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11.8: Introduction to Asexual Reproduction in Plants - Biology

11.8: Introduction to Asexual Reproduction in Plants - Biology


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What you’ll learn to do: Describe plants that reproduce asexually

Many plants reproduce asexually as well as sexually. Asexually reproducing plants thrive well in stable environments.


  • Most plants have roots, stems and leaves. These are called the vegetative parts of a plant.
  • Vegetative Propagation is a type of asexual reproduction in which new plants are produced from roots, stems, leaves and buds. Since reproduction is through the vegetative parts of the plant, it is known as vegetative propagation.
  • Bryophyllum (sprout leaf plant) has buds in the margins of leaves. If a leaf of this plant falls on a moist soil, each bud can give rise to a new plant.

Bryophyllum (sprout leaf plant)

  • The roots of some plants can also give rise to new plants. Sweet potato and dahlia are examples.
  • Plants such as cacti produce new plants when their parts get detached from the main plant body. Each detached part can grow into a new plant.
  • Plants produced by vegetative propagation take less time to grow and bear flowers and fruits earlier than those produced from seeds.
  • The new plants are exact copies of the parent plant, as they are produced from a single parent.

References

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Resources

Ellstrand, Norman C. Dangerous Liaisons?: When Cultivated Plants Mate with their Wild Relatives. Baltimore: The Johns Hopkins University Press, 2003.

Majerus, Michael E. N. Sex Wars: Genes, Bacteria, and Biased Sex Ratios. Princeton: Princeton Univerity Press, 2003.

O ’ Neill, Sherman, and Jeremy A. Roberts. Plant Reproduction. New York: Blackwell, 2001.

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Contents

Fission Edit

Prokaryotes (Archaea and Bacteria) reproduce asexually through binary fission, in which the parent organism divides in two to produce two genetically identical daughter organisms. Eukaryotes (such as protists and unicellular fungi) may reproduce in a functionally similar manner by mitosis most of these are also capable of sexual reproduction.

Multiple fission at the cellular level occurs in many protists, e.g. sporozoans and algae. The nucleus of the parent cell divides several times by mitosis, producing several nuclei. The cytoplasm then separates, creating multiple daughter cells. [4] [5] [6]

In apicomplexans, multiple fission, or schizogony appears either as merogony, sporogony or gametogony. Merogony results in merozoites, which are multiple daughter cells, that originate within the same cell membrane, [7] [8] sporogony results in sporozoites, and gametogony results in microgametes.

Budding Edit

Some cells divide by budding (for example baker's yeast), resulting in a "mother" and a "daughter" cell that is initially smaller than the parent. Budding is also known on a multicellular level an animal example is the hydra, [9] which reproduces by budding. The buds grow into fully matured individuals which eventually break away from the parent organism.

Internal budding is a process of asexual reproduction, favoured by parasites such as Toxoplasma gondii. It involves an unusual process in which two (endodyogeny) or more (endopolygeny) daughter cells are produced inside a mother cell, which is then consumed by the offspring prior to their separation. [10]

Also, budding (external or internal) occurs in some worms like Taenia or Echinococcus these worms produce cysts and then produce (invaginated or evaginated) protoscolex with budding.

Vegetative propagation Edit

Vegetative propagation is a type of asexual reproduction found in plants where new individuals are formed without the production of seeds or spores and thus without syngamy or meiosis. [11] Examples of vegetative reproduction include the formation of miniaturized plants called plantlets on specialized leaves, for example in kalanchoe (Bryophyllum daigremontianum) and many produce new plants from rhizomes or stolon (for example in strawberry). Other plants reproduce by forming bulbs or tubers (for example tulip bulbs and Dahlia tubers). Some plants produce adventitious shoots and may form a clonal colony. In these examples, all the individuals are clones, and the clonal population may cover a large area. [12]

Spore formation Edit

Many multicellular organisms form spores during their biological life cycle in a process called sporogenesis. Exceptions are animals and some protists, which undergo meiosis immediately followed by fertilization. Plants and many algae on the other hand undergo sporic meiosis where meiosis leads to the formation of haploid spores rather than gametes. These spores grow into multicellular individuals (called gametophytes in the case of plants) without a fertilization event. These haploid individuals give rise to gametes through mitosis. Meiosis and gamete formation therefore occur in separate generations or "phases" of the life cycle, referred to as alternation of generations. Since sexual reproduction is often more narrowly defined as the fusion of gametes (fertilization), spore formation in plant sporophytes and algae might be considered a form of asexual reproduction (agamogenesis) despite being the result of meiosis and undergoing a reduction in ploidy. However, both events (spore formation and fertilization) are necessary to complete sexual reproduction in the plant life cycle.

Fungi and some algae can also utilize true asexual spore formation, which involves mitosis giving rise to reproductive cells called mitospores that develop into a new organism after dispersal. This method of reproduction is found for example in conidial fungi and the red algae Polysiphonia, and involves sporogenesis without meiosis. Thus the chromosome number of the spore cell is the same as that of the parent producing the spores. However, mitotic sporogenesis is an exception and most spores, such as those of plants, most Basidiomycota, and many algae, are produced by meiosis. [ citation needed ]

Fragmentation Edit

Fragmentation is a form of asexual reproduction where a new organism grows from a fragment of the parent. Each fragment develops into a mature, fully grown individual. Fragmentation is seen in many organisms. Animals that reproduce asexually include planarians, many annelid worms including polychaetes [13] and some oligochaetes, [14] turbellarians and sea stars. Many fungi and plants reproduce asexually. Some plants have specialized structures for reproduction via fragmentation, such as gemmae in liverworts. Most lichens, which are a symbiotic union of a fungus and photosynthetic algae or cyanobacteria, reproduce through fragmentation to ensure that new individuals contain both symbionts. These fragments can take the form of soredia, dust-like particles consisting of fungal hyphen wrapped around photobiont cells.

Clonal Fragmentation in multicellular or colonial organisms is a form of asexual reproduction or cloning where an organism is split into fragments. Each of these fragments develop into mature, fully grown individuals that are clones of the original organism. In echinoderms, this method of reproduction is usually known as fissiparity. [15] Due to many environmental and epigenetic differences, clones originating from the same ancestor might actually be genetically and epigenetically different. [16]

Agamogenesis Edit

Agamogenesis is any form of reproduction that does not involve a male gamete. Examples are parthenogenesis and apomixis.

Parthenogenesis Edit

Parthenogenesis is a form of agamogenesis in which an unfertilized egg develops into a new individual. It has been documented in over 2,000 species. [17] Parthenogenesis occurs in the wild in many invertebrates (e.g. water fleas, rotifers, aphids, stick insects, some ants, bees and parasitic wasps) and vertebrates (mostly reptiles, amphibians, and fish). It has also been documented in domestic birds and in genetically altered lab mice. [18] [19] Plants can engage in parthenogenesis as well through a process called apomixis. However this process is considered by many to not be an independent reproduction method, but instead a breakdown of the mechanisms behind sexual reproduction. [20] Parthenogenetic organisms can be split into two main categories: facultative and obligate.

Facultative Parthenogenesis Edit

In facultative parthenogenesis, females can reproduce both sexually and asexually. [17] Because of the many advantages of sexual reproduction, most facultative parthenotes only reproduce asexually when forced to. This typically occurs in instances when finding a mate becomes difficult. For example, female Zebra Sharks will reproduce asexually if they are unable to find a mate in their ocean habitats. [21]

Parthenogenesis was previously believed to rarely occur in vertebrates, and only be possible in very small animals. However, it has been discovered in many more species in recent years. Today, the largest species that has been documented reproducing parthenogenically is the Komodo Dragon at 10 feet long and over 300 pounds. [22] [23]

Heterogony is a form of facultative parthenogenesis where females alternate between sexual and asexual reproduction at regular intervals (see Alternation between sexual and asexual reproduction). Aphids are one group of organism that engages in this type of reproduction. They use asexual reproduction to reproduce quickly and create winged offspring that can colonize new plants and reproduce sexually in the fall to lay eggs for the next season. [24] However, some aphid species are obligate parthenotes. [25]

Obligate Parthenogenesis Edit

In obligate parthenogenesis, females only reproduce asexually. [17] One example of this is the desert grassland whiptail lizard, a hybrid of two other species. Typically hybrids are infertile but through parthenogenesis this species has been able to develop stable populations. [26]

Gynogenesis is a form of obligate parthenogenesis where a sperm cell is used to initiate reproduction. However, the sperm's genes never get incorporated into the egg cell. The best known example of this is the Amazon Molly. Because they are obligate parthenotes, there are no males in their species so they depend on males from a closely related species (the Sailfin Molly) for sperm. [27]

Apomixis and nucellar embryony Edit

Apomixis in plants is the formation of a new sporophyte without fertilization. It is important in ferns and in flowering plants, but is very rare in other seed plants. In flowering plants, the term "apomixis" is now most often used for agamospermy, the formation of seeds without fertilization, but was once used to include vegetative reproduction. An example of an apomictic plant would be the triploid European dandelion. Apomixis mainly occurs in two forms: In gametophytic apomixis, the embryo arises from an unfertilized egg within a diploid embryo sac that was formed without completing meiosis. In nucellar embryony, the embryo is formed from the diploid nucellus tissue surrounding the embryo sac. Nucellar embryony occurs in some citrus seeds. Male apomixis can occur in rare cases, such as the Saharan Cypress Cupressus dupreziana, where the genetic material of the embryo are derived entirely from pollen.

Some species can alternate between sexual and asexual strategies, an ability known as heterogamy, depending on many conditions. Alternation is observed in several rotifer species (cyclical parthenogenesis e.g. in Brachionus species) and a few types of insects.

One example of this is aphids which can engage in heterogony. In this system, females are born pregnant and produce only female offspring. This cycle allows them to reproduce very quickly. However, most species reproduce sexually once a year. This switch is triggered by environmental changes in the fall and causes females to develop eggs instead of embryos. This dynamic reproductive cycle allows them to produce specialized offspring with polyphenism, a type of polymorphism where different phenotypes have evolved to carry out specific tasks. [28]

The cape bee Apis mellifera subsp. capensis can reproduce asexually through a process called thelytoky. The freshwater crustacean Daphnia reproduces by parthenogenesis in the spring to rapidly populate ponds, then switches to sexual reproduction as the intensity of competition and predation increases. Monogonont rotifers of the genus Brachionus reproduce via cyclical parthenogenesis: at low population densities females produce asexually and at higher densities a chemical cue accumulates and induces the transition to sexual reproduction. Many protists and fungi alternate between sexual and asexual reproduction. A few species of amphibians, reptiles, and birds have a similar ability. [which?] [ which? ]

The slime mold Dictyostelium undergoes binary fission (mitosis) as single-celled amoebae under favorable conditions. However, when conditions turn unfavorable, the cells aggregate and follow one of two different developmental pathways, depending on conditions. In the social pathway, they form a multi-cellular slug which then forms a fruiting body with asexually generated spores. In the sexual pathway, two cells fuse to form a giant cell that develops into a large cyst. When this macrocyst germinates, it releases hundreds of amoebic cells that are the product of meiotic recombination between the original two cells. [29]

The hyphae of the common mold (Rhizopus) are capable of producing both mitotic as well as meiotic spores. Many algae similarly switch between sexual and asexual reproduction. [30] A number of plants use both sexual and asexual means to produce new plants, some species alter their primary modes of reproduction from sexual to asexual under varying environmental conditions. [31]

In the rotifer Brachionus calyciflorus asexual reproduction (obligate parthenogenesis) can be inherited by a recessive allele, which leads to loss of sexual reproduction in homozygous offspring. [32] [33]
Inheritance of asexual reproduction by a single recessive locus has also been found in the parasitoid wasp Lysiphlebus fabarum. [34]

Asexual reproduction is found in nearly half of the animal phyla. [35] Parthenogenesis occurs in the hammerhead shark [36] and the blacktip shark. [37] In both cases, the sharks had reached sexual maturity in captivity in the absence of males, and in both cases the offspring were shown to be genetically identical to the mothers. The New Mexico whiptail is another example.

Some reptiles use the ZW sex-determination system, which produces either males (with ZZ sex chromosomes) or females (with ZW or WW sex chromosomes). Until 2010, it was thought that the ZW chromosome system used by reptiles was incapable of producing viable WW offspring, but a (ZW) female boa constrictor was discovered to have produced viable female offspring with WW chromosomes. [38] The female boa could have chosen any number of male partners (and had successfully in the past) but on these occasions she reproduced asexually, creating 22 female babies with WW sex-chromosomes.

Polyembryony is a widespread form of asexual reproduction in animals, whereby the fertilized egg or a later stage of embryonic development splits to form genetically identical clones. Within animals, this phenomenon has been best studied in the parasitic Hymenoptera. In the 9-banded armadillos, this process is obligatory and usually gives rise to genetically identical quadruplets. In other mammals, monozygotic twinning has no apparent genetic basis, though its occurrence is common. There are at least 10 million identical human twins and triplets in the world today.

Bdelloid rotifers reproduce exclusively asexually, and all individuals in the class Bdelloidea are females. Asexuality evolved in these animals millions of years ago and has persisted since. There is evidence to suggest that asexual reproduction has allowed the animals to evolve new proteins through the Meselson effect that have allowed them to survive better in periods of dehydration. [39] Bdelloid rotifers are extraordinarily resistant to damage from ionizing radiation due to the same DNA-preserving adaptations used to survive dormancy. [40] These adaptations include an extremely efficient mechanism for repairing DNA double-strand breaks. [41] This repair mechanism was studied in two Bdelloidea species, Adineta vaga, [41] and Philodina roseola. [42] and appears to involve mitotic recombination between homologous DNA regions within each species.

Molecular evidence strongly suggests that several species of the stick insect genus Timema have used only asexual (parthenogenetic) reproduction for millions of years, the longest period known for any insect. [43]

In the grass thrips genus Aptinothrips there have been several transitions to asexuality, likely due to different causes. [44]

A complete lack of sexual reproduction is relatively rare among multicellular organisms, particularly animals. It is not entirely understood why the ability to reproduce sexually is so common among them. Current hypotheses [45] suggest that asexual reproduction may have short term benefits when rapid population growth is important or in stable environments, while sexual reproduction offers a net advantage by allowing more rapid generation of genetic diversity, allowing adaptation to changing environments. Developmental constraints [46] may underlie why few animals have relinquished sexual reproduction completely in their life-cycles. Almost all asexual modes of reproduction maintain meiosis either in a modified form or as an alternative pathway. [47] Facultatively apomictic plants increase frequencies of sexuality relative to apomixis after abiotic stress. [47] Another constraint on switching from sexual to asexual reproduction would be the concomitant loss of meiosis and the protective recombinational repair of DNA damage afforded as one function of meiosis. [48] [49]


Asexual reproduction

Algae

…female gametes (sex cells), by asexual reproduction, or by both ways.

Animals

Asexual reproduction (i.e., reproduction not involving the union of gametes), however, occurs only in the invertebrates, in which it is common, occurring in animals as highly evolved as the sea squirts, which are closely related to the vertebrates. Temporary gonads are common among lower animals…

Apicomplexans

Asexual reproduction is by binary or multiple fission (schizogony).

Echinoderms

Asexual reproduction in echinoderms usually involves the division of the body into two or more parts (fragmentation) and the regeneration of missing body parts. Fragmentation is a common method of reproduction used by some species of asteroids, ophiuroids, and holothurians, and in some…

Fungi

Typically in asexual reproduction, a single individual gives rise to a genetic duplicate of the progenitor without a genetic contribution from another individual. Perhaps the simplest method of reproduction of fungi is by fragmentation of the thallus, the body of a fungus. Some…

Growth and development

…in plants that reproduce by vegetative division, the breaking off of a part that can grow into another complete plant. The possibilities for debate that arise in these special cases, however, do not in any way invalidate the general usefulness of the distinctions as conventionally made in biology.

Major references

In asexual reproduction the new individual is derived from a blastema, a group of cells from the parent body, sometimes, as in Hydra and other coelenterates, in the form of a “bud” on the body surface. In sponges and bryozoans, the cell groups from which new…

Multicellular organisms also reproduce asexually and sexually asexual, or vegetative, reproduction can take a great variety of forms. Many multicellular lower plants give off asexual spores, either aerial or motile and aquatic (zoospores), which may be uninucleate or multinucleate. In some cases the reproductive body is multicellular, as in…

…higher plants also reproduce by nonsexual means. Bulbs bud off new bulbs from the side. Certain jellyfish, sea anemones, marine worms, and other lowly creatures bud off parts of the body during one season or another, each thereby giving rise to populations of new, though identical, individuals. At the microscopic…

Plants

Both homosporous and heterosporous life histories may exhibit various types of asexual reproduction (vegetative reproduction, somatic reproduction). Asexual reproduction is any reproductive process that does not involve meiosis or the union of nuclei, sex cells, or sex organs. Depending on the type of…

Asexual reproduction involves no union of cells or nuclei of cells and, therefore, no mingling of genetic traits, since the nucleus contains the genetic material (chromosomes) of the cell. Only those systems of asexual reproduction that are not really modifications of sexual reproduction are considered…

Population ecology

In sexual populations, genes are recombined in each generation, and new genotypes may result. Offspring in most sexual species inherit half their genes from their mother and half from their father, and their genetic makeup is therefore different from either parent or any other…

Protozoans

Asexual reproduction is the most common means of replication by protozoans. The ability to undergo a sexual phase is confined to the ciliates, the apicomplexans, and restricted taxa among the flagellated and amoeboid organisms. Moreover, sexual reproduction does not always result in an immediate increase…

Spores

Spores are agents of asexual reproduction, whereas gametes are agents of sexual reproduction. Spores are produced by bacteria, fungi, algae, and plants.


Life Cycle of Rhizome

The annual cycle of a rhizome is similar to that of a corm. In late spring/early summer, food from the leaves passes back to the rhizome, and a lateral bud uses it, grows horizontally underground, and so continues the rhizome. Other lateral buds produce new rhizomes which branch from the parent stem. The terminal buds of these branches curve upwards and produce new leafy shoots and flowers. Contractile, adventitious roots grow from the nodes of the underground stem and keep it at a constant depth.


Understand the sexual and asexual reproduction from the bramble and the daffodil plant

NARRATOR: Sexual activity in the plant world normally goes unnoticed by humans. But we don't have to look far to realize that all kinds of plants are constantly increasing their numbers.

Most flowering plants grow from seeds, but why do plants produce so many seeds?

The rigors of the terrestrial environment mean that many plants will not be able to reproduce [music out]. By making a large number of seeds, plants increase the chances that some of them will survive and reproduce again.

Not all plants need to reproduce sexually. The bramble plant produces roots at the end of this long stem from here a new plant will grow. In spite of this method, however, sexual reproduction is still a major factor in the reproductive cycle of the bramble plant.

In order for sexual reproduction to take place, male and female gametes must fuse to produce the first cell of the new embryonic plant.

What is the value of sexual reproduction to plants?

Sexual reproduction ensures diversity, new genes, and a new blueprint. If the bees like it, this color could become dominant in the future, and if they don't, then this plant will become another casualty of natural selection.

In either case, the outcome will work to the advantage of the species as a whole.

The manner in which sexual reproduction is achieved varies from plant to plant, but the sexual reproductive cycle for all plants involves two stages, or generations. Botanists call this phenomenon the alternation of generations.

Consider the moss Mnium hornum. The leafy plant is the gametophyte generation and produces sperm and eggs. The antheridia produce motile sperms. In wet conditions, mature antheridia swell and burst, releasing sperm onto the surface of the leaves. Attracted by the sucrose secretion at the neck of the female sex organs, sperm swim into the neck of the archegonia, where fertilization takes place. The zygote cell will grow into the sporophyte generation. Inside the capsule, diploid spore mother cells divide by meiosis to produce haploid spores. Under the right conditions, the spores will be released and will germinate into the embryonic gametophyte plant called the protonema.

Mosses provide a clear illustration of how the alternation of generations works. The ferns show the same pattern but with a different dominant generation. Here, the spore-producing sporophyte is what we mainly associate with ferns, while the gamete-making gametophyte is a tiny unnoticed plant near the ground.

In flowering plants, like these daffodils, the alternation of generations is far less obvious because the gametophyte generation is even further reduced, while the sexual apparatus is far more sophisticated.

The sexual organs of this daffodil plant are concentrated in the flower.

Colored petals function to attract insects, important agents for pollination. The trumpet structure in the middle is called the corolla and consists of fused petals. What kind of an advantage might the corolla be to a plant?

Inside the corolla, we can see the sexual structures of the plant. The female sex organ consists of the stigma, which is elevated on the style and terminates at the ovary, where the female egg cells are contained within ovules. The female egg cells contained within the ovules constitute the female gametophyte generation.

Arranged around the stigma are the male sex organs or stamens. Each stamen consists of an anther and a filament. The anthers contain microspore mother cells that eventually produce the gametophyte generation, also known as pollen grains.

When the flower is mature, the corolla opens to reveal the pistil and stamen inside. At the same time, the top of the stamen is releasing millions of pollen grains.

The daffodil is called an entomophilous flower because insects transfer the pollen from one flower to another. But why do insects do this?

Flowers have evolved to produce the colors, scent, and food sources that will be most attractive to insects. In their quest for food, insects brush against anthers and stigmas, effectively cross-pollinating the flowers. Insects are blissfully unaware of their vital role in the life cycles of the plants they pollinate.

Some flowers, such as these foxgloves, have evolved in parallel with their insect pollinators. The size and shape of the flowers ideally suits the bumblebee. The markings and hairs on the lower petals serve as a landing strip to guide the pollinators straight to the nectaries.

Insects are not the only agents of pollination used by plants. For plants that rely on the wind to carry their pollen, there is no need for insect attractors such as conspicuous flowers, petals, sepals, nectaries, or other temptations. The tiny flowers suspend their anthers and stigmas into the wind to promote cross-pollination.

The pollen grains of anemophilous species are smaller and lighter than those of insect-pollinated flowers. They are also produced in extremely large numbers. How might this help the plant achieve pollination?

However it is achieved, pollination is an all-important process for most terrestrial plants since it ensures that fertilization will take place and that there will be a new generation of plants.

But how do plants ensure that the right pollen gets to the right stigma at the right time?

The dandelion uses a special mechanism to ensure that the correct pollen is transferred to its stigma. The flower head is actually made up of many individual flowers, called florets. The florets are cross-pollinated by insects but can also self-pollinate.

When the florets were growing, the closed stigma of the dandelion flower grew through the middle of the anthers so that pollen was transferred onto the style as it elongated. After a period of time, if cross-pollination has not taken place, the stigma curls back on itself to pick up its own pollen from the style below.

The passionflower has evolved a most interesting method for ensuring cross-pollination. When the flower opens, the anthers flip over. Foraging bees brush against the anthers taking pollen away on their backs. Sometime afterwards, the stigmas descend. Bees who are already carrying pollen from other flowers then transfer pollen to the stigmas as they continue their search for the nectar.

Despite some elaborate mechanisms to prevent it, self-pollination is sometimes unavoidable. But self-pollination doesn't have to mean self-fertilization. Plants can chemically recognize their own pollen and inhibit its further development in favor of pollen from another source.

Flowers such as this tiger lily in which both male and female sex organs are located are called perfect flowers. Where the plant produces separate male and female flowers, the flowers are said to be imperfect.

Here, the male flower is the catkin and is called the staminate flower. The small red flower is the female, or pistillate, flower.

How do completely separate male and female flowers ensure that they are in sync?

Many plants have ensured that cross-pollination takes place by deliberately keeping male and female flowers on the same plant out of sync. In this corn plant, the pollen is ripe long before the stigmas are receptive therefore, the only way the ovules can be fertilized is by the pollen from another corn plant.

Once the pollen season starts, there's no escaping it. Pollen seems to get everywhere. But what happens when it finally reaches the tip of a receptive stigma?

Each tiny pollen grain contains two nuclei. One nucleus is called the generative nucleus and will divide to produce two male sperm nuclei. The other nucleus is called the tube nucleus.

The receptive stigma chemically switches on the pollen grain which begins to generate a long tube with its own nucleus. It is important that this happens because the male gametes have to reach to ovules, which may be a considerable distance from the tip of the stigma.

The pollen tube grows down into the tissues of the stigma and down the length of the style. The tube eventually penetrates the ovule by passing through a small hole called the micropyle.

At the micropyle end of the ovule there is an egg cell nucleus flanked by two other nuclei. At the other end of the ovule are three nuclei left over from the previous meiotic divisions. In the center there is a diploid nucleus formed by the fusion of two polar nuclei.

The pollen tube releases one male gamete nucleus which fuses with the female egg cell nucleus. This is the moment of fertilization and produces a diploid zygote. This is the first cell of the new sporophyte, and it will divide repeatedly to produce the embryo.

The other male gamete nucleus fuses with the diploid polar nucleus to produce a unique triploid nucleus. From this new triploid nucleus a tissue called the endosperm develops.

It is in the endosperm tissue of wheat, barley, corn, oats, and rice that most of the seed's food reserves are stored. Thus, it is easy to see that endosperm is the single most important food source for mankind.

Not all plants produce a separate food store like endosperm for their embryos. It is a feature of most monocotyledons.

In dicotyledons, however, such as this young kidney bean plant, no such endosperm food store exists. The first two leaves of the embryo become swollen with food from the mother plant before the beans split the pod. This food store supports the young plant until it can make its own food through photosynthesis.

The seed is a powerhouse of potential.

Within a tough seed coat there is an embryonic plant with its own special food store.

When a seed is ready to germinate, it takes in water, and metabolic activity begins. Food stores, which mostly consist of starch, are mobilized by enzymes produced by the embryo.

Within this dicotyledonous seed, the young root, known as the radical, can be seen clearly. The young shoot is known as the plumule and emerges after the root.

On some monocotyledons, the plumule is protected by a coleoptile. This protective cap is clearly visible on young corn plants since it is left behind once the shoot reaches the surface.

After winter has thawed, the first flowers of early spring appear.

But these plants have not grown from seeds they have reproduced from bulbs. Bulbs enable plants to reproduce asexually, that is, without producing gametes. It usually results in the production of identical offspring, although there are random mutations.

Bulbs are known as perennating organs. They allow plants to survive in adverse conditions and then to grow quickly when the time is right.

The swollen rhizomes of irises have a similar function, but asexual reproduction does not rely solely on perennating organs.

This liverwort can reproduce asexually via gemmae. Gemmae are small disks of green tissue that grow inside special cups. When mature they break off from the parent plant, often due to the action of raindrops. They scatter away from the parent plant and will eventually grow into new gametophyte plants.

Plants like this Bryophyllum can also reproduce asexually. Miniature plantlets develop at the edges of its leaves. In time, these will drop off and develop into independent plants.

Mature strawberry plants are able to establish new plantlets on the end of long runners.

Gardeners are able to cultivate plants asexually via cuttings. This is possible because stem cells like these are able to trigger the formation of root cells and will start to grow roots.

The ability of many plants to reproduce asexually helps commercial growers because it's quicker and more reliable than growing plants from seeds. It also ensures growers that quality is consistent.

Asexual reproduction is all about exploiting a good niche. In such circumstances the value of sexual reproduction [music in] with its resultant diversity may actually weaken the dominance of an established group. But in a changing environment, diversity means survival.


General overview of metabolic cycles

Metabolism denotes the sum of the chemical reactions in the cell that provide the energy and synthesized materials required for growth, reproduction, and maintenance of structure and function. In plants the ultimate source of all organic chemicals and the energy stored in their chemical bonds is the conversion of CO2 into organic compounds (CO2 fixation) by either photosynthesis or chemosynthesis. The general and specific features of plant metabolism ultimately derive from oxygenic photosynthesis, which underlies the autotrophic nutrition of plants.