Chapter 1 — Sexual Reproduction in Flowering Plants

Chapter 1 of the Class 12 Biology NCERT textbook, "Sexual Reproduction in Flowering Plants", covers sexual reproduction in angiosperms, explaining how flowers serve as the site of reproduction through processes including pollination, double fertilisation, and seed and fruit formation.

• The flower as a reproductive machine — The chapter treats the flower as the seat of sexual reproduction, with the androecium (stamens) forming the male apparatus and the gynoecium (pistils) the female. Both build specialised gametophytes that house the gametes.
• Building the gametophytes — Inside anthers, meiosis (microsporogenesis) generates pollen grains, the male gametophyte; the ovule develops the embryo sac, the female gametophyte. This links spore formation to the eventual delivery of gametes.
• Pollination and its agents — Reaching the stigma requires transfer of pollen, which the chapter frames as pollination — mediated by abiotic agents like wind and water or biotic agents such as insects, birds and bats, each shaping floral adaptations.
• Double fertilisation and its outcomes — A hallmark of angiosperms is double fertilisation, producing both the zygote and the nutritive endosperm. Fertilisation then converts ovules into seeds and the ovary into fruit, tying the whole process to offspring dispersal.
• Reproduction without fertilisation — The chapter also broadens the idea of seed formation by introducing apomixis, where seeds arise without fertilisation, showing that sexual structures can be bypassed in some plant reproductive strategies.

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.