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

In an angiosperm, the male gametes are developed within the anther. On the contrary, the female gametes are developed inside the ovules.

The important differences between microsporogenesis and megasporogenesis are mentioned below:

Meiotic cell division occurs during megasporogenesis and microsporogenesis. It is also known as reductional division that leads to the production of haploid gametes.
The structures formed at the end of these events are:
Microsporogenesis – Pollen grain
Megasporogenesis – Embryo sac

The following is the correct developmental sequence: Sporogenous tissue → pollen mother cell → microspore tetrad → pollen grain → male gamete. When the microsporangium is developing, every cell of the sporogenous tissue serves as a pollen mother cell, giving rise to a microspore tetrad possessing four haploid microspores through the meiosis process (microsporogenesis). When the anthers mature, these microspores dissociate and develop into pollen grains. The pollen grains mature and give rise to male gametes.

Monosporic development is the development of a female gametophyte from one functional megaspore. In angiosperms, a single diploid mother megaspore undergoes meiotic division to form four megaspores (haploid). Only one out of the four megaspores is functional, which forms a female gametophyte, while the rest of the three degenerate.

5. The female gametophyte is formed by the mitotic division of the mother megaspore. The megaspore divides mitotically thrice to form 8 nucleate embryo sacs. The process of formation of the 7-celled, 8-nucleate nature of female gametophyte is mentioned below:

4. Two nuclei are formed after the cell undergoes the first mitotic division.

5. These two nuclei move towards the micropylar end and the chalazal end, respectively.

6. They divide and redivide to form an 8-nucleate stage.

7. Consequently, there are four nuclei each on either end.

8. At the micropylar end, three out of the four nuclei differentiate into an egg cell and synergids.

9. At the chalazal end, three out of the four nuclei differentiate as antipodal cells.

10. The remaining cells, each from either end move towards the centre and is known as polar nuclei.

11. Therefore, on maturation, the female gametophyte looks like a 7-celled structure.

Chasmogamous flowers are flowers that have open petals such that the reproductive organs are exposed to allow cross-pollination. Cleistogamous flowers are small, closed flowers with unexposed reproductive organs. Therefore, they cannot undergo cross-pollination. However, they undergo self-pollination since the stigma and anther are present near each other.

There are two strategies involved in preventing self-pollination in flowers:

i) Dichogamy – It refers to the production of male and female reproductive organs at different times in order to prevent self-fertilization.

ii) Self-incompatibility – It is a genetically controlled mechanism in which pollen grains of a flower are unable to grow completely on the stigma of the same flower.

Self-incompatibility refers to a genetically controlled mechanism that prevents self-pollination and promotes cross-pollination in flowers. Self-pollination cannot lead to the formation of seeds in a self-incompatible species. This happens because the pollens are unable to fertilize the ovules that would develop into an embryo and hence form seeds.

The bagging technique helps prevent fertilization of the stigma by any undesired pollen by covering the emasculated flower (flower whose anther is removed) with a polybag or butter paper. The bagging technique is beneficial in the plant breeding programme. In this, only desired pollens are made to fertilize the stigma in order to produce plants with desired characteristics.

Triple fusion occurs when a male gamete fuses with two polar nuclei within the embryo sac of flowering plants. The following events take place in triple fusion:

i) The pollen grains get dusted on the stigma and germinate, giving rise to a pollen tube that enters the ovule.

ii) The pollen tube passes into one of the synergids and releases two male gametes.

iii) One out of the two gametes fuses with the egg nucleus and forms a zygote.

iv) The other gamete fuses with the two polar nuclei located in the central cell and forms a triploid endosperm nucleus.

v) The nuclei involved in triple fusion are:

1. A male gamete nucleus

2. Two polar nuclei

The zygote remains inactive until the endosperm is formed as a result of triple fusion. The endosperm provides nutrition to the developing embryo and is formed from the primary endosperm cell that results from triple fusion.

i) Epicotyl and hypocotyl

ii) Coleoptile and coleorrhiza

iii) Integument and testa

iv) Perisperm and pericarp

A false fruit is derived from some secondary parts and not from the ovary. Apple is derived from the thalamus and is hence called a false fruit.

Emasculation refers to the removal of stamens from bisexual flowers before the maturation of the anther in order to avoid self-pollination in the flowers. This technique is employed when the breeder wants plants of desired characteristics. The flowers are bagged even before the anther matures. When the anther matures, the pollen grains are shed on the covered stigma and are allowed to pollinate with the flowers of the desired characteristics.

Parthenocarpy refers to the development of fruits without fertilization. Fruits devoid of seeds, such as watermelon and muskmelon, are in great demand. Therefore, these varieties will be developed by parthenocarpy.

Tapetum is the internal layer of microsporangium and plays an important role in the formation of pollen grain walls. It provides nutrition to the maturing pollen grains. Various amino acids, enzymes, and hormones are produced by tapetum cells that are essential for the maturation of pollen grains. Tapetum also forms the exine layer of pollen grains.

Apomixis is the process of seed formation without the occurrence of fertilization (the process of meiosis and syngamy, to be precise). It plays a vital role in the production of hybrid seeds. Apomixis inhibits the loss of desired characters in a hybrid. Also, the process of production of hybrid seeds through cultivation is expensive. Therefore, apomixis is used for the production of hybrid seeds.