Sexual Reproduction in Flowering Plants
Flowers are the site of sexual reproduction in flowering plants. In a flower, androecium consists of stamens and gynoecium consists of pistils.
A typical anther is bilobed, dithecous and tetrasporangiate. Inside the microsporangia, pollen grains develop.
In angiosperm, the male gametophytic generation is represented by pollen grain. The pollen grain consists of two layers: the outer exine and inner intine.
Pollen grains contain either two cells that is a vegetative cell and generative cell or three cells that is vegetative cell and two male gametes.
The pistil consists of three parts – the stigma, style and the ovary. The ovules are present in the ovary. The ovule is made up of a stalk called funicle, protective integument, and an opening called micropyle.
The Pollination is the process of transfer of pollen grains from the anther to the stigma. The agents that help in the process of pollination are called pollinating agents. The pollinating agents can be: abiotic (wind and water) or biotic (animals).
After compatible pollination, pollen grain germinates on the stigma. The pollen tube grows through the style, enters the ovules and releases two male gametes.
In angiosperms, double fertilization and triple fusion occurs.
The products of double fertilization and triple fusion are the diploid zygote and the triploid primary endosperm nucleus. After fertilization the zygote develops into the embryo and the primary endosperm cell forms the endosperm tissue. The mature dicotyledonous embryo consists of two cotyledons while embryos of monocotyledonous plant has single cotyledon. After fertilisation, ovary develops into fruit and ovules develop into seeds.
The process of formation of seeds without fertilization is called apomixes. It is found in some angiosperms, particularly in grasses.
Some angiosperms produce more than one embryo in their seed. This phenomenon is called polyembryony.