Ovule

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Ovule, in botany, is the name applied to immature seeds, which are produced within the ovary of a flower. In flowering plants, the development of the ovule is generally as follows.

At the site of the future seed, an outgrowth, the nucellus or megasporangium, develops; this becomes covered by two integuments that grow up from its base, leaving an opening at the top called the micropyle. Within the nucellus is the megaspore mother cell. It divides into two and then into four; one of these megaspores then typically divides into eight nuclei to become the embryo sac of the female gametophyte. It is in this sac that the plant embryo will develop.

The young male plants, or male gametophytes, are popularly referred to as pollen grains; these are contained in modified leaves called stamens. When a pollen grain is placed on the stifma, it sends out a tube that grows down to the ovary and eventually enters the ovule. Two sperms are then discharged into the embryo sac; one of these fuses with the egg nucleus at the micropylar end of the embryo sac, and this fertilized egg then develops into the embryo of the seed. The other sperm fuses with two nuclei near the middle of the embryo sac; the resulting triple-fusion nucleus develops into the endosperm, which usually remains as the food storage tissue of the seed.

See Fertilization; Flower; Pollination.

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