seed, fertilized and ripened ovule, consisting of the plant embryo, varying amounts of stored food material, and a protective outer seed coat. Seeds are frequently confused with the fruit enclosing them in flowering plants as in the grains and nuts. The seed-bearing plants are the highest in the evolutionary scale; in lower plants (e.g., mosses and ferns) the spore is the agent of propagation. True seeds vary in size from the dustlike seeds of some orchids to the large seed contained in the coconut. The period of dormancy undergone by many seeds before germination also varies; the mangrove seed may sprout inside a fruit still hanging on the tree, while a seed of a sacred lotus dated at about 1,200 years and one of a date palm about 2,000 years old have been germinated. Long dormancy in some seeds is ensured by their extremely hard coats, which have to be scratched or split to force sprouting. In plant breeding, the source of pollen for fertilization is carefully controlled to produce the desired qualities in seed; under natural conditions a plant grown from seed may be quite different genetically from its maternal plant (see fertilization).

psyllium seed: see plantain.

or spermatophyte

Any of the flowering plants (angiosperms) and the conifers and related plants (gymnosperms). Seed plants share many features with ferns, including the presence of vascular tissue (see xylem and phloem), but unlike ferns, they have stems that branch sideways and vascular tissue that is arranged in strands (bundles) around the core. Seed plants have generally more complex plant bodies and reproduce via seeds. As the main dispersal unit of seed plants, the seed represents a significant improvement over the spore, with its limited capacity for survival. Seed plants also differ from ferns in having gametophytes that are reduced in size and are embedded in the sporophytes (and thus are less vulnerable to environmental stress). Another land-based adaptation of seed plants is pollen dispersed by wind or animals. The dispersal of pollen, in addition to dispersal of seeds, promotes genetic recombination and distribution of the species over a wide geographic area.

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Seed leaf within the embryo of a seed that provides energy and nutrients for the developing seedling. After the first true leaves have formed, they wither and fall off. Flowering plants whose embryos have a single cotyledon are grouped as monocots, or monocotyledonous plants; embryos with two cotyledons are grouped as dicots, or dicotyledonous plants. Unlike flowering plants, gymnosperms usually have several cotyledons rather than one or two.

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Reproductive structure in plants that consists of a plant embryo, usually accompanied by a supply of food (endosperm, which is produced during fertilization) and enclosed in a protective coat. Seed embryos contain one or more cotyledons. In typical flowering plants, seed production follows pollination and fertilization. As seeds mature, the ovary that enclosed the ovules develops into a fruit containing the seeds. Most seeds are small, weighing less than a gram; the smallest contain no food reserve. At the opposite extreme, the seed of the double coconut palm may weigh up to about 60 lb (27 kg). Seeds are highly adapted to transportation by animals, wind, and water. When circumstances are favorable, water and oxygen penetrate the seed coat, and the new plant begins to grow (see germination). The longevity of seeds varies widely: some remain viable for only about a week; others have been known to germinate after hundreds or even thousands of years.

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