Wednesday, February 16, 2011

Long and Glorious Life Cycles of Angiosperms

Reproduction in angiosperms takes place within the flower. Following pollination and fertilization, the seeds develop inside protective structures.
The angiosperm life cycle begins when the mature sporophyte produces flowers. Each flower contains anthers and an ovary. Inside the anthers, the male part of the flower, each cell undergoes meiosis and produces four haploid spore cells. Each of these cells becomes a  single pollen grain. The wall of each pollen grain thickens, protecting protecting the contents of the pollen grain from dryness and physical damage when it is released from the anther.
The nucleus of each pollen grain undergoes one mitotic division to produce two haploid nuclei. The pollen grain, which is the entire male gametophyte, usually tops growing until it is released from the anther and deposited on a stigma.
The ovary of the flower contains the ovules in which the female gametophyte develops. A single diploid cell goes through meiosis to produce eight nuclei. These eight nuclei and the surrounding membrane are called the embryo sac. The embryo sac, contained within the ovule, is the female gametophyte of a flowering plant. One of the eight nuclei, near the base of the gametophyte, is the egg nucleus, the female gamete. If fertilization takes place, this cell will become the zygote that grows into a new sporophyte plant.

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