Class 12-science NCERT Solutions Biology Chapter 1 - Sexual Reproduction in Flowering Plants

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.  

Differences between microsporogenesis and megasporogenesis:

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.

Sporogenous tissue Pollen mother cell Microspore tetrad Pollen grain Male gametes

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

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.

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. 

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). 

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.

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.

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.

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.

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.

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.

Differences between coleoptile and coleorhiza: 

Differences between integument and testa: 

Differences between perisperm and pericarp:

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.

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.

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.

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.

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).