NCERT Solutions for Class 12 Biology Chapter 2 Sexual Reproduction in Flowering Plants

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NCERT Solutions for Class 12 Biology Chapter 2, sexual reproduction in flowering plants Class 12, sexual reproduction in flowering plants Class 12 notes, sexual reproduction in flowering plants notes, ncert solutions for Class 12 biology chapter 2


NCERT Solutions for Class 12 Biology Chapter 2

Class12th 
Chapter No02
ProvidingQuestions And Answers, Notes & Numericals PDF
Chapter NameSexual Reproduction in Flowering Plants
BoardCBSE
Book NCERT
SubjectBiology
Medium English
Study MaterialsFree VVI Study Materials are Available
Download PDF Class 12 Biology Ncert Solutions PDF CH-2

sexual reproduction in flowering plants notes


1. All flowering plants (angiosperms) show sexual reproduction. Their reproductive cells originate from two types of gamete mother cells in the reproductive organs. 

(i) Microspores that give rise to pollen grains. Pollen grains contain sperm and carry it safely to the egg. 

(ii) Megaspores that give rise to the ovule or egg cell. In flowers, the male reproductive organ; the androecium consists of stamens and the female reproductive organ; the gynoecium consists of the pistil.

2. Stamens These are the male reproductive structures. Each stamen has two parts:

(i) Filament, a long slender stalk. 

(ii) Anther, terminal generally bilobed structure. dotcom asilim dim etiam

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(a) Anther is generally bilobed having two theca i.e., they are dithecous. 

(b) A longitudinal groove runs lengthwise separating the theca. 

(c) In the transverse section, the anther is a four-side (tetragonal) structure consisting of microsporangia located at the corners, two in each lobe. 

(d) Microsporangia develop further and become pollen sacs.

3. Microsporangium

(i) Each microsporangium is covered with a four-layered wall (epidermis, endothecium, middle layers and tapetum). The outer three layers help in the protection and dehiscence of anther.

(ii) The innermost wall layer of tapetum nourishes the developing pollen grains. Cells of tapetum possess dense cytoplasm and generally have more than one nucleus. 

(iii) In the centre, cells of sporogenous tissue are present that transform into microspores of pollen mother cells.

(iv) These diploid cells undergo meiosis, forming tetrads of haploid microspores. 

(v) Individual microspores mature into pollen grains. 

4. Microsporogenesis

(1) This process of formation of haploid microspores from diploid pollen mother cells is called microsporogenesis.

(i) Each cell of sporogenous tissue is capable of giving rise to a microspore tetrad. 

(ii) The microspores are arranged in a cluster of four cells-the microspore tetrad.

(iii) Microspores/pollen grains are released with the dehiscence of anther.

5. Structure of Pollen Grain or microspore is the first cell or the male gametophyte. In fact, pollen grain is like a box that protects its contents and delivers it safely to the eggs. The pollen grain is spherical in shape

(i) It has two layers-outer exine, made of sporopollenin which is the most resistant organic material and inner intine which is pectocellulosic in nature (pectin and cellulose).

(ii) Exine has germ pores, through which the pollen tube grows, at the time of fertilisation. si ut notion 

6. Development of Male Gametophytes

Its protoplast undergoes mitotic division to make two unequal-sized cells 

(i) Bigger, vegetative cells with a large irregularly-shaped nucleus and abundant food reserves and cell organelles.

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(ii) Smaller, generative cell is spindle-shaped with dense cytoplasm around its nucleus. Initially, there is a wall around this cell that separates it from the pollen grain wall, but eventually, it dissolves and the generative cell floats in the cytoplasm of the vegetative cell. 

In many angiosperms, pollen grains are shed at this 2-celled stage. But in some species, the generative cell divides mitotically to make two male gametes. So, the pollen grains are cleared at the 3-celled stage (one vegetative cell and two gametes).

7. Pollen Allergies Large numbers of pollen grains produced float in the air. They may enter the respiratory tract and cause severe allergies in some people. It may lead to chronic (long-term) disorders like asthma, rhinitis and bronchitis. 

Parthenium or carrot grass came to India as a contaminant with imported wheat has spread vastly and is a major cause of pollen allergies. Amaranthus and Cynodon are a few other examples.

Uses of pollen grains

(i) Pollen is rich in nutrients so its tablets are used as food supplements.

(ii) Also used as nutrient supplements to increase the performance of athletes and race horses. 

(iii) Pollen grains can be stored in pollen banks in liquid nitrogen (-196°C) for years.

8. The Pistil (gynoecium) is the female reproductive part of the flower. It may consist of a single pistil, the condition referred to as monocarpellary or more pistils (multicarpellary). When more than one pistils are present they may be : 

(i) fused together-syncarpous pistils

(ii) free-apocarpous pistils. 

9. The pistil has three parts

(i) Stigma The platform that receives the pollen grains and where their compatibility is determined. 

(ii) Style Elongated tube that connects stigma with ovary.

(iii) Ovary The swollen part of the pistil at the base.

(a) Inside the ovary is a cavity called a locule. Placenta is located inside this locule.

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(b) From placenta, megasporangia arise. Megaspores (that form female gametophytes) are produced in megasporangia. Megaspores develop into ovules. Ovules later mature into seeds.

(c) An ovary may contain one ovule (wheat, paddy, mango) or many (papaya, watermelon).

(d) An ovule contains an embryo sac formed from the reduction division of the megaspore.

(e) Ovules are small structures attached to the placenta by a fb stalk called a funicle.

(f) The body of the ovule fuses with the funicle in the region called the hilum.

(g) The ovule is encircled with protective envelopes; integuments. 

(h) One end of the ovule exists as a small opening, uncovered by integuments, that is called the micropyle. 

(i) Enclosed in the integuments, is a mass of cells with abundant food reserves, called nucellus. Located in the nucleus is the embryo sac or female gametophyte.



10. Megasporogenesis 

(i) In the micropylar end of the nucellus is present the sporogenous tissue.

(ii) Cells of sporogenous tissue are potential megaspore mother cells. A Megaspore Mother Cell divides meiotically to form four megaspores. Three of the megaspores degenerate and one remains functional which develops into an embryo sac or the female gametophyte. 

(iii) This method of female gametophyte/embryo sac formation from a single megaspore is termed monosporic

11. Process of Formation of Embryo Sac/ Female Development. Gametophyte The nucleus of the functional megaspore undergoes mitosis resulting in 2 nuclei that move to the opposite poles forming a 2-nucleate embryo sac. Two more mitotic nuclear divisions result in the 4-nucleate and later 8-nucleate stages of the embryo sac. No cell wall formation takes place.

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Only after 8 nuclei have formed, do the cell walls start to build, leading to the organisation of female gametophytes or embryo sacs.

Six of the 8 nuclei are bound by the cell wall and the remaining 2-celled polar nuclei lie free below the egg apparatus in the large central cell.

12. Characteristic Distribution of Cells in the Embryo Sac Three cells are grouped together at the micropylar end and constitute the egg apparatus.

(i) The egg apparatus consists of two more syn- gids and one egg cell. 

(ii) The special cellular of the polar end is called The filiform apparatus which guides the pollen tube into synergid. 

(iii) Three cells at the chalazal end are called A diagrammatic representation of the antipodals. The large central cell has two polar nuclei. 

Thus, a typical angiosperm embryo sac at maturity is 7-celled but 8-nucleated as the central cell has 2 nuclei.

13. Pollination is the mechanism of transfer of pollen grains from the anther to the stigma. Agents that bring about pollination are abiotic (water and air) or biotic (animals). Pollination is of different types:

(i) Autogamy e.g., Viola, Oxalis and Commelina.) 

(a) Pollen goes to the stigma of the same flower.

(b) Requires synchrony between pollen release and stigma receptivity and proximity of stigma and anthers.

Chasmogamous flowers are those that have significant similarities to flowers of other species and have exposed anthers and stigma.

Cleistogamous flowers are those that remain closed, never open to expose their anthers and – stigma. 

Their anthers and stigma lie close to each other and undergo autogamous pollination. Oxalis, Commelina and Viola produce both the above types of flowers.

(ii) Geitonogamy (e.g., Cucurbits)

(a) Transfer of pollen grains from anther to the stigma of another flower of the same plant. 

(b) It is functionally a cross-pollination, involving pollinating agents but genetically similar to autogamy (pollination within the same flower) since pollen grains come from the same plant. 

(iii) Xenogamy (eg., Data palm, papaya, maize, etc.) It is the transfer of pollen grains from anther to the stigma of a different plant. This is the only pollination that brings genetically different pollen grains to the stigma.

14. Agents of Pollination 

(i) Abiotic (wind and water)

(a) Pollination by wind 

Pollen grains are light and non-sticky. 

They often possess well-exposed stamens and large often feathery stigmas to easily trap air-borne pollen grains. 

Often have a single ovary in each ovary and numerous flowers packed into an inflorescence, e.g.Corn cob, grasses.

Pollination by water

Common in lower plant groups such as algae, bryophytes and pteridophytes, e.g., Vallisneria and Hydrilla.

Pollen grains are long, ribbon-like, carried passively inside water, and protected from wetting by mucilaginous covering.

(ii) Biotic (Animals)

Insects, bees, butterflies, birds, ants and wasps. 

Adaptation

Flowers are large, colourful, fragrant and rich in nectar. The inflorescence is clustered to make animals conspicuous, eg, Yucca, Amorphophallus.

15. Pollen-Pistil Interaction This interaction involves a series of events to determine the compatibility/non-compatibility of pollen and to ensure that only the correct pollen of the same species is recognised by the pistil. If incompatible pollen lands on the pistil, it is inhibited and if correct, then the pollen tube germinates.

16. Fertilisation Two fusion events, i.e., syngamy and triple fusion, occur in each embryo sac. This phenomenon is called double fertilisation. Double fertilisation ensures that the endosperm (the nutritive tissue) develops only after the successful formation of the zygote.

(i) Syngamy One gamete and an egg cell fuse to form a diploid zygote. The vacuole and plasmodesmata connections of the egg degenerate and it becomes ready to produce the embryo.

(ii) Triple fusion The nucleus of the second gamete fuses with two haploid polar nuclei and a triploid primary endosperm nucleus results. 

After triple fusion, the central cell becomes the primary endosperm cell, while the endosperm is developing, it undergoes repeated nuclear divisions to form free nuclei. This stage is called free nuclear endosperm, e.g., the water from the tender coconut. Later, cell walls develop around free nuclei and the endosperm becomes cellular.

Endosperm tissue provides nutrition to the developing embryo. 

17. Embryo Development and Maturation Embryo develops from the zygote at the micropylar end of the embryo sac, only after the endosperm has developed fully.

This is to ensure a proper nutrient supply for growing embryos. 

(i) Zygote makes a pro-embryo that matures into a globular heart-shaped embryo.

(ii) A dicot embryo has an embryonal axis and two cotyledons (hence the name dicots).

(iii) The portion of the embryonal axis above the cotyledons is epicotyl which terminates with plumule. 

Stages of development of a dicot embryo from the fertilized egg cell zygote as 1

(iv) The portion of the embryonal axis below the cotyledons hypocotyl which terminates in the radicle.

(v) Monocot embryos possess only one cotyledon. In the grass family, it is called scutellum.

(vi) At its lower end the embryonal axis has a radical and root cap enclosed in coleorhiza (undifferentiated covering). 

(vii) Portion of the embryonal axis above the scutellum is epicotyl which has a shoot apex and a few leaf primordials enclosed in a coleoptile (hollow foliar structure).

18. Seed It is the fertilised ovule formed inside the fruit. It consists of a seed coat (hardened ovule integuments), cotyledons and an embryonal axis.

Two types of mature seeds are 

(i) Non-albuminous in which consumed, e.g., In peas and groundnut. endosperm is completely

(ii) Albuminous that retains part of endosperm, e.g., Wheat, maize and barley.

In some seeds, remnants of nucellus are persistent which is called perisperm, e.g., Black pepper and beetroot. The Micropyle of the ovule remains as the small pore in the seed coat. It facilitates the entry of oxygen and water into the seed during germination.

19. Dormancy In this condition, water content reduces, the seed becomes dry, metabolic activities of the embryo slow down and the seed may enter into a state of inactivity called dormancy. If conditions are not favourable it will not germinate and it may germinate if conditions are favourable. 

20. Ovary develops into fruit and ovule matures into seeds. The wall of the ovary becomes a wall of the fruit, i.e., the Pericarp. 

(i) In actual fruits, the ovary contributes to the fruit formation. 

(ii) Fruits may be fleshy such as guava, orange, or mango may be or may be dry, i.e., Groundnut, mustard, etc. 

(iii) In false fruits, the thalamus (swollen end of stems that bears floral parts) also contributes to the fruit formation.

(iv) Parthenocarpic fruits develop without fertilisation, so they are seedless, e.g., Bananas. 

(v) Seedless fruits can be reproduced artificially through the application of growth hormones.

21. Advantages of Seed Seed-bearing plants make pollen grains that drift with air currents and reach the eggs, ensuring reproductive success.

(i) Seed formed after fertilisation dries

and so, can be stored for longer periods without rotting. 

(ii) It has reserve food to help the germination of seedlings till it achieves nutritional independence. 

(iii) Seeds are formed through sexual reproduction, so they carry genetic variation that may help in adaptations in the resulting plant, to the changing environmental conditions.

(iv) Seeds help in agriculture.

(v) Seeds help in the propagation of desired traits, once artificially introduced in plants.

(vi) Apomixis refers to the phenomenon of the formation of seeds without fertilisation, e.g., Asteraceae and grasses. 

(vii) Polyembryony is the phenomenon of the formation of more than one embryo in the seed, e.g., Citrus and mango.


sexual reproduction in flowering plants class 12 notes


Question 1. Name the parts of an angiosperm flower in which the development of male and female gametophyte take place. 

In an angiosperm flower, male gametophyte development occurs in the pollen sac of the anther and female gametophyte in the nucellus of the ovule.

Question 2. Differentiate between microsporogenesis and megasporogenesis. Which type of cell division occurs during these events? Name the structures formed at the end of these two events. 

Answer In both processes, the type of cell division is meiosis.

S.N.MicrosporogenesisMegasporogenesis
1It is the process of formation of haploid microspores (pollen grains) from diploid pollen mother cell through meiosis.It is the process of formation of haploid megaspores from diploid megaspore mother cell through meiosis.
2The mother cell first forms a tetrad (four cells), which dissociate from each other to form four pollen grains. The mother cell forms four cells. Out of the tetrad, one megaspore is functional and develops into an embryo sac, while the other three degenerates. 
3The structure formed at the end of megasporogenesis is the embryo sac (female gametophyte).3. The structure formed at the end of microsporogenesis is pollen grain (male gametophyte),

Question 3. Arrange the following terms in the correct developmental sequence Pollen grain, sporogenous tissue, microspore tetrad, pollen mother cell, and male gametes.

Answer The correct developmental sequence is as follows:

(I) Sporogenous tissue 

(ii) Pollen mother cell

(iii) Microspore tetrad 

(iv) Pollen grains

(v) Male gametes

Question 4. With a neat, labelled diagram, describe the parts of a typical angiosperm ovule.

Answer An ovule consists of a female gametophyte (embryo sac), nutrient-rich tissue (nucellus) and a jacket of cell layers (integuments). On fertilisation, the ovule ripens into a seed and the integuments make the seed coat.

Description

An ovule has the following parts

(i) Attachment points

Funicle is the stalk that attaches the ovule to the placenta. Hilum is the point, where the funicle is attached to the ovule body. Raphe is a ridge-like structure formed, where the funicle fuses with the body of the ovule.

(ii) Integuments Integuments are one or two layers, having a cuticle on the outside.

Colton will to dut nollersle Basal part of the ovule from where integuments originate is called chalaza. It lies opposite of micropyle. via ho A pore-like structure on one side of the ovule, where integuments are absent is called the micropyle

(iii) Nucellus 

A mass of cells that lies enclosed within the integuments is called the nucellus. Cells of nucellus have abundant reserves of food material. 

(iv) Embryo sac/ female gametophyte

A female gametophyte covered by a thin membrane is called an embryo sac. It is located in the nucellus. An ovule generally has a single embryo sac formed from the megaspore through reduction division

(meiosis).

Question 5. What is meant by the monosporic development of female gametophytes?

Answer The method of embryo sac formation from a single megaspore is called monosporic development. Out of the megaspore tetrad formed at the end of megasporogenesis, only one megaspore is functional while the other three degenerate. swank sings on ever tins bebop eve

Question 6. With a neat diagram explain the 7-celled, 8-nucleate nature of the female gametophyte.

Answer

Explanation Nucleus of the functional megaspore undergoes mitosis resulting in 2-nuclei that move to two opposite poles forming a 2-nucleate embryo sac.

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Two more mitotic nuclear divisions result in the 4-nucleate and later 8-nucleate stages of the embryo sac. So, far no cytokinesis (cytoplasmic division) has taken place.

Now cell walls start to build leading to the organisation of gametophyte or embryo sac.

Six of the 8 nuclei are bound by the cell wall and the remaining 2-celled polar nuclei lie below the egg apparatus in the large central cell.

Seven-celled stage

Three cells are grouped together at the micropylar end and constitute the egg apparatus, which is constituted of two synergids and one egg cell. Three cells at the chalazal end are called antipodals. The large central cell

has 2-polar nuclei. Thus, a typical angiosperm embryo sac at maturity is 7-celled and 8-nucleated as the central cell has 2 nuclei.

Question 7. What are chasmogamous flowers? Can cross-pollination occur in cleistogamous flowers? Give reasons for your answer. 

Answer Chasmogamous flowers resemble flowers of other species and have exposed anthers and stigma. Cross-pollination cannot occur in cleistogamous flowers as they don’t open at all. So, their anthers and stigma remain inaccessible. However, they do produce seeds through self-pollination.

Question 8. Mention Two strategies evolved to prevent self-pollination in flowers

Answer Two strategies evolved to prevent self-pollination are:

(1) Pollen release and stigma receptivity is not synchronized. 

(ii) Anthers and stigma are placed at such positions that pollen doesn’t reach the stigma.

Question 9. What is self-incompatibility? Why does self-pollination not lead to seed formation in self-incompatible species? 

Answer Self-incompatibility is a genetic mechanism to prevent self-pollen from fertilizing the ovules by inhibiting pollen germination or pollen tube growth in the pistil.

In these cases, self-pollination does not lead to seed formation because fertilization is inhibited.

Question 10. What is the bagging technique? How is it useful in a plant breeding programme?

Answer Bagging is a technique of covering the female reproductive parts of flowers to prevent stigma from contamination with undesired pollen. This is useful in artificial hybridisation when desired pollen grains are required for breeding the plants. Following steps are carried out

(i) Anthers are removed with forceps, from the flower bud before dehiscence (in case of bisexual flower).

(ii) The flower is then covered with a bag of suitable size. 

(iii) Mature pollen from the desired male plant is dusted on the receptive stigma.

(iv) Flowers are re-bagged again and fruits are allowed to develop.

Question 11. What is triple fusion? Where and how does it take place? Name the nuclei involved in triple fusion. 

Answer Triple fusion refers to the process of fusion of three haploid nuclei. It takes place in the embryo sac. The 3 nuclei that fuse together are the nucleus of the male gamete and the 2-polar nuclei of the central cell to produce a triploid primary endosperm nucleus.

Question 12. Why do you think the zygote is dormant for some time in a fertilised ovule?

Answer The zygote is dormant in fertilized ovules for some time because, at this time, the endosperm needs to develop. As endosperm is the source of nutrition for the developing embryo, nature ensures the formation of enough endosperm tissue before starting the process of embryogenesis,

Question 13. Differentiate between

(a) Hypocotyl and epicotyl 

(b) Coleoptile and coleorhiza

(c) Integument and testa

(d) Perisperm and pericarp

Answer

(a) Hypocotyl is the cylindrical portion below the level of cotyledons terminating at its lower end in the radical or root tip. Epicotyl is the portion of the embryonal axis above the level of cotyledons. 

(b) Coleoptile is a hollow foliar sheath enclosing the shoot apex. Coleorhiza is an undifferentiated sheath enclosing the radical and root cap. 

(c) Integument is the outer covering of ovules. Testa is the outer protective covering of the seed coat.

(d) Perisperm is residual, persistent nucellus. Pericarp refers to the wall of fruit developed from the wall of the ovary.

Question 14. Why is an apple called a false fruit? Which part(s) of the flower forms the fruit?

Answer Apple is called a false fruit because it develops from the thalamus instead of the ovary (the thalamus is the enlarged structure at the base of the flower).

Question 15. What is meant by emasculation? When and why does a plant breeder employs this technique?

Answer Emasculation means the removal of anthers, with forceps, from the flower bud before dehiscence.

Plant breeder employs this technique to prevent contamination of stigma with undesired pollen. This is useful in artificial hybridisation, where desired pollen is required.

Question 16. If one can induce parthenocarpy through the application of growth substances, which fruits would you select to induce parthenocarpy and why?

Answer Oranges, lemons, and litchis could be potential fruits for inducing the parthenocarpy because a seedless variety of these fruits would be much appreciated by the consumers.

Question 17. Explain the role of tapetum in the formation of pollen grain walls.

Answer Tapetum is the innermost layer of the microsporangium. It produces the exine layer of the pollen grains, which is composed of sporopollenin, the most resistant fatty substance. 

During microsporogenesis, the cells of tapetum produce various enzymes, hormones, amino acids and other nutritious materials required for the development of pollen grains. 

Question 18. What is apomixis and what is its importance?

Answer Apomixis is the process of asexual production of seeds, without fertilisation.

The plants that grow from these seeds are identical to the mother plant.

Uses

(1) It is a cost-effective method for producing seeds. 

(ii) It has great use for plant breeding when specific traits of a plant have to be preserved.


ncert solutions for class 12 biology chapter 2


Question 1. Name the component cells of the ‘egg apparatus’ in an embryo sac.

Answer Two synergids, one egg cell and the filiform apparatus.

Question 2. Name the part of the gynoecium that determines the compatible nature of pollen grain. 

Answer The part of gynoecium that determines the compatible nature of pollen grain is the pistil. 

Question 3. Name the common function that cotyledons and nucellus perform.

Answer The function of nucellus (parenchymatous cells in ovules) is to provide nourishment to the sporogenous cells.

Question 4. Indicate the stages where meiosis and mitosis occur (1, 2 or 3) in the flow chart.

Megaspore mother cell Megaspores

Question 5. How is pollination carried out in water plants?

Answer Pollination in water plants occurs by

(i) Water currents, e.g., Vallisneria. 

(ii) Wind, e.g.. Water hyacinth and water lily.

(iii) Insects, e.g., Yucca. The last two means are used in plants that come out of water pollen.

Question 6. What is the function of the two male gametes produced by each pollen grain in angiosperms?

Answer Function of two gametes

(1) One of the male gametes fuses with the nucleus of the egg cell (syngamy) to produce a diploid zygote that develops into an embryo.

(ii) The other male gamete fuses with two polar nuclei in the central cell to produce a triploid primary endosperm nucleus, that develops into nutritive tissue (triple fusion).


sexual reproduction in flowering plants neet questions


Question 1. The generative cell of two-celled pollen divides in the pollen tube but not in three-celled pollen. Give reasons.

Answer In plants, where pollen grains are shed at the two-celled stage, the generative cell divides to form the two male gametes inside the pollen tube. 

But, in plants that shed pollen at the three-celled stage, pollen tubes carry the male gametes from the beginning so there is no need for any division.

Question 2. What are the possible types of pollinations in chasmogamous flowers? Give reasons.

Answer The types of pollinations possible in chasmogamous flowers are 

(i) Geitonogamy is the transfer of pollen grains from anther to the stigma of another flower of the same plant.

It is functionally cross-pollination involving pollinating agents but genetically similar to autogamy (pollination within the same flower), since pollen grains come from the same plant. 

(ii) Xenogamy is the transfer of pollen grains from another to the stigma of a different plant. This is the only pollination that brings genetically different pollen grains to the stigma.

Question 3. With a neat, labelled diagram, describe the parts of a natural angiosperm embryo sac. Mention the role of synergids. 

Answer Synergids have special cellular thickenings at the micropylar tip Called filiform apparatus which play an important role in guiding pollen tubes into synergies.

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Question 4. Draw the diagram of a microsporangium and label its wall layers. Write briefly on the role of the endothecium. 

Answer

Enlarged view of one microsporangium showing wall layers Endothecium protects and helps in dehiscence of anther to release the pollen.

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