The lifecycle of angiosperms follows the alternation of generations explained previously. The haploid gametophyte alternates with the diploid sporophyte during the sexual reproduction process of angiosperms.
Male Gametophyte: The Pollen Grain
The male gametophyte develops and reaches maturity in an immature anther. In a plant’s male reproductive organs, development of pollen takes place in a structure known as the microsporangium (Figure 1). The microsporangia, which are usually bi-lobed, are pollen sacs in which the microspores develop into pollen grains. These are found in the anther, which is at the end of the stamen—the long filament that supports the anther.
Within the microsporangium, the microspore mother cell divides by meiosis to give rise to four microspores, each of which will ultimately form a pollen grain (Figure 2). An inner layer of cells, known as the tapetum, provides nutrition to the developing microspores and contributes key components to the pollen wall. Mature pollen grains contain two cells: a generative cell and a pollen tube cell. The generative cell is contained within the larger pollen tube cell. Upon germination, the tube cell forms the pollen tube through which the generative cell migrates to enter the ovary. During its transit inside the pollen tube, the generative cell divides to form two male gametes (sperm cells). Upon maturity, the microsporangia burst, releasing the pollen grains from the anther.
Female Gametophyte: The Embryo Sac
While the details may vary between species, the overall development of the female gametophyte has two distinct phases. First, in the process of megasporogenesis, a single cell in the diploid megasporangium—an area of tissue in the ovules—undergoes meiosis to produce four megaspores, only one of which survives. During the second phase, megagametogenesis, the surviving haploid megaspore undergoes mitosis to produce an eight-nucleate, seven-cell female gametophyte, also known as the megagametophyte or embryo sac. Two of the nuclei—the polar nuclei—move to the equator and fuse, forming a single, diploid central cell. This central cell later fuses with a sperm to form the triploid endosperm. Three nuclei position themselves on the end of the embryo sac opposite the micropyle and develop into the antipodal cells, which later degenerate. The nucleus closest to the micropyle becomes the female gamete, or egg cell, and the two adjacent nuclei develop into synergid cells (Figure 3). The synergids help guide the pollen tube for successful fertilization, after which they disintegrate. Once fertilization is complete, the resulting diploid zygote develops into the embryo, and the fertilized ovule forms the other tissues of the seed.
A double-layered integument protects the megasporangium and, later, the embryo sac. The integument will develop into the seed coat after fertilization and protect the entire seed. The ovule wall will become part of the fruit. The integuments, while protecting the megasporangium, do not enclose it completely, but leave an opening called the micropyle. The micropyle allows the pollen tube to enter the female gametophyte for fertilization.
An embryo sac is missing the synergids. What specific impact would you expect this to have on fertilization?
- The pollen tube will be unable to form.
- The pollen tube will form but will not be guided toward the egg.
- Fertilization will not occur because the synergid is the egg.
- Fertilization will occur but the embryo will not be able to grow.
[reveal-answer q=”673553″]Show Answer[/reveal-answer]
[hidden-answer a=”673553″]Statement b should be expected.[/hidden-answer]
60 Seed Plants: Angiosperms
Figure 1: These flowers grow in a botanical garden border in Bellevue, WA. Flowering plants dominate terrestrial landscapes. The vivid colors of flowers are an adaptation to pollination by insects and birds. (credit: Myriam Feldman)
From their humble and still obscure beginning during the early Jurassic period (202–145.5 MYA), the angiosperms, or flowering plants, have successfully evolved to dominate most terrestrial ecosystems. Angiosperms include a staggering number of genera and species with more than 260,000 species, the division is second only to insects in terms of diversification ([Figure 1]).
Angiosperm success is a result of two novel structures that ensure reproductive success: flowers and fruit. Flowers allowed plants to form cooperative evolutionary relationships with animals, in particular insects, to disperse their pollen to female gametophytes in a highly targeted way. Fruit protect the developing embryo and serve as an agent of dispersal. Different structures on fruit reflect the dispersal strategies that help with the spreading of seeds.
Sexual Reproduction in Flowering Plants (12th biology NCERT ch 2)
A typical Angiosperm Flower
Some important terms:
•Essential whorl: Gynoecium, Androecium.
•Accessory whorl: calyx, corolla.
•Pistillate: contains only pistil, female flower.
•Staminate: contains only stamen, male flower
•Bisexual flower: hermaphrodites.
•Complete flower: contains all four whorl.
•Incomplete flower: one or more whorl are absent.
•Neuter flower: neither pistil nor stamen present.
•Monoecious plants: is one that has male and female flowers on the same plant, or that has flowers on every plant that contain both male and female reproductive components.
•Dioecious plants: when male and female flower grows on different plants, eg: papaya.
•Perienth: undifferentiated calyx and corolla
•Petaloid: non green perienth.
•Bract: leaf in the Axil of flower.
•Nectaries: nectar producing cells.
Stamen microsporangium and pollen grain :
Microsporangium (pollen sac)
- Epidermis: provide protection to underlying tissue.
- Endothecium: provide protection to underlying tissue.
- Middle layer: provide protection to underlying tissue.
- Tapetum: provide nourishment to the growing embryo.
•Cells of tapetum have more cytoplasm and are multi nucleate.
•Sporogenous tissue occupies the center of each microsporangium.
•sporogenous tissue undergo meiotic division and forms micro-spore tetrad.
•As the anther matures and dehydrate micro-spores dissociate and forms pollen grain.
•Measuring about 25 to 50 μ m in diameter.
•Two layered wall: exine and intine.
•Exine is made up of sporopollenin (the most resistive organic material)
•Exine has prominent apertures where sporopolleninis absent known as germ pores.
•Intine is continuous inner layer made up of cellulose and pectin.
•Mature pollen grain contains two cells: vegetative cell and generative cell.
•Vegetative cell: bigger, has abundant food reserve, irregularly shaped nucleus.
•Generative cell: spindle shaped, floats in the cytoplasm of vegetative cell.
•Some species release pollen grain in 2 celled state, while in others generative cell divides mitotically and 3 celled pollen grain is released.
structure of pollen grain
The pistil, mega sporangium and Embryo sac:
•Monocarpellary: gynoecium having one pistil.
•Multicarpellary: gynoecium having multiple pistil.
•Apocarpous: when the several pistils of the same flower are separate. Eg: lotus, rose.
•Syncarpous: when the several pistils of the same flower are fused. Eg: tomato.
•Epigynousflower: inferior ovary
•Hypogynousflower: superior ovary
•Perigynousflower: semi inferior ovary
•Hilum: junction between hilum and ovule.
•Integument : protective coverings.
•Micropyle: entry point for pollen tube.
•Chalaza: basal part of ovule.
•Nucellus: nutritive tissue.
•Embryosac: female gametophyte.
Types of ovule :
•Orthrotropous: chalaza, micropyle and funicle situated in one line.
•Anatropous: ovule bent at 180 degree
•Hemianatropous: ovule bent at 90 degree
•Campylotropous: ovule circled more or less 90 degree to funicle, micropyle bent down slightly.
•Amphitropous: ovule curves more, embryo sac become horse shoe shape.
•Circinotropous: the funicle is long creating nearly a full circle around the ovule with micropyle pointing to upward direction.
Megasporogenesis and Megagametogenesis
- A hypodermal cell of nucellus toward micropyler end differentiate and behaves like Primary Archesporial Cell.
- After mitosis, the basal cell toward micropyle behaves as primary parietal cell which after further divisions develop wall cells.
- The inner cell behaves as Primary Sporogenous cell or Megaspore mother cell, which undergoes meiosis and form four Haploid megaspore.
- Out of four megaspore, only one remain functional, mostly the one which is toward chalazal end.
- Functional mega spore undergoes mitosis thrice, and as a result 8 nucleus forms.
- out of these 8 nucleus, 3 toward chalzal end diffrentiate to form Antipodial cells.
- 3 cells toward micropyler end forms 2 synergid and 1 egg cell (female gamete)
- and central cell contains 2 polar nuclei.
Pollination : the process of transfer of pollen grains from anther to stigma of the same flower or different flower on the same plant or on the different plant.
•Autogamy : Autogamy, or self-fertilization, refers to the fusion of two gametes that come from one individual.
•Geitonogamy: (from Greek geiton = neighbor+ gamein= to marry) is a type of self-pollination. … In flowering plants, pollen is transferred from a flower to another flower on the same plant.
•Xenogamy: Xenogamy is the transfer of pollen grains from the anther to the stigma of a different plant.
•Chasmogamy: Chasmogamy, is a plant reproductive mechanism in which pollination occurs in chasmogamous flowers. Chasmogamous flowers are commonly showy with open petals encircling exposed reproductive parts.
•Cleistogamy: self-fertilization that occurs within a permanently closed flower.
Modes of cross pollination
Pollen Pistil Interaction:
Fertilization: Fusion of male and female gametes to produce zygote.
•Siphonogamy: Siphonogamy is a condition in plants in which pollen tubes are developed for the transfer of the male cells to the eggs.
•Pollen tube eats its way through the solid part of stigma and style by secreting pectinase, hydrolytic enzymes.
•Pollen tube travels intercellularly and chemotropically along the concentration gradient of Ca+ boron inositol sugar complex.
•Mode of entry of pollen tube:
•Porogamy : when pollen tube enters the ovule through micropyle.
•Chalazogamy: when pollen tube enters the ovule through Chalaza.
•Mesogamy: when pollen tube enters the ovule some where between micropyle and chalaza.
Double fertilization and triple Fusion:
•Syngamy: Generative fertilization (male and female gametes) –> 2nZygote
•Vegetative fertilization: male gamete + polar nuclei –> 3n endosperm
Post Fertilization Changes
•The primary endosperm cell divides repeatedly and forms a triploid endosperm tissue.
•The cells of this tissue are filled with reserved food materials and are used for the nutrition of the developing embryo.
•Types of endosperm development:
•Nuclear endosperm eg: coconut
•Cellular endosperm eg. Datura
•Helobial endosperm eg. Monocots.
Embryo: Structure, type and Development:
Embryo develops at micropylar end of the embryo sac.
Development of a Dicot Embryo:
Development of a Monocot Embryo:
Dicot Vs Monocot Embryo
Application of Plant Biotechnology
188.8.131.52 Induction of Haploidy
Haploids are sporophytes of higher plants with a gametophytic chromosomal constitution, that is, they possess a single set of chromosomes that forms the alternation of generation (haploid and diploid as mentioned in Fig. 5.9 ). These types of plants are obtained from androgenesis, gynogenesis, parthenogenesis, semigamy, and polyembryony. In 1922, Belling and Blakeslee identified the first haploids in flowering plants (D. stramonium) [96,97] . The first culture of haploid plants (immature anthers of D. innoxia) was successfully established by Guha and Maheswari in Delhi University, which attracted worldwide attention in the application of tissue culture to synthesize haploid plants  . Normally somatic cells of higher plants have a diploid chromosome number while reproductive cells are haploid. They are succeeded in raising haploid embryoids and plantlets from developed culture (from microspores within the anthers). This opens the field of androgenesis. In the following year Bourgin and Nitsch confirmed the totipotency of pollen grains raising full haploid plants of tobacco, rice, and wheat  . Haploids by anther culture are now reported to have been raised in 247 species belonging to 34 families. Induction of haploid plants from pollinated ovaries and ovules (gynogenesis) is another recent advancement in plant tissue culture and experimental embryology. San Noeum reported her first result on in vitro culture of ovary isolated from Hordeum vulgare  . This demonstrates that not only the microspores but also the megaspores or female gametophytes of angiosperms can be triggered in vitro to saprophytic development, thus making way for an alternative approach to haploid plant breeding. It may be noted that anther culture has given rise to diploid, polyploid, and aneuploid plants. Thus, haploid induction or haploid represents significant applications for plant breeders and genetics in agriculture and plant tissue culture science.
Figure 5.9 . Plant life cycle alternation of generations.
CHAPTER-1 Reproduction in Organisms
1. Which is not true for life span of an organism?
(a) Period from birth to natural death of an organism
(b) It is necessarily correlated with size of organisms
(c) It may be very short
(d) Senescence is a phase of life span
2. In asexual reproduction
(a) Single parent is involved
(b) Gametic fusion is present
(c) Variations are produced
(d) Both (a) and (c)
3. Simple plants such as algae reproduce through special reproductive structure i.e.,
4. In Penicillium special asexual reproductive structure produced is
5. The most significant value of vegetative propagation is that
(a) It enables rapid production of genetic variation
(b) It is a means of producing a large population of individuals genetically identical to the parent
(c) It ensures that the progeny is safe from attack of pests and diseases
(d) It involves reduction division
6. Which of the following vegetative propagule is produced in Agave?
7. The plant/s propagated through root is/are
(a) Sweet potato
(d) All of the above
8. A leaf cell of a flowering plant has 22 chromosomes, then the number of chromosome would be
(a) 11 in gametes
(b) 22 in gametes
(c) 44 in embryo
(c) All of these
9. Which one of the following shows clear cut vegetative, reproductive and senescent phases?
(a) Annuals only
(b) All perennials
(c) Annuals and biennials
(d) All flowering plants
10. Which one of the following statement is incorrect?
(a) Unicellular sex organs are present in Marchantia
(b) Internal fertilization occurs in all embryophytes
(c) Development of endosperm is post-fertilisation event in angiosperms
(d) In monoecious plant, flowers are unisexual
11. In chara, male sex organ is
12. Which of the following plant is monocarpic?
13. Which of the following is the vital link that ensures continuity of species between organisms of one generation and the next?
(a) Male gamete
(b) Female gamete
14. Pick odd one out w.r.t. monoecious plants
15. Sexual reproduction is
(b) Less complex
(c) Less elaborate
(d) Less rapid
16. Leaf buds are found in
17. Which one of the following is correctly matched?
(a) Chlamydomonas – conidia
(b) Yeast – zoospores
(c) Onion – bulb
(d) Ginger – sucker
18. Meiosis takes place in
19. Which of the following propagate through leaf-tip?
(a) Walking fern
(d) Sprout-leaf plant
20. Embryo in sunflower has
(a) Two cotyledons
(b) Many cotyledons
(c) No cotyledons
(d) One cotyledon
21. Which of the following is a polygamous plant?
22. Vegetative propagation in mint occurs by
23. Which one of the following plants is monoecious?
24. Why is vivipary an undesirable character for annual crop plants?
(a) It reduces the vigour of the plant
(b) It adversely affects the fertility of the plant
(c) The seeds exhibit long dormancy
(d) The seeds cannot be stored under normal conditions for the next season
25. Vegetative propagation in Pistiaoccurs by
26. Endosperm is consumed by developing embryo in the seed of
27. Which of the following is resistant to enzyme action?
(a) Leaf cuticle
(c) Wood fibre
(d) Pollen exine
28. Which one of the following pairs of plant structures has haploid number of chromosomes?
(a) Egg nucleus and secondary nucleus
(b) Megaspore mother cell and antipodal cells
(c) Egg cell and antipodal cells
(d) Nucellus and antipodal cells
29. The role of double fertilization in angiosperms is to produce
30. In a cereal grain, the single cotyledon of embryo is represented by
31. The eyes of potato tuber are
(a) Axillary buds
(b) Root buds
(c) Flower buds
(d) Shoot buds
32. Wind pollination is common in
33. Nucellar polyembryony is reported in species of
34. What would be the number of chromosomes of aleurone cells of a plant with 42 chromosomes in its root tip cells?
35. Which one of the following pair is wrongly matched?
(a) Bryophyllum – leaf buds
(b) Agave – bulbil
(c) Penicillium – conidia
(d) Water hyacinth – runner
36. What is common between vegetative reproduction and apomixes?
(a) Both occur round the year
(b) Both produces progeny identical to the parent
(c) Both are applicable to only dicot plants
(d) Both bypass the flowering phase
37. Vegetative propagation through root does not occur in
38. Find odd one out w.r.t. plants where grafting is performed
39. The asexual production of seed is called
40. Hormogonia are the vegetative reproductive structures of
41. A scion is grafted to a stock. The quality of fruits produced will be determined by the genotype of
(c) Both stock and scion
(d) Neither stock nor scion
42. Adventitious buds are present at tip of leaves in
43. The vital link that ensure the continuity of species is
44. Cell division is itself a mode of reproduction in
(a) Protista and all fungi
(b) Monera, fungi and higher plants
(c) Protista and monera
(d) Protista only
45. The process of fusion of two vegetative nuclei is called
46. Which of the following is not an artificial method of vegetative reproduction?
47. In grafting, scion belongs to
(a) Plant having superior or desirable characters
(b) Plant having well developed root system
(c) Plant resistant to diseases
(d) Both (b) and (c)
48. A piece of plant part which is used in tissue culture is called as
49. The study of formation, growth and development of a new individual from an egg is known as
50. Parthenogenesis is defined as the development of an organism directly from
(a) Egg without fertilization
(b) Synergids without fertilization
(c) Fruits without fertilization
(d) Fruits without pollination
12.15: Sexual Reproduction in Angiosperms - Biology
what is the life spans of :
Elephant, Rose, Dog, Butter fly, Crow, Banana tree, Cow, Parrot, Crocodile, Horse, Fruit fly,
Rice plant, Tortoise & Banyan tree ??
How does penicillium reproduce asexually
Differentiate between parthenocarpy and parthenogenesis?
What are the examples of Sucker and Offset in Vegetative Propagation.
can u name a plant in which sepals remain attached after fertilization?
Bryophytes and Pteridophytes produces a large number of male gametes but relatively very few female gametes. Why?
What is the life span of a mango tree and a peepal tree?
EXPLAIN HAPLONTIC, DIPONTIC AND HAPLO DIPLONTIC LIFE CYCLE
Explain why internal fertilization is found to be more successful than the external fertilization in the living bodies.
in some plants , where flowering occurs more than once, what would you call the inter flowering period - juvenile or mature.
Why is fertilisation the most vital event in sexual reproduction?
how long does it take for marigold / rice / wheat / coconut / mango plants to come flower?
amoeba is immortal, explain .
the meiocyte of an onion plant contains 32 chromosomes.work out the no. of chromosomesfound in its endosperm?
how sexual reproduction enables organisms like algae and fungi to survive during unfavourable conditions? why is sexual reproduction favoured under such conditions/
Why is sexual reproduction favoured under such adverse conditions?
what are the examples of continuous breeders and seasonal breeders?
Why is it difficult to get rid of water hyacinth from a water body? Name one biotic and abiotic component that gets affected by its spread in the water body.
Does meiosis never occur in organisms that are haploid?
which type of plants show successful grafting & why it is so?
Name the type of gametes that are formed in staminate and pistillate flowers.
pattern of sexual reproduction in diverse types of organisms
why do wesay there is no natural death in single celled organism.
what would happen if syngamy does not occur?
explain haplontic and diplontic life cycle? [biology chapter 1 lesson 4 post fertilisation events]
give examples of monothecous and dithecous anther ?
why do we say there is no natural death in single celled organisms?
what is encystation n sporulation.
What is the kind of development that takes place in the zygote in organisms with diplontic and haplo- diplontic life cycle?
Explain briefly the vegetative reproduction in potato?
q.1. what is the difference between zoospores and conidia??with reference to their mobility..
q.2.why the clones are considered as genetically identical to its parent ..write examples
Explain vegetative propagation.
can i get difference between oestrus cycle and menstrual cycle please
how does gamete transfer takes place in:
explin the mada janan tantra
examples of flowers showing interflowering period
what do you mean by physical education.
Explain that embryonal protection and care are comparatively better in viviparous then oviparous organism?
Name the progenitor of the next generation in a seed.
What is protandry and protogyny?
difference between exalbuminous seeds and albuminous seeds
does sweet potato produces bisexual flowers?
what is the difference between self pollination and cross pollination?
all papaya plants bear flowers but fruits are seen only in some. explain
what are the differences between seasonal breeder and continous breeders?//
difference between bulb and bulbils
what is the difference between isogametes, anisogametes and heterogamates
Unicellular organisms are immortal whereas multicellular organisms are not.Justify
Which of the following pairs of asexual reproductive structures is found in animals?also explain reason..and why the answer is not zoospores please explain.
Differentiate between (a) oestrus and menstrual cycles (b) ovipary and vivipary. Cite an example for each type.
Give me some examples of fungi and algae in which both gametes are motile.