Chapter 2, Sexual Reproduction in Flowering Plants - CBSE Class 12 Biology - NCERT Solutions

CBSE Board - Class 12 Biology - NCERT Solutions

Chapter 2, Sexual Reproduction in Flowering Plants

Solutions of NCERT Biology chapter exercise questions

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

Solution: In an angiosperm flower, the male gametophyte is represented by pollen grains. Pollen grains develop inside the microsporangia. In angiosperms, female gametophyte or embryosac is formed from a megaspore through reduction division. It lies inside the nucellus of 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.

Solution:

Microsporogenesis	Megasporogenesis
1. It occurs inside the microsporangia of anther. 2.  Many microspore mother cells are differentiated in a microsporangium. 3. All the four cells of the microspore tetrads continue to grow.	1. It occurs in the nucellus of the ovule / megasporangium. 2. Only one megaspore mother cell is differentiated in a megasporangium. 3. Only one of the four cells in the megaspore tetrad continues to grow.

Meiosis (reductional cell division) occurs during sporogenesis. The structures formed at the end are microspores and megaspores respectively.

Question 3: Arrange the following terms in a correct developmental sequence:

Pollen grain, sporogenous tissue, microspore tetrad, pollen mother cell, male gametes.

Solution: The correct developmental sequence for the above are as follows:

Sporogenous tissue, microspore tetrad, pollen mother cell, pollen grain, male gametes.

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

Solution:

Figure: Longitudinal section of an anatropous ovule with the two integuments and micropyle

Question 5: What is meant by monosporic development of female gametophyte?

Solution: During megasporogenesis, one megaspore mother cell (MMC) divides mitotically to produce four megaspores. In majority of angiosperms, only one megaspore is functional (situated towards chalazal end) and develops into female gametophyte while other three degenerate. This is called as "monosporic development of female gametophyte", e.g. polygonum.

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

Solution:

Fig: A fully developed Embryo Sac

1. Female gametophyte develops from functional megaspore.

2. In angiosperm, the female gametophyte is also called "embryo sac".

3. In majority of angiosperms embryo sac develops from single functional, chalazal megaspore and hence, is described as monosporic type.

4. A typical embryo sac is 8-nucleate and 7-celled.

5. It was first identified by Straussburger in polygonum, which is 7-celled, 8-nucleated structure.

6. Embryo sac consist of 3 parts namely,

(A). Egg Apparatus

(B). Central Cell

(C). Antipodals

(A) Egg Apparatus

1. A group of three cells present at micropylar end is called "Egg Apparatus".

2. It consist of 1 egg cell and 2 synergids.

3. Egg Cell is larger, present at the middle and contains basal nucleus and upper vacuole.

4. Synergids are present on either side of the egg cell, which are smaller and contain special finger-like cellular thickenings called "Filiform Apparatus". It helps in directing the pollen tube towards synergids due to secretion of chemicals.

(B) Central Cell

1. It is the largest cell of embryo sac.

2. It is dikaryotic with two haploid polar nuclei.

3. At the time of fertilisation, two polar nuclei unites and form diploid "secondary nuclei".

(C) Antipodals

1. A group of 3-cells present at chalazal end is called "Antipodals".

2. Antipodals are smallest cell of embryo sac.

3. They are ephemeral (short lived) and degenerates before or after fertilisation.

4. They are also called "vegetative cells" of embryo sac.

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

Solution: Chasmogamous flowers are those bisexual flowers, which open at maturity and expose the stamens and stigma. These may be cross-pollinated or self-pollinated. Cleistogamous flowers are bisexual and do not open at all even at maturity to ensure complete self-pollination. In cleistogamous flowers there is no scope of cross-pollination because of the following reasons:

(i) flowers do not open at all, (ii) anthers and stigma lie close to each other for self-pollination to occur.

So, they are autogamous and cannot be cross-pollinated.

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

Solution: Majority of flowering plants produce bisexual flowers, which favours self-pollination. Continuous self-pollination results in "inbreeding depression". To avoid self-pollination, flowering plants developed many devices to promote cross-pollination. Two of these are as follows:

1. Dichogamy: Maturation of androecium and gynoecium in bisexual flowers occurs at different intervals is called "Dichogamy".

It prevents self-pollution because flower is structurally bisexual but functionally unisexual. It is of two types:

(a) Protandry: Maturation of androecium earlier than gynoecium in bisexual flower is called "Protandry" e.g. China Rose, Sunflower.

(b) Protogymy: Maturation of gynoecium earlier than androecium in bisexual flower is called "Protogymy” e.g. Colchicum.

2. Herkogamy: In some bisexual flowers, anthers and stigma are present at different heights (China Rose, Sunflower) or in different directions (Gloriosa) is called "Herkogamy".

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

Solution: Self-incompatibility is a genetic mechanism that prevents the self-pollen from fertilising the ovules by inhibiting pollen germination or pollen tube growth on the pistil. It occurs due to prevention of some physiological or morphological mechanisms. Self-incompatibility is a natural barrier to prevent self-pollination. It means even if the pollen grains come in contact with stigma of the same flower or flowers of same plant, they are unable to grow and produce thier own clones without any genetic variations. It includes several complex mechanisms related with interactions of pollen and stigmatic tissues. It can be sporophytic or gametophytic incompatibility. It occurs due to prevention of pollen germination, retardation of growth, deorientation of pollen tube. It may take place due to failure of nuclear fusion. Due to which there is no seed formation in self-incompatible species.

Question 10: What is bagging technique? How is it useful in a plant-breeding programme?

Solution: Covering the emasculated flower in a female parent with a butter paper of suitable size, to prevent contamination of stigma of the bagged flower with unwanted pollen grains is called "bagging". Only female flowers are bagged in the case of unisexual flower.

Bagging technique is very useful for plant breeding programmes as it prevents the contamination of stigma with unwanted pollen.

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