The mature, fertilized ovule of a plant, containing the embryo and a food store to sustain the seedling during germination, enclosed within a protective coat, the testa. In gymnosperms the seeds lie exposed on the cone scales; in flowering plants they are protected within the ovary. Some seeds are very large and are produced in small numbers (eg the coconut); others are very small and are produced in prodigious numbers (eg the orchid). Seeds, aided by the fruit, offer a means of dispersal, and in annuals a means of surviving harsh seasons. Some seeds germinate immediately after dispersal, but others remain dormant until exposed to specific conditions, such as low temperatures or high light intensities.
The importance of the seed relative to more primitive forms of reproduction and dispersal is attested to by the success of these two groups of plants in dominating the landscape.Seed structure
A seed contains the embryo from which a new plant will grow under proper conditions. Seeds also usually contain a supply of stored food and is wrapped in the seed coat or testa. The dust-like orchid seeds are the smallest with about one million seeds per gram. Orchid seeds have immature embryos and no significant food reserves.
The embryo has one cotyledon or seed leaf in monocotyledons, two cotyledons in almost all dicotyledons and two or more in gymnosperms. In others, the endosperm is absorbed by the embryo as the latter grows within the developing seed, and the cotyledons of the embryo become filled with this stored food. At maturity, seeds of these species have no endosperm and are termed exalbuminous seeds. Seeds with an endosperm at maturity are termed albuminous seeds.
The seed coat develops from the tissue, the integument, originally surrounding the ovule. The seed coat in the mature seed can be a paper-thin layer (e.g. The seed coat helps protect the embryo from mechanical injury and from drying out.
The seeds of angiosperms are contained in a hard or fleshy (or with layers of both) structure called a fruit. Gymnosperm seeds begin their development "naked" on the bracts of cones, although the seeds do become covered by the cone scales as they develop.
Some seeds have an appendage on the seed coat such an aril (as in yew and nutmeg) or an elaiosome (as in Corydalis) or hairs (as in cotton). The hilum is the scar on the seed coat where the seed was attached to the ovary wall by the funiculus.
In order for the seed coat to split, the embryo must imbibe (soak up water), which causes it to swell, splitting the seed coat. However, the nature of the seed coat determines how rapidly water can penetrate and subsequently initiate germination. For seeds with a very thick coat, scarification of the seed coat may be necessary before water can reach the embryo. In the latter case, the seed coat protects the seed from digestion, while perhaps weakening the seed coat such that the embryo is ready to sprout when it gets deposited (along with a bit of fertilizer) far from the parent plant. In species with thin seed coats, light may be able to penetrate into the dormant embryo. The presence of light or the absence of light may trigger the germination process, inhibiting germination in some seeds buried too deeply or in others not buried in the soil.
Seed functions
Seeds protect and nourish the embryo or baby plant. Seeds usually give a seedling a faster start than a sporling from a spore gets because of the larger food reserves in the seed. As a consequence, plants have evolved many ways to disperse their population through their seeds (see also vegetative reproduction). Seed dispersal is often attributed mainly to fruits, however many seeds aid in their own dispersal, for example:
Many seeds (e.g. Seeds (nuts) which are an attractive long-term storable food resource for animals (e.g. the seeds are stored some distance from the parent plant, and some escape being eaten if the animal stores more than it needs, forgets where it has stored the seed, or dies. Elaiosomes provide food for ants, which usually disperse such seeds. Some plants, such as Mucuna and Dioclea, produce buoyant seeds termed sea-beans or drift seeds because they float in rivers to the oceans and wash up on beaches .For annuals, seeds are a way for the species to survive dry or cold seasons. Ephemeral plants are usually annuals that can go from seed to seed in as few as six weeks.
One important function of most seeds is delaying germination to allow time for dispersal and to prevent all seeds from germinating at once when conditions are favorable. Seed dormancy is defined as a seed failing to germinate under environmental conditions optimal for germination. It is often confused with seed quiescence, which is a seed failing to germinate because environmental conditions are inappropriate for germination. Many cultivated seeds lack dormancy but do not germinate in seed packets simply because there is insufficient moisture.
Economic importance
Many seeds are edible. Seeds also provide most cooking oils, many beverages and spices and some important food additives.
Seeds are used to propagate many crops such as cereals, legumes, forest trees, turfgrasses and pasture grasses.
Some seeds are also poisonous.
The world's most important clothing fiber grows attached to cotton seed.
Many important nonfood oils are extracted from seeds.
Seeds are the source of some medicines including castor oil, tea tree oil and the discredited cancer drug, Laetrile.
Many seeds have been used as beads in necklaces and rosaries including Job's tears, Chinaberry and rosary pea.
Other seed uses include:
Seeds once used as weights for balances.Oldest viable seeds
The oldest seed datable by carbon 14 that has been germinated into a viable plant was a date palm seed about 2000 years old, recovered from excavations at Herod the Great's palace on Masada in Israel; If the 2,000 year old Judean date palm seed longevity claim is confirmed by a report in a refereed scientific journal, then the second oldest viable seed would be the 1,300 year old sacred lotus (Nelumbo nucifera) from China There is a persistent myth that seeds from Egyptian tombs with ages of over 3000 years were viable .Seed packets and seed information
Generally, seed packets labels includes:
Common plant name and the botanical name (in parentheses). Space and deep: how deep to place the seeds in the soil, space between plants (from one row to the other one and from one plant to the other one in the same row).
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