Seeds

Seeds develop from the fertilized ovules of female (pistillate) floral parts, following fertilization by pollen released from the male (staminate) floral parts. If ovules and pollen come from different individual plants, then the genetic makeup of the seed represents a mixture of the two parent plants, and sexual reproduction is said to have occurred.

A close-up of grass seed on grass. Photograph by Roy Morsch. Stock Market. Reproduced by permission.

In some plant species (known as monoecious plants), pollen from a plant may fertilize its own ovules, a phenomenon that is known as self-pollination. This can occur when flowers contain both pistillate and staminate organs (these are known as "perfect" flowers). Self-fertilization can also occur when the same flowers on the same plant are either male or female. Although self-pollination results in genetic mixing, the degree of mixing is much less than in true, sexual reproduction. If self-fertilization occurs frequently, the eventual result is a loss of genetic variation through inbreeding, which may have deleterious consequences on the evolutionary fitness of the plant.

Most plant species avoid self-pollination, and encourage cross-pollination among genetically different individuals of the species. One such adaptation involves individual plants that produce only male flowers or only female flowers (these are known as dioecious plants). In addition, many plant species have pollination systems that encourage out-crossing, such as pollination by the wind. Other plants are pollinated by insects or birds that carry the pollen to the receptive stigmatic surfaces of other plants of the same species. The benefit of outcrossing is to reap the evolutionary benefits of sexual reproduction by producing genetically diverse seeds.

A seed is more than just a fertilized ovule; it also contains the embryonic tissues of the adult plant, including a rudimentary root, shoot, and leaves. These structures are surrounded by tissues containing starch and/or oil that are intended to provide nourishment for germination and the early growth of the seedling. The walls of the ovule develop into a hard seed coat, intended to provide protection for the tender, internal tissues.

The above description gives an idea of the basic, anatomical structure of seeds. However, the actual proportion of the various tissues in the seed varies according to species. Orchids (family Orchidaceae), for example, have tiny, dust-like seeds that consist of little more than core embryonic tissues, with very little in the way of energy reserves. In contrast, the gigantic seeds of the Seychelles Islands coconut (Lodoicea maldivica) can weigh more than 11.5 lb (25 kg), most of which is nutritional reserve surrounded by fibrous, protective husk.

The seeds of many plant species are dispersed as individual units throughout the environment, while those of other species are encased as groups of seeds inside of fruits of various sorts. These fruits are usually intended for ingestion by animals, which then disperse the seeds widely (see below).