Chapter 1 — Sexual Reproduction in Flowering Plants

Chapter 1 of Class 12 Biology covers sexual reproduction in flowering plants (angiosperms), explaining how flowers serve as the site of reproduction through processes including pollination, double fertilisation, and seed and fruit formation.

Sexual reproduction in flowering plants involves the androecium (stamens) as the male reproductive organ and the gynoecium (pistils) as the female reproductive organ. Pollen grains develop inside microsporangia through microsporogenesis and carry two cells — a vegetative cell and a generative cell. The female gametophyte, or embryo sac, is 7-celled and 8-nucleate at maturity. Pollination transfers pollen from anther to stigma via abiotic agents (wind, water) or biotic agents (insects, birds, bats). Double fertilisation — unique to angiosperms — produces a diploid zygote and a triploid primary endosperm nucleus. Ovules mature into seeds and the ovary develops into fruit. Apomixis allows some plants to produce seeds without fertilisation.

1. 01A typical anther is bilobed, dithecous, and tetrasporangiate; pollen grains develop inside four microsporangia through meiosis (microsporogenesis).
2. 02Pollen grains have a tough outer exine made of sporopollenin and an inner intine of cellulose and pectin; they contain a vegetative cell and a generative cell.
3. 03The mature embryo sac (female gametophyte) is 7-celled and 8-nucleate: egg apparatus (egg + 2 synergids) at the micropylar end, 3 antipodals at the chalazal end, and 2 polar nuclei in the large central cell.
4. 04Double fertilisation — unique to angiosperms — involves syngamy (male gamete + egg = diploid zygote) and triple fusion (male gamete + 2 polar nuclei = triploid primary endosperm nucleus).
5. 05After fertilisation, the ovary develops into a fruit and ovules develop into seeds; endosperm development always precedes embryo development.
6. 06Apomixis is the formation of seeds without fertilisation, found in some grasses and Asteraceae; polyembryony refers to the occurrence of more than one embryo in a seed.