One of these fuses with the ovum to form a zygote while the other fuses with two further cells of the embryo sac to form a triploid nutritive tissue called endosperm. This is called double fertilization and is unique to plants. The zygote divides to give rise to two cells. Embryonic differentiation starts but does not proceed for long. When the development ceases, the embryo becomes packaged in a seed, specialized for dispersal.
Most plants reproduce by both self and cross-pollination.
One feature that developed as a result of insect pollination is pollen-tube competition . When a number of pollen grains is deposited on a stigma, each pollen grows a pollen tube to reach the ovule. Whichever reaches first, it fertilizes the ovule. The fastest growing pollen tube usually carries the best genes and results in a more vigorous offspring. See also Pollination Adaptations .