Some plants do not produce seeds that have functional complete embryos, or the seed may have no embryo at all, often called empty seeds. Predators and pathogens can damage or kill the seed while it is still in the fruit or after it is dispersed. Environmental conditions like flooding or heat can kill the seed before or during germination. The age of the seed affects its health and germination ability: since the seed has a living embryo, over time cells die and cannot be replaced. Some seeds can live for a long time before germination, while others can only survive for a short period after dispersal before they die.
Seed vigor is a measure of the quality of seed, and involves the viability of the seed, the germination percentage, germination rate and the strength of the seedlings produced. The germination percentage is simply the proportion of seeds that germinate from all seeds subject to the right conditions for growth. The germination rate is the length of time it takes for the seeds to germinate.
Germination percentages and rates are affected by seed viability, dormancy and environmental effects that impact on the seed and seedling. In agriculture and horticulture quality seeds have high viability, measured by germination percentage plus the rate of germination. There is great variation amongst plants and a dormant seed is still a viable seed even though the germination rate might be very low. Environmental conditions affecting seed germination include; water, oxygen, temperature and light. Three distinct phases of seed germination occur: water imbibition; lag phase; and radicle emergence.
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. The rate of imbibition is dependent on the permeability of the seed coat, amount of water in the environment and the area of contact the seed has to the source of water.
For some seeds, imbibing too much water too quickly can kill the seed. For some seeds, once water is imbibed the germination process cannot be stopped, and drying then becomes fatal. Other seeds can imbibe and lose water a few times without causing ill effects, but drying can cause secondary dormancy. During seed dormancy , often associated with unpredictable and stressful environments, DNA damage accumulates as the seeds age.
A number of different strategies are used by gardeners and horticulturists to break seed dormancy. Scarification allows water and gases to penetrate into the seed; it includes methods to physically break the hard seed coats or soften them by chemicals, such as soaking in hot water or poking holes in the seed with a pin or rubbing them on sandpaper or cracking with a press or hammer. Sometimes fruits are harvested while the seeds are still immature and the seed coat is not fully developed and sown right away before the seed coat become impermeable.
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Under natural conditions, seed coats are worn down by rodents chewing on the seed, the seeds rubbing against rocks seeds are moved by the wind or water currents , by undergoing freezing and thawing of surface water, or passing through an animal's digestive tract. In the latter case, the seed coat protects the seed from digestion , while often weakening the seed coat such that the embryo is ready to sprout when it is deposited, along with a bit of fecal matter that acts as fertilizer, far from the parent plant.
Microorganisms are often effective in breaking down hard seed coats and are sometimes used by people as a treatment; the seeds are stored in a moist warm sandy medium for several months under nonsterile conditions. Stratification , also called moist-chilling, breaks down physiological dormancy, and involves the addition of moisture to the seeds so they absorb water, and they are then subjected to a period of moist chilling to after-ripen the embryo.
Sowing in late summer and fall and allowing to overwinter under cool conditions is an effective way to stratify seeds; some seeds respond more favorably to periods of oscillating temperatures which are a part of the natural environment. Leaching or the soaking in water removes chemical inhibitors in some seeds that prevent germination. Rain and melting snow naturally accomplish this task. Soaking longer, especially in stagnant water, can result in oxygen starvation and seed death.
Seeds with hard seed coats can be soaked in hot water to break open the impermeable cell layers that prevent water intake. Other methods used to assist in the germination of seeds that have dormancy include prechilling, predrying, daily alternation of temperature, light exposure, potassium nitrate, the use of plant growth regulators, such as gibberellins, cytokinins, ethylene, thiourea, sodium hypochlorite, and others. For some seeds, fire cracks hard seed coats, while in others, chemical dormancy is broken in reaction to the presence of smoke.
Liquid smoke is often used by gardeners to assist in the germination of these species. Seeds may be sterile for few reasons: they may have been irradiated, unpollinated, cells lived past expectancy, or bred for the purpose. The issue of the origin of seed plants remains unsolved. However, more and more data tends to place this origin in the middle Devonian. The description in of the proto-seed Runcaria heinzelinii in the Givetian of Belgium is an indication of that ancient origin of seed-plants.
As with modern ferns, most land plants before this time reproduced by sending into the air spores that would land and become whole new plants. Taxonomists have described early "true" seeds from the upper Devonian, which probably became the theater of their true first evolutionary radiation. With this radiation came an evolution of seed size , shape, dispersal and eventually the radiation of gymnosperms and angiosperms and monocotyledons and dicotyledons.
Seed plants progressively became one of the major elements of nearly all ecosystems. DowDuPont and Monsanto account for 72 percent of corn and soybean seed sales in the U. Many seeds are edible and the majority of human calories comes from seeds,  especially from cereals , legumes and nuts.
Seeds also provide most cooking oils , many beverages and spices and some important food additives. In different seeds the seed embryo or the endosperm dominates and provides most of the nutrients. The storage proteins of the embryo and endosperm differ in their amino acid content and physical properties. For example, the gluten of wheat, important in providing the elastic property to bread dough is strictly an endosperm protein.
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Seeds are used to propagate many crops such as cereals, legumes, forest trees , turfgrasses , and pasture grasses. Particularly in developing countries, a major constraint faced is the inadequacy of the marketing channels to get the seed to poor farmers. Seeds are also eaten by animals seed predation , and are also fed to livestock or provided as birdseed.
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While some seeds are edible, others are harmful, poisonous or deadly. In some cases, these compounds simply taste bad such as in mustard , but other compounds are toxic or break down into toxic compounds within the digestive system. Children, being smaller than adults, are more susceptible to poisoning by plants and seeds. A deadly poison, ricin , comes from seeds of the castor bean. Reported lethal doses are anywhere from two to eight seeds,   though only a few deaths have been reported when castor beans have been ingested by animals.
The seeds of many legumes, including the common bean Phaseolus vulgaris , contain proteins called lectins which can cause gastric distress if the beans are eaten without cooking.
The common bean and many others, including the soybean , also contain trypsin inhibitors which interfere with the action of the digestive enzyme trypsin. Normal cooking processes degrade lectins and trypsin inhibitors to harmless forms. Please see the category plant toxins for further relevant articles.
Cotton fiber grows attached to cotton plant seeds.
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Other seed fibers are from kapok and milkweed. Many important nonfood oils are extracted from seeds. Linseed oil is used in paints. Oil from jojoba and crambe are similar to whale oil. Seeds are the source of some medicines including castor oil , tea tree oil and the quack cancer drug Laetrile. Many seeds have been used as beads in necklaces and rosaries including Job's tears , Chinaberry , rosary pea , and castor bean. However, the latter three are also poisonous.
The Book of Genesis in the Old Testament begins with an explanation of how all plant forms began:. And God said, Let the earth bring forth grass, the herb yielding seed, and the fruit tree yielding fruit after his kind, whose seed is in itself, upon the earth: and it was so. And the earth brought forth grass, and herb yielding seed after its kind, and the tree yielding fruit, whose seed was in itself, after its kind: and God saw that it was good.
And the evening and the morning were the third day. It is Allah Who causeth the seed-grain and the date-stone to split and sprout. He causeth the living to issue from the dead, and He is the one to cause the dead to issue from the living.