SEED

reproductive structure in plants that consists of a miniature plant, or embryo, usually accompanied with a supply of food and enclosed in a protective coat. In typical flowering plants, seed production follows pollination and fertilization. Grains of pollen that fall on a flower's pistil germinate and form pollen tubes that grow downward to reach the ovules at the base of the pistil. There fertilization is accomplished as a sperm cell from a pollen tube unites with the egg cell of an ovule; the fertilized egg divides and gives rise to the plant embryo. In a process of double fertilization, another sperm cell unites with two nuclei in the ovule to produce a nutrient tissue called the endosperm. A protective coat grows around the embryo and endosperm, and the seed is formed. As the seeds mature, the ovary that enclosed the ovules develops into a fruit containing the seeds. In many plants, seed and fruit become fused into one entity and are dispersed together as a so-called diaspore; in other plants, the seeds are discrete units. Gymnosperms have ovules that are exposed rather than being enclosed in an ovary. This group of plants is characterized by a long time span between pollination and fertilization, and the ovules begin to develop into seeds long before fertilization takes place. In one species of pine, for example, the cones containing the ovules start to develop in the winter and are pollinated in the spring. The egg cells are not fertilized until the second growing season, and the mature seeds are not ready for release until the third year. Most seeds are small in size, weighing less than a gram. The smallest are those that contain no food reserve, belonging to orchids and other plants that obtain their nutrients from other organisms. At the opposite extreme is the seed of the double coconut palm, which may weigh up to 27 kilograms (about 60 pounds). Because seeds constitute the chief food of many animals, the survival of plant species is enhanced by the production of many small seeds rather than a few large ones. Both the size and the shape of seeds and diaspores are important to their mode of dispersal. The roundness or flatness of seeds helps determine whether they will roll or germinate where they land. Some species produce seeds that differ in lengths of dormancy and means of dispersal, providing an insurance against environmental disasters. Seeds are highly adapted to transportation by animals, wind, and water. Fleshy fruits and berries attract birds and animals by their inviting odours, colours, and tastes and contain seeds with a tough covering that resists digestion. Mammals ranging in size from mice to elephants feed on seeds and excrete undigested seeds in another spot. Furry mammals are the most frequent carriers of burrlike diaspores that cling to them with spines, hooks, barbs, or bristles. Many birds carry fruits, nuts, and seeds in their beaks. Some of these are eaten; others are dropped or cached and left behind. Wind is the major dispersal agent for plants with tiny seeds or with diaspores that have special structures to catch air currents. Some seeds travel in the wind by means of tufts, plumes, or hairs; others have wings for gliding or are inflated to resemble tiny balloons. With tumbleweeds, the whole plant breaks off and blows across the country, scattering seeds as it goes. Many beach, pond, and swamp plants have seeds enclosed in corky or air-containing fruits that float on water currents to a new site for germination. A number of plants are characterized by self-dispersal. Some of these use mechanisms to forcibly eject their seeds; others have creeping diaspores with appendages capable of coiling and flexing. Seeds require different amounts of moisture, light, and temperature before germination can take place. When the circumstances are favourable, water and oxygen penetrate the seed coat, and the new plant begins to grow. The longevity of seeds varies widely; some seeds remain viable for only about a week, while others have been known to germinate after hundreds or even thousands of years.