campechianum. Because of the absence of resin, gums and colouring substances, sapwood is preferred for pulpwood, and for wood to be impregnated with preservatives.

Secondary Phloem:

The cambial cells divide tangentially and produce secondary phloem elements towards outside of it. Normally, the amount of secondary phloem is lesser than the amount of secondary xylem. In most of the dicotyledons usually the primary phloem becomes crushed and functionless and the secondary phloem performs all physiological activities for sufficiently a long period of time.

This is a complex tissue made up of various types of cells having common origin in the cambium. These cells are quite similar to the cells of primary phloem. However, the secondary phloem possesses a more regular arrangement of the cells in radial rows. The sieve tubes are comparatively larger in number and possess thicker walls.

The elements of secondary phloem are sieve tubes, companion cells, and phloem parenchyma and phloem ray cells. Sometimes sclerenchyma is also found. Presence of sieve tubes is characteristic of angiosperms, however, they are not found in gymnosperms. In gymnosperms, sieve cells are present.

The companion cells are not found in gymnosperms but probably they are present in all types of angiosperms. The companion cells are usually found accompanied with the sieve tubes. Phloem parenchyma cells are also found in the secondary phloem of all plants except few primitive types.

Phloem parenchyma cells are formed directly from parenchyma mother cells, which are formed from cambial cells. Sclerenchyma is also found in the secondary phloem of several plants. Usually the fibres occur in tangential bands. In certain plants which possess a hard or tough bark, the fibres consist the greater part of the secondary phloem and surround the softer tissues.

Sieve tubes are series of sieve-tube elements attached end to end with certain sieve areas more highly specialized than others. The sieve tubes of the secondary phloem of dicotyledons are of many types as regards the shape and nature of the end and side walls.

In many woody species (e.g., Carya cordiformis), the oblique end walls of the sieve tube elements frequently extend for about half the length of the element.

These oblique walls possess many areas which together make compound sieve plates. The other type, i.e., simple sieve plate is found in Robinia, Madura and some species of Ulmus. Here the terminal walls of the sieve tube elements are transverse and there is a single specialized sieve area. In the majority of species, the sieve tube elements of the secondary phloem possess simple sieve plates.

Sclerenchyma of one type or another is a characteristic of the secondary phloem of several species. Fibres occur frequently in definite tangential bands (e.g., in Liriodendron and Populus). In Cephalanthus, the fibres are found singly. However, in Carya cordiformis, the fibres constitute the greater part of the secondary phloem and surround the groups of softer tissues.

All conditions have been reported in gymnosperms. The phloem of Pinus strobus lacks sclerenchyma well developed tangential bands of fibres are found to be present in Juniperus, and large masses of sclereids are present in Tsuga. In Thuja occidentalis, the fibres are arranged in uniseriate tangential rows. These rows of fibres alternate with rows of sieve cells and phloem parenchyma.

In Platanus and Fagus sclereids are the only type of sclerenchyma present in the phloem. The sclereids are found abundantly in the older, living, but non-conducting phloem of the woody plants.

The phloem rays are usually present in the vascular tissues developed by the cambium. The vascular rays are formed in the cambium and develop on either side of it with the secondary xylem and secondary phloem of which they are a part. The phloem rays may be one to several cells in width. Normally they are of uniform width throughout their length.

They may increase in width outwardly, the increase being due to the multiplication of the cells or to the increase in size of cells toward the outer end of the ray. The phloem rays may be one cell wide (e.g., in Castanea and Salix), two or three cells wide (e.g., in Malus pumila) or many cells wide (e.g., in Robinia and Liriodendron).

However, in oaks there are two types of phloem rays—one very broad and the other uniseriate.

Commonly the phloem ray cells in woody plants, as seen in transverse section, are rectangular and radially elongated. In herbaceous plants, commonly the ray cells are globose. In Cephalanthus, Agrimonia and Potentilla the ray cells closely resemble the phloem parenchyma cells. All phloem ray cells are parenchymatous with active protoplasm, but as they become older many of them become sclereids.

A special type of ray cell known as albuminous cell is found in gymnosperms. These albuminous cells are found to be situated at the upper and lower margins of the phloem rays. The albuminous cells differ from the ordinary ray cells both structurally and functionally.

They are joined directly with the sieve cells by sieve areas. They do not contain starch, and are of much greater vertical diameter than the normal ray cells. They retain their protoplasts as long as the sieve cells with which they are connected function. It is thought that they function like companion cells of angiosperms.

Seasonal Rings in Secondary Phloem:

The tissues of the secondary phloem are generally arranged in definite tangential bands. These layers of tissue have the appearance of annual rings. However, these ring like bands do not possess definite seasonal limits like those of secondary xylem, because there is no sharp distinction between the phloem cells formed in the early and late growing season.

Seasonal formation of sclerenchyma bands may exist, but this is not constant feature. In tropical plants new layers of phloem and xylem are formed with each period of new growth.

The functions of secondary phloem are normally the same as that of primary phloem. The various cells of secondary phloem are structurally adapted for the function of translocation of food. The sieve tubes, companion cells and some phloem parenchyma cells are especially adapted for lengthwise conduction, and certain phloem rays help in horizontal conduction to and from the xylem and the cambium.

Some of the phloem parenchyma cells in some plants act as storage tissue of starch, crystals and other organic materials.

The secondary phloem of various trees and shrubs of the Malvaceae, Tiliaceae, Moraceae has provided bast fibres for economic purposes. The tapa cloth of Pacific islands is composed of mainly of phloem fibres.

Tannin obtained from the secondary phloem of various plants is utilized for the preparation of spices and drugs. Secretory canals are abundantly found in the secondary phloem, and the secretions are of much economic value—such as rubber is obtained from the latex of Hevea brasiliensis, and resins from various gymnosperms.

Periderm:

Due to continued formation of secondary tissues, in the older stem, and roots, however, the epidermis gets stretched and ultimately tends to rupture and followed by the death of epidermal cells and outer tissues, and a new protective layer is developed called periderm. The formation of periderm is a common phenomenon in stems and roots of dicotyledons and gymnosperms that increase in thickness by secondary growth.

Structurally, the periderm consists of three parts:

1. A meristem known as phellogen or cork cambium,

2. The layer of cells cut off by phellogen on the outer side, the phellem or cork, and

3. The cells cut off by phellogen towards inner side, the phelloderm.

The periderm appears on the surface of those plant parts that possess a continuous increase in thickness by secondary growth. Usually the periderm occurs in the roots and stems and their branches in gymnosperms and woody dicotyledons. It occurs in herbaceous dicotyledons, sometimes limited to the oldest parts of stem or root.

In contrast to the vascular cambium, the phellogen is relatively simple in structure, and composed of one type of cells. The cells of phellogen appear rectangular in cross- section, and somewhat flattened radially. Their protoplasts are vacuolated and may contain tannins and chloroplasts, except in the lenticels, intercellular spaces lacking.

When we consider the place of origin of the meristem forming the periderm, it becomes necessary to distinguish between the first periderm, and the subsequent periderms, which arise beneath the first and replace it as the axis increases in circumference. In most stems the first phellogen arises in the sub-epidermal layer.

In a few plants the phellogen arises in the epidermal cells (e.g., Nerium, Pyrus). Sometimes only a part of the phellogen is developed from epidermis while the other part arises in sub-epidermal cells (e.g., Pyrus). In some stems the second or third cortical layer initiates the development of periderm (e.g., Robinia, Aristolochia, Pinus, Larix, etc.).

In still other plants the phellogen arises near the vascular region or directly in the phloem (e.g., in Caryophyllaceae, Cupressaceae, Ericaceae, Punica, Vitis, etc.). If the first periderm is followed by the formation of others, these are formed repeatedly, in successively deeper layers of the cortex or phloem.

At the time of the beginning of the development of a phellogen in epidermal cells, the protoplasts lose their central vacuoles and the cytoplasm increases in amount and becomes more richly granular. As soon as this initial layer develops, it divides tangentially and, to a lesser extent radially, in the similar way as division takes place in true cambium. The derivative cells are normally arranged in radial rows.

Generally, several to many times as many cells are cut off toward the outside (phellem-cork cells) as toward the inside (phelloderm). Phelloderm cells are few or absent rarely phelloderm is greater in amount than phellem.

The cells that constitute phellem are commonly known as cork cells. They are like the phellogen cells from which they are derived. As seen in tangential section, they are polygonal and uniform in shape, and often radially thin as seen in cross section of the stem.

The cells of the commercial cork (Quercus suber) are radially elongated as seen in transverse section. In the periderm of Betula and Prunus, the cork cells are elongated tangentially as seen in cross-section. There are no intercellular spaces among cork cells.

The development of the periderm layers in the cork oaks (Quercus suber) is of special interest. The ability of the plant produce phellogen in deeper layers when the superficial periderm is removed is utilized in the production of commercial cork from the cork oak (Quercus suber).

At the age of about twenty years, when the tree is about 40 cm in circumference, this outer layer, known as virgin cork is removed by stripping to the phellogen. The exposed tissue dries out to about 1/8 in. in depth. A new phellogen is established beneath the dry layer and rapidly produces a massive cork of a better quality than the first.

After nine or ten years the new cork layer has attained sufficient thickness to be commercially valuable and is in turn removed. Of course, this cork is of better quality than the virgin cork, but of inferior quality than the cork obtained at the third and subsequent stripping’s. These stripping’s take place at intervals of about nine years until the tree is 150 or more years old.

After the successive stripping’s the new phellogen layers develop at greater depth in the living tissue. The cortex is lost after few stripping’s and the subsequent cork layers are formed in the secondary phloem. The important properties of the commercial cork are its imperviousness, its lightness, toughness and elasticity.

The phellogen cuts off the phelloderm cells towards inner side. The phelloderm cells are living cells with cellulose walls. In most plants, they resemble cortical cells in wall structure and contents. Their shape is similar to that of phellogen cells. They may be distinguished from cortical cells by their arrangement in radial series resulting from their origin from the tangentially dividing phellogen.

In some species they act as photosynthetic tissue and aid in starch storage. They are pitted like other parenchyma cells. Occasionally, the sclereids and other such specialized cells occur in phelloderm. The term secondary cortex is sometimes applied to phelloderm, which does not seem to be appropriate.

Bark:

The term bark is commonly applied to all tissues outside the vascular cambium of the stem, in either primary or secondary state of growth. In this way, bark includes primary phloem and cortex in stem with primary tissues only, and primary and secondary phloem, cortex and periderm in stem with secondary tissues.

This term is also used to denote the tissue that is accumulated on the surface of the stem as a result of the activity of cork cambium.

As the periderm develops, it becomes separated, by a non-living layer of cork cells from the living tissues. The tissue layers thus separated become dead. The term bark in restricted sense is applied to these dead tissues together with the cork layers. In wider sense the term is applied to denote the tissues outside the vascular cambium. However, the term bark is loose and non-technical.

Rhytidome:

In most of plants, as soon as the first phellogen ceases to function, second phellogen develops in the tissue below the first one. In this way additional layers of periderm are formed in the progressively deeper regions of the stem, thus new phellogen layers arise in deeper regions of the cortex which may exceed even upto phloem.

As the phellogen arises in deeper region and cuts cork cells or phellem towards outside, all the living cells outside the phellogen do not get water supply and nutrients, and become dead. These dead tissues formed outside the phellogen constitute the rhytidome.

In some rhytidomes parenchyma and soft cork cells predominate whereas others contain large amounts of fibres usually derived from the phloem. The manners in which the successive layers of periderm originate possesses a characteristic effect upon the appearance of the rhytidome.

When the sequent periderms develop as overlapping scale-like layers, the outer tissue breaks up into units related to the layers of periderm, and thus formed outer bark is termed scale- bark. On the other hand, if the phellogen arises around the whole circumference of the stem, a ring bark is formed, which shows the separation of hollow cylinders or rings from the stem.

Lenticels:

Usually in the periderm of most plants, certain areas with loosely arranged cells have been found, which possess more or less raised and corky spots where the underneath tissues break through the epidermis. Such areas are universally found on the stems of woody plants. These broken areas are called the lenticels.

Wutz (1955) defined a lenticel as a small portion of the periderm where the activity of the phellogen is more than elsewhere, and the cork cells produced by it are loosely arranged and possess numerous intercellular spaces.

These areas are thicker radially than rest of the periderm because of the presence of loose complementary cells. The lenticels perform the function of exchange of gases during night or when the stomata are closed.

Lenticels are first formed immediately beneath the stomata or group of stomata and the number of lenticels, therefore, depends upon the number of stomata or groups of stomata. The lenticels may be scattered on the stems or they may be arranged in vertical or horizontal rows. The lenticels also occur on the roots.

The lenticels originate beneath the stomata, either just before or simultaneously with the initiation of the first layer of the periderm. In most of plants, lenticel formation takes place in the first growing season and sometimes previous to the growth in length has stopped.

As the lenticel formation begins, the parenchyma cells found near about the sub-stomatal cavity lose their chlorophyll and divide irregularly in different planes giving rise to a mass of colourless, rounded, thin walled, loose cells called complementary cells.

Such cells are also produced by phellogen towards outside instead of cork cells. As the complementary cells increase in number, pressure is caused against the epidermis and it ruptures. Very often, the outer most cells die due to exposure to outer atmosphere and are replaced by the cells cut off by cork cambium or phellogen.

The thin walled loose complementary cells may alternate with masses of more dense and compact cells called the closing cells. These cells together form a layer called closing layer. With the continuous formation of new loose complementary cells, the closing layers are ruptured.

The lenticels are filled up with complementary cells completely in the spring season whereas in the end of the spring season the lenticel becomes closed by the formation of closing layer.

The complementary cells are thin-walled, rounded and loose with sufficiently developed intercellular spaces among them. Their cell walls are not suberized. Due to the presence of profuse intercellular spaces, the lenticels perform the function of exchange of gases between the atmosphere and internal tissues of the plant.

Sometimes, lenticels develop independent of the stomata. In such cases the phellogen cuts for sometime the cork cells and then loose complementary cells which ultimately break the cork and rise to a new lenticel.