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

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Chapter 2 - Sexual Reproduction in Flowering Plants Exercise 40

Solution 1

In an angiosperm flower, the development of the male gametophyte (microgametogenesis) occurs in the pollen sac of the anther. The development of the female gametophyte (megagametogenesis) takes place in the nucellus of the ovule.  

Solution 2

Differences between microsporogenesis and megasporogenesis:


Character

Microsporogenesis

Megasporogenesis

Process

It is the formation of haploid microspores from the diploid microspore mother cell.

It is the formation of haploid megaspores from the diploid megaspore mother cell.

Number of spore mother cells involved

A large number of microspore mother cells are involved.

Only a single megaspore mother cell is involved.

Arrangement of spores

The arrangement of microspores in a tetrad is generally tetrahedral.

The arrangement of megaspores in a tetrad is commonly linear.

Fate of spores

The microspore leads to the formation of the male gametophyte.

The megaspore leads to the formation of the female gametophyte.

Functionality of spores

All the four microspores of a spore tetrad are functional.

Only one megaspore of a spore tetrad is functional.

Site of occurrence

It occurs inside the microsporangium.

It occurs inside the megasporangium.

 

Type of cell division: Meiosis or meiotic cell division occurs during microsporogenesis and megasporogenesis.

Microsporogenesis: Each diploid microspore mother cell divides meiotically (sporic meiosis) to produce four haploid pollen grains.

Megasporogenesis: Each diploid megaspore mother cell divides meiotically (sporic meiosis) to produce four haploid megaspores.


Solution 3

Sporogenous tissue Ncert Solutions Cbse Class 12-science Biology Chapter - Sexual Reproduction In Flowering Plants Pollen mother cell Ncert Solutions Cbse Class 12-science Biology Chapter - Sexual Reproduction In Flowering Plants Microspore tetrad Ncert Solutions Cbse Class 12-science Biology Chapter - Sexual Reproduction In Flowering Plants Pollen grain Ncert Solutions Cbse Class 12-science Biology Chapter - Sexual Reproduction In Flowering Plants Male gametes

Solution 4

The ovule is an integumented megasporangium which encloses an embryo sac. The anatropous ovule is the most common type of ovule found in angiosperms.

Ncert Solutions Cbse Class 12-science Biology Chapter - Sexual Reproduction In Flowering Plants

Structure of the anatropous ovule:

  • Funicle: It is a short, multicellular structure which attaches the main body of the ovule to the placenta.
  • Hilum: The point of attachment of the funicle with the main body of the ovule is called the hilum.
  • Nucellus: The main body of the ovule is composed of thin-walled parenchymatous cells and is known as the nucellus. It forms the nourishing tissue for the developing embryo.
  • Integuments: The nucellus is surrounded by one or two cellular coats called integuments. They provide protection to the inner tissues.
  • Chalaza: The basal part of the nucellus is swollen and is known as the chalaza.
  • Micropyle: There is a small opening called the micropyle at the apex of the integuments. The pollen tube enters the embryo sac through the micropyle.
  • Embryo sac: An oval sac called the embryo sac is found embedded in the nucellus situated towards the micropylar end. It contains the egg apparatus and develops inside the nucellus.

Solution 5

In majority of the angiosperms, out of the four megaspores in a linear tetrad, three degenerate and only one remains functional. The functional uninucleated megaspore undergoes three mitotic divisions to form an 8-nucleated, 7-celled embryo sac. This development of the female gametophyte or the embryo sac from a single uninucleated functional megaspore is called monosporic development.

Of the eight nuclei formed in the embryo sac, four are situated at the micropylar end and four are situated at the chalazal end. One nucleus from each end shifts inwards to form two polar nuclei in the centre of the embryo sac. 

Solution 6

The female gametophyte in angiosperms is called the embryo sac. It is a small oval structure which contains a 3-celled egg apparatus, 3 antipodal cells and 1 binucleate central cell. Hence, the embryo sac is called the 7-celled and 8-nucleated structure. The cells within the embryo sac are characteristically arranged.

  • Egg apparatus: Three cells are grouped together at the micropylar end and constitute the egg apparatus. The egg apparatus in turn consists of two synergids and one egg cell. The egg is connected to synergids through plasmodesmata. The synergids are special cellular thickenings at the micropylar tip called the filiform apparatus which play an important role in guiding the pollen tubes into the synergids.
  • Antipodal cells: Three other cells of various shapes and sizes are present at the chalazal end and are called antipodals. They are the vegetative cells of the embryo sac.
  • Central cell: The larger central cell initially has two polar nuclei which fuse just before fertilisation to form a secondary nucleus or definitive nucleus (2n). 

 

Ncert Solutions Cbse Class 12-science Biology Chapter - Sexual Reproduction In Flowering Plants


Chapter 2 - Sexual Reproduction in Flowering Plants Exercise 41

Solution 7

Chasmogamous flowers are open flowers with exposed anthers and stigma which facilitate cross-pollination. Example: Oxalis

Cross-pollination cannot occur in cleistogamous flowers as such flowers are closed and do not open at all. Therefore, their anthers and stigma are never exposed. Thus, the transfer of pollen grains from outside to the stigma of the flower is not possible.

Solution 8

Self-pollination causes a decline in the vigour and vitality of the race. Hence, flowering plants have developed many devices to avoid self-pollination and to promote cross-pollination.

  • Dichogamy:It involves the maturation of the anthers and stigmas of a flower at different times. It is of two types:
    • Protandry: Anthers mature before the stigma. Example: Sunflower
    • Protogyny: Stigmas mature before the anthers. Example: Plantago
  • Self-incompatibility/self-sterility: The pollen grains of a flower are incapable of completing growth on the stigma of the same flower because of the presence of similar self-sterile genes. Example: Potato

Unisexuality or dicliny, prepotency, heterostyly and herkogamy are few strategies evolved by flowering plants to prevent self-pollination.


Solution 9

The inability of certain functional male and female gametes, even from genetically similar plant species, to fuse with each other and produce viable seeds is called self-incompatibility. It is a genetic mechanism to prevent self-pollination resulting in highly homozygous individuals with low survival value.

Self-pollination does not lead to seed formation in self-incompatible species because of the interaction of the chemical substances produced by the male gametophyte and the tissue of the style which obstruct fertilisation (pollen–pistil interaction). The chemicals inhibit the germination of pollen grains and the further growth of the pollen tube so as to avoid inbreeding and to help outbreeding.

Solution 10

During artificial hybridisation, the anthers of bisexual flowers are removed in the bud condition. These emasculated flowers are then covered with butter paper or polythene to prevent contamination of their stigmas from foreign unwanted pollen. This process is called bagging. When the stigmas of the bagged flowers attain receptivity, mature pollen grains are collected from the anther of the desired male parents and dusted on the stigmas. The flowers are then rebagged and fruits are allowed to develop. 

Advantages of the bagging technique:

  • It prevents contamination of the stigma of a flower with unwanted pollen.
  • It prevents damage caused by animals during grazing.
  • It promotes cross-breeding.
  • It is useful to obtain commercially superior varieties of plants.

Solution 11

During double fertilisation in an angiospermic flower, one of the male gametes fuses with two polar nuclei located in the central cell and forms the triploid primary endosperm nucleus (PEN) As this involves the fusion of three haploid nuclei, it is termed triple fusion.

This process occurs in the central cell of the embryo sac of an ovule. The pollen tube containing two male gametes enters the ovule through the micropyle. Of the two gametes, one joins with the egg cell resulting in the production of zygote and the other with the secondary nucleus (formed by the fusion of two polar nuclei) producing a triploid primary endosperm nucleus. This is called triple fusion. In triple fusion, two polar nuclei and one male gamete are involved.

Solution 12

The development of the zygote does not start before the development of the primary endosperm nucleus into the endosperm. The endosperm provides nourishment to the embryo developing from the zygote. Hence, the zygote is dormant for sometime in a fertilised ovule. The development of the embryo from the zygote begins only when a certain amount of endosperm is formed. This is an adaptation in flowering plants to provide assured nutrition to the developing embryo.


Solution 13 (a)

Hypocotyl

Epicotyl

  • The portion of the embryonal axis below the level of cotyledons is called the hypocotyl.
  • The portion of the embryonal axis above the level of cotyledons is called epicotyl.
  • It terminates at its lower end in the radicle or root tip covered with a root cap.
  • It terminates with the plumule.
  • In epigeal germination, the hypocotyl elongates so that the cotyledons come out of the soil.
  • In hypogeal germination, the epicotyl elongates so that the cotyledons remain in the soil.
Solution 13 (b)

Differences between coleoptile and coleorhiza: 

Coleoptile 

Coleorhiza 

 It is a conical protective sheath over the epicotyl bearing the shoot apex and leaf primordia.

 It is a protective sheath over the radicle and root cap.

 It has a terminal pore for the emergence of the first leaf.

 It is a solid structure.

 It comes out of the soil.

 It remains inside the soil.

 It protects the plumule during its emergence from the soil.

 It does not protect the radicle during its passage into the soil.

 It grows much beyond the grain.

 It stops growing after its emergence from the grain.

 It emerges from the soil, turns green in colour and carries out photosynthesis.

 It remains inside the soil and is non-green.

 


Solution 13 (c)

Differences between integument and testa: 

Integument

Testa 

 It is the protective covering of the ovule.

 It is the protective covering of the seed.

 It is a thin covering.

 It is a thick covering.

 The cells are living.

 The cells are dead.

 Sclereids are absent in the cells.

 Sclereids are present in abundance in the cells.

 It arises from the chalazal end of the ovule.

 It is derived from the outer integument of the ovule.

 It is a pre-fertilisation structure.

 It is a post-fertilisation structure.

 

Solution 13 (d)

Differences between perisperm and pericarp:

Perisperm 

Pericarp 

 It represents residual persistent nucellus.

 It represents the fruit wall derived from the ovary wall.

 It is part of the seed.

 It is part of the fruit.

 It is generally dry.

 It can be dry or fleshy.

 It is often non-functional for the seed.

 It acts like a protective covering, helps in dispersal and providing nutrition.

 It is present only in few seeds.

 It is found in all fruits.

 


Solution 14

When the fruit is derived from the ovary along with other accessory floral parts, it is called a false fruit or pseudocarpic fruit. This is true for apple where the thalamus grows to form the fleshy edible part of the fruit along with the ovary.

In apple and cashew, false fruit formation involves the thalamus of the flower, while true fruits develop only from the ovary.

Solution 15

Emasculation is the process of removal of anthers from the flower bud before the anther dehisces from bisexual flowers without affecting the female reproductive structures.

Plant breeders employ this technique for artificial hybridisation for crop improvement programmes to prevent self-pollination and promote cross-pollination. To remove anthers, the flowers are covered with a bag before they open to avoid self-pollination. This ensures that the flower is pollinated by pollen grains obtained from desirable varieties only. The matured, viable and stored grains are dusted on the bagged stigma to allow artificial pollination to occur and to obtain the desired plant variety.


Solution 16

Parthenocarpic fruits are seedless. They develop from the ovary without fertilisation. Fleshy fruits such as watermelon, orange, banana, grapes and lemon should be selected to induce parthenocarpy because the seeds are scattered within the edible parts in these fruits. These seeds act as irritants while consuming these fruits. Hence, if the seeds of these fruits are removed, they become even more valuable.

Solution 17

The tapetum is the innermost wall layer of the microsporangium.

Role of the tapetum in the formation of the pollen grain wall:

  • It provides nutrition to the developing pollen grains and helps in the formation of the pollen grain wall.
  • It secretes enzymes, hormones and special proteins for the pollen grains to recognise compatibility.
  • It produces Ubisch granules which provide sporopollenin and other materials for the formation of the exine of pollen grain.
  • The tapetum secretes pollenkitt over the outer side of mature pollen.
  • The tapetal cells provide nourishment to the young microspore mother cells.

Solution 18

Apomixis is a form of asexual reproduction which mimics sexual reproduction by the production of seeds directly from the diploid cells of the nucellus or integuments without fertilisation. It is common in grasses and species of the Asteraceae family.

Importance of apomixis:

  • It is of commercial value in horticulture and agriculture, particularly the hybrid seed industry. It can be used for the cultivation of hybrid seeds.
  • It prevents the loss of specific characters in the hybrid because the embryos produced apomictically are genetically similar to their parents.
  • Embryos formed through apomixis are generally free from infection.
  • It is a cost-effective method. The cost of production of hybrid seeds can be minimised if apomixis is introduced in hybrid seeds as apomixis is genetically controlled.
  • It maintains hybrid vigour in crop plants.
  • It results in the formation of more than one embryo in a seed (polyembryony).
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