A mature seed typically consists of a mature plant ovum containing a minute, partially developed young plant, the embryo, surrounded by an abundant supply of food and enclosed by a protective seed coat. Seed plants are divided into two main groups: the gymnosperms, primarily conebearing plants such as pine, spruce, and fir trees, and the angiosperms, the flowering plants. The gymnosperms have naked ovules which, at the time of pollination, are exposed directly to the pollen grains. Their food supply in the seed is composed of a female gametophyte, rather than the endosperm found in angiosperms.
In angiosperms, seeds develop from ovules that are enclosed in a protective ovary. The ovary is the basal portion of the carpel, typically a vaseshaped structure located at the center of a flower. The top of the carpel, the stigma, is sticky, and when a pollen grain lands upon it, the grain is firmly held. The germinating pollen grain produces a pollen tube that grows down through the stigma and style into the ovary and pierces the ovule.
Two male sperm nuclei are released from the pollen grain and travel down the pollen tube into the ovule. One of the sperm nuclei fuses with an egg cell inside the ovule. This fertilized egg divides many times and develops into the embryo. The second male nucleus unites with other parts of the ovule and develops into the endosperm, a starchy or fatty tissue that is used by the embryo as a source of food during germination. Angiosperm seeds remain protected at maturity. While the seed develops, the enclosing ovary also develops into a hard shell, called a seed coat or testa, often enclosed in a fibrous or fleshy fruit.
Structure
Although the characteristics of different plant seeds vary greatly, some structural features are common to all seeds.
Continue of the article: Structure
Size and Chemistry
The range of seed size is extreme more than nine orders of magnitude.
Continue of the article: Size and Chemistry
Dispersal
A seed can be regarded as a vessel in which lies a partially developed young plant in a condition of arrested growth, waiting for the correct conditions for growth to resume.
Continue of the article: Dispersal
Dormancy
Seeds are adapted for conditions other than geographic dispersal.
Continue of the article: Dormancy
|
