ICSE Class 9 Answered

Double fertilization is a characteristic feature of flowering plants. In flowering plants, the act of germination of pollen grains occurs soon after pollination.

• The pollen grain gets deposited on the stigma, which is the receptive surface of the pistil.  
• On the stigma, the germination of pollen grains begins by the absorption of water and nutrients. The stigma of the flower provides a solution of sugars to feed the pollen cell.
• A tiny pollen tube grows out of the pollen grain by breaking its outer wall called exine. The generative nucleus divides into two male gamete nuclei or sperm nuclei. At this stage, there are 3 nuclei in the pollen tube- 2 sperm nuclei and 1 tube nucleus.
• The pollen tube then grows through the stigma, passes down through the style by dissolving the tissues with the help of enzymes and reaches the ovary. There, it pushes through the micropyle and reaches the embryo sac. The tube nucleus which had directed the growth of pollen tube all the way down, now disintegrates.
• The fully developed embryo sac consists of 3 antipodal cells, 2 synergids, 1 egg cell and 1 secondary nucleus formed from two polar nuclei.
• Now, the pollen tube enters through one of the synergids and releases its two sperm nuclei. The tip of the pollen tube disintegrates and the 2 male gametes are liberated in the embryo sac. One or both synergids which help in directing the pollen tube towards the egg disintegrates.
• Of the two 2 sperm nuclei, one sperm nucleus enters the egg cell and fuses with its nucleus to form the zygote. The other sperm nucleus moves towards the two polar nuclei or secondary nucleus in the central cell and fuses with them to form the endosperm. This implies that fertilisation has occurred twice. This process is called double fertilisation.
• Union of one sperm nucleus with the egg cell nucleus is called syngamy while the union of the other sperm nucleus with the two polar nuclei is termed triple fusion.