FERTILIZATION IN FLOWERING PLANTS
Immediately after pollination, fertilization follows. Fertilization is the union of male and female gametes to form a zygote. In a flower, the male gametes develop from the pollen grains while the female gametes develop from the ovules which are retained in the ovary.
Development of pollen grain
When a pollen grain develops from an anther, it contains only one nucleus. This nucleus later divides to form a generative nucleus and tube nucleus. At the time of pollination therefore, a pollen grain contains two nuclei. A pollen grain has an outer thick wall or exine, surrounding an inner thin wall or intine. Immediately after pollination, the stigma secretes a sugary substance which stimulates further development of the pollen grain.
The outer thick wall dissolves at a point of stimulus. From this point the inner thin wall protrudes and develops into a tube. The tube nucleus moves into the tube now known as pollen tube. It is believed that the tube nucleus directs the growth of the pollen tube through the tissues of the style towards the ovary.
Meanwhile the generative or male nucleus also moves into the pollen tube where it divides into two male gametes. The two male gametes move closely behind the tube nucleus. They will later be used for fertilization when they reach the ovary.
Development of ovule
The ovules are contained in the ovary attached to the placenta by a short funicle. An immature ovule contains nucleus and a cytoplasm sur- rounded by two protective coats known as integuments, leaving a tiny hole or micropyle through which the pollen tube enters the ovule.
At maturity the ovule develops by three nuclear divisions resulting in the production of eight nuclei in an embryo sac.
Four of the nuclei occur at the micropylar end and the other four occur at the opposite end known as chalaza. One nucleus from each end moves to the centre where they unite to form a polar or secondary nucleus. The remaining three at the chalaza end acquire a wall each to become the three antipodal cells.
The three at the micropylar end become arranged into an ovum or egg cell at the top and two guard cells below it. The ovule is now ready for fertilization.
The pollen tube growing through the tissues of the style gets to the ovary and enters the ovule through the micropylar end.
Process of fertilization in flowering plant
As soon as it makes contact with the wall of the embryo sac, the wall dissolves to create a free passage for the male gametes. The tube nucleus degenerates. One of the male gametes moves in and unites with the ovum to form a zygote. Thus fertilization has taken place. The second male gamete unites with the secondary or polar nucleus to form an endosperm nucleus. This is known as “double fertilization” characteristic of flowering plants alone.
Changes after fertilization of flower
Immediately after fertilization, the flower is deemed to have completed its function. The stamens, corolla and calyx wither off. . In some flowers like Crotalaria, tomato, garden egg and Tridax, the calyx is persistent, that is, they do not fall off but are retained in the fruit. In Tridax and related family, the calyx becomes modified into hairy pappus for dispersal by wind. The stigma and style wither to a point on the ovary where it is seen as “remains of style”. The endosperm nucleus develops into a food reserve or endosperm used for nourishing the developing embryo. The zygote develops into an embryo. The antipodal and guard cells degenerate. The ovule becomes the seed while the ovary enlarges and grows into a fruit. The integuments form a protective coat or testa over the seed, while the wall of the ovary becomes a pericarp.
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