The fusion of two dissimilar sexual reproductive units such as male and female gametes is called fertilization. This process was discovered by Strasburg in 1884. It begins with pollination and completed with fusion of male gamete nuclei derived from the pollen grain or microspore of the anther and sac present in the ovule.
Double fertilization and triple fusion In angiosperms, fertilization occurs in the embryo sac. So, the pollen grains received by the stigma have to germinate, produce pollen tube and carry the male gamete
One of the two male gamete nuclei of the pollen tube fuses with the egg of embryo sac forming a diploid zygote. The process is called syngamy or true fertilization. The other male gamete fuses with the definite nucleus or secondary nucleus (2n) and results in the formation of a triploid primary endosperm nucleus. As there are two separate fusions by the two male gametes, the process is called double fertilization.
As soon as fertilization is over, the cells other than the fusion products of embryo sac gets disintegrated. Antipodal cells disappear even before fertilization, since they have no function in the process.
The time involved between pollination and fertilization varies among different plants. Generally, it is between few hours to a few days, but in some cases, it may take even many months. It depends on the rate of growth of the pollen tube.
Although many pollen grains are deposited on the stigma and some produce germ tubes, only one becomes effective to reach the ovule prior to others. Other pollen tubes dry up and become non functional.
Post-fertilization changes formation of seeds and fruits
The withering and shedding of corolla usually indicate that fertilization has been completed. As a result of first fertilization between one of the two male gametes and egg cell, oospores are formed, which later develops into embryo.
The fusion product of the other male gamete with definite nucleus (resulted from triple fusion) is the first cell nucleus of the endosperm During post- fertilization period the embryo and the endosperm are seen to develop simultaneously. The ovule; containing the embryo is transformed into the seed while the ovary becomes a fruit.