Explain the changes involved in the formation of seed from Ovule.
A. Process of double fertilization :
1) Fertilization is the process of fusion of male and female gametes. For the fusion pollen grains have to reach the surface of the stigma. This is called pollination.
2) Pollen grain received by the stigma, germinate and give rise to pollen tubes. The pollen tube has two male nuclei.
3) Usually the pollen tube enters the ovule through microphyle and discharges the two male gametes into the embryo sac.
4) One male nucleus (garnet) approaches the egg and fuses with it to form diploid (2n) zygote this is called first fertilization.
5) The other male nucleus reaches the secondary nucleus (2n) (polar nucleus) and fuses with it to form endosperm nucleus which will be triploid. This is second fertilization. Thus double fertilization occurs in embryosac.
Changes after double fertilization :
6) After double fertilization, the ovule increases in size rapidly as a result of formation of endosperm tissue by mitosis and the development of new embryo.
7) The embryo consists of cotyledons an epicotyl and a hypocotyl. The cotyledons become greatly enlarged because of stored food for the seedling.
8) The zygote divides several times to form an embryo within the ovule. The ovule develops a tough coat and is converted into a seed. The ovary grows to form a fruit.