Chapter 2 Sexual Reproduction In Flowering Plants
- Flowers are the seat of sexual reproduction in angiosperms. In the flower, androecium consisting of stamens represents the male reproductive organs and gynoecium consisting of pistils represents the female reproductive organs.
- Pollen grains represents the male gametophytic generation. The pollen grains have a two-layered wall, the outer exine and the inner intine. The exine is made up of sporopollenin and has germ pores.
- The pistil has three parts- the stigma, style and the ovary. Ovules are present in the ovary. The ovules have a stalk called funicle, protective integument(s), and an opening called micropyle. The central tissue is the nucleus in which the archesporium differentiates.
- Pollination is the mechanism to transfer pollen grains from the anther to the stigma. Pollinating agents help in pollination. Pollinating agents are either abiotic (wind and water) or biotic (animals).
- Pollen-pistil interaction involves all events from the landing of pollen grains on the stigma until the pollen tube enters the embryo sac (when the pollen is compatible) or pollen inhibition (when the pollen is incompatible). Following compatible pollination, pollen grain germinates on the stigma and resulting pollen tube grows through the style, enters the ovules and finally discharges two male gametes in one of the synergids. Angiosperms exhibit double fertilization because two fusion events occur in each embryo sac, namely syngamy and triple fusion. The products of these fusions are the diploid zygote and the triploid primary endosperm nucleus. Zygote develops into the embryo and the primary endosperm cell forms the endosperm tissue. Formation of endosperm always precedes development of the embryo.
- The developing embryo passes through different stages as the proembryo, globular and heart-shaped stages before maturation. After fertilization, ovary develops into fruit and ovules develop into seeds.