Fertilisation in Plants

After pollination, a pollen grain on the stigma absorbs water and germinates. A pollen tube grows out through a pore in the pollen wall and down through the style towards the ovary. The tube secretes digestive (hydrolase) enzymes that break down the tissue of the style as it grows. It grows towards chemicals released by the ovule, a response called chemotropism.The tube nucleus stays near the tip and controls growth of the tube. The generative nucleus divides by mitosis to form two male gamete nuclei, which follow behind it.The pollen tube enters the ovule through a small gap in the integuments called the micropyle. The tube nucleus degenerates, the tip of the tube bursts, and the two male nuclei are released into the embryo sac.Double fertilisation then takes place, because both male nuclei fuse with nuclei in the embryo sac:
• one male nucleus fuses with the egg cell nucleus to form a diploid zygote, which develops into the embryo.
• the other male nucleus fuses with the two polar nuclei to form a triploid (3n) endosperm nucleus, which develops into the endosperm, a food store for the growing embryo.After fertilisation, the ovule develops into the seed and the ovary develops into the fruit.