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Director: Charles Wiedman. Writer: Charles Wiedman.
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New York Times. In very small seeds, the importance of shape can be judged only by taking into account soil clod size and microtopography of the soils onto which they are dropped. Sign In Don't have an account? Once this process of lifting was over, I remained in a state of simple love, understanding, and pure joy. In Germany an average jay may transport about 4, acorns per season, over distances of up to 4 km 2. Meet Our Educators. Return to Sunnydale, Part 2 Absent.
Alan Nelson Edit Storyline Buck Conner is a small town Texas corn farmer who still uses his own seed while every other farmer around him is using GMO seeds. Taglines: A small town Texas farmer is threatened by a multi-national GMO seed company when he's falsely accused of planting their corn seed without a license. Country: USA. The inflated indehiscent pods of Colutea arborea , a steppe plant, represent balloons capable of limited air travel before they hit the ground and become windblown tumbleweeds. Winged fruits are most common in trees and shrubs , such as maple , ash , elm , birch , alder , and dipterocarps a family of about species of Old World tropical trees.
The one-winged propeller type, as found in maple, is called a samara. When fruits have several wings on their sides, rotation may result, as in rhubarb and dock species. Sometimes accessory parts form the wings—for example, the bracts small green leaflike structures that grow just below flowers in Tilia linden. Seeds with a thin wing formed by the testa are likewise most common in trees and shrubs, particularly in climbers— jacaranda , trumpet vine , catalpa , yams , butter-and-eggs.
Most famous of these is the seed with a giant membranaceous wing 15 cm [6 inches] long of the Javan cucumber Alsomitra macrocarpa , a tropical climber. Many fruits form plumes, some derived from persisting and ultimately hairy styles, as in clematis , avens , and anemones ; some from the perianth, as in the sedge family Cyperaceae ; and some from the pappus, a calyx structure, as in dandelion and Jack-go-to-bed-at-noon Tragopogon.
Plumed seeds usually have tufts of light, silky hairs at one end rarely both ends of the seeds—e. In woolly fruits and seeds, the pericarp or the seed coat is covered with cottonlike hairs—e. In some cases, the hairs may serve double duty, in that they function in water dispersal as well as in wind dispersal. Poppies have a mechanism in which the wind has to swing the slender fruitstalk back and forth before the seeds are thrown out through pores near the top of the capsule.
Many marine, beach, pond, and swamp plants have waterborne seeds, which are buoyant by being enclosed in corky fruits or air-containing fruits or both; examples of these plants include water plantain , yellow flag, sea kale , sea rocket , sea beet, and all species of Rhizophoraceae, a family of mangrove plants. Sea dispersal of the coconut palm has been well proved; the fibrous mesocarp of the fruit, a giant drupe , provides buoyancy.
A sea rocket species with seeds highly resistant to seawater is gaining a foothold on volcanic Surtsey Island, south of Iceland. Purple loosestrife , monkey flower , Aster tripolium , and Juncus species rushes are often transported by water in the seedling stage. Rainwash down mountain slopes may be important in tropical forests.
Hygrochasy, the opening of fruits in moist weather, is displayed by species of Mesembryanthemum , Sedum , and other plants of dry environments. Best known in this category are the active ballists, which forcibly eject their seeds by means of various mechanisms. In the fruit of the dwarf mistletoe Arceuthobium of the western United States, a very high osmotic pressure pressure accumulated by movement of water across cell membranes principally in only one direction builds up that ultimately leads to a lateral blasting out of the seeds over distances of up to 15 metres 49 feet with an initial velocity of about 95 km 60 miles per hour.
Squirting cucumber Ecballium elaterium also employs an osmotic mechanism. In Scotch broom and gorse , however, drying out of the already dead tissues in the two valves of the seed pod causes a tendency to warp, which, on hot summer days, culminates in an explosive and audible separation of these valves, with violent seed release. Such methods may be coupled with secondary dispersal mechanisms, mediated by ants in the case of Scotch broom and gorse or by birds and mammals, to which sticky seeds may adhere, in the case of Arceuthobium and squirting cucumber.
Other active ballists are species of geranium , violet, wood sorrel , witch hazel , touch-me-not Impatiens , and acanthus ; probable champions are Bauhinia purpurea , with a distance of 15 metres, and the sandbox tree Hura crepitans , with 14 metres. Barochory, the dispersal of seeds and fruits by gravity alone, is demonstrated by the heavy fruits of horse chestnut.
Creeping diaspores are found in grasses such as Avena sterilis and Aegilops ovata , the grains of which are provided with bristles capable of hygroscopic movements coiling and flexing in response to changes in moisture. The mericarps fruit fragments of a schizocarp of storksbill Erodium species , when moistened, bury themselves with a corkscrew motion by unwinding a multiple-barbed, beak-shaped appendage, which, in the dry state, was coiled. This strategy is typical in old, nutrient-impoverished landscapes, such as those of southwestern Australia.
The aim is often achieved by synaptospermy, the sticking together of several diaspores, which makes them less mobile, as in beet and spinach , and by geocarpy. Geocarpy is defined as either the production of fruits underground, as in the arum lilies Stylochiton and Biarum , in which the flowers are already subterranean, or the active burying of fruits by the mother plant, as in the peanut , Arachis hypogaea.
In the American hog peanut Amphicarpa bracteata , pods of a special type are buried by the plant and are cached by squirrels later on. Kenilworth ivy Cymbalaria , which normally grows on stone or brick walls, stashes its fruits away in crevices after strikingly extending the flower stalks. Not surprisingly, geocarpy, like synaptospermy, is most often encountered in desert plants; however, it also occurs in violet species, in subterranean clover Trifolium subterraneum —even when it grows in France and England—and in begonias Begonia hypogaea of the African rainforest.
Dormancy has at least three functions: 1 immediate germination must be prevented even when circumstances are optimal so as to avoid exposure of the seedling to an unfavourable period e. Accordingly, the wide variation in seed and diaspore longevity can be appreciated only by linking it with the various dispersal mechanisms employed as well as with the climate and its seasonal changes. Thus, the downy seeds of willows , blown up and down rivers in early summer with a chance of quick establishment on newly exposed sandbars , have a life span of only one week. Tropical rainforest trees frequently have seeds of low life expectancy also.
Intermediate are seeds of sugarcane , tea , and coconut palm, among others, with life spans of up to a year. In general, viability is better retained in air of low moisture content. Some seeds, however, remain viable underwater—those of certain rush Juncus species and Sium cicutaefolium for at least 7 years. Salt water can be tolerated for years by the pebblelike but floating seeds of Guilandina bonduc , which in consequence possess an almost pantropical distribution.
In , seeds of the arctic tundra lupine Lupinus arcticus found in a frozen lemming burrow with animal remains established to be at least 10, years old germinated within 48 hours when returned to favourable conditions. The problem of differential seed viability has been approached experimentally by various workers, one of whom buried 20 species of common Michigan weed seeds, mixed with sand , in inverted open-mouthed bottles for periodic inspection.
After 80 years, 3 species still had viable seeds.
See also soil seed bank. In some plants, the seeds are able to germinate as soon as they have matured on the plant, as demonstrated by papaya and by wheat , peas , and beans in a very rainy season. Certain mangrove species normally form foot-long embryos on the trees; these later drop down into the mud or sea water. Such cases, however, are exceptional. The lack of dormancy in cultivated species, contrasting with the situation in most wild plants, is undoubtedly the result of conscious selection by humans.
In plants whose seeds ripen and are shed from the mother plant before the embryo has undergone much development beyond the fertilized egg stage orchids , broomrapes , ginkgo , ash , winter aconite , and buttercups , there is an understandable delay of several weeks or months, even under optimal conditions, before the seedling emerges. There are at least three ways in which a hard testa may be responsible for seed dormancy: it may 1 prevent expansion of the embryo mechanically, 2 block the entrance of water, or 3 impede gas exchange so that the embryos lack oxygen.
Resistance of the testa to water uptake is most widespread in the bean family, the seed coats of which, usually hard, smooth, or even glassy, may, in addition, possess a waxy covering. In some cases water entry is controlled by a small opening, the strophiolar cleft , which is provided with a corklike plug; only removal or loosening of the plug will permit water entry. Similar seeds not possessing a strophiolar cleft must depend on abrasion, which in nature may be brought about by microbial attack, passage through an animal, freezing and thawing, or mechanical means.
In horticulture and agriculture , the coats of such seeds are deliberately damaged or weakened by humans scarification. In chemical scarification, seeds are dipped into strong sulfuric acid , organic solvents such as acetone or alcohol , or even boiling water. In mechanical scarification, they may be shaken with some abrasive material such as sand or be scratched with a knife. Frequently seed coats are permeable to water yet block entrance of oxygen; this applies, for example, to the upper of the two seeds normally found in each burr of the cocklebur plant. The lower seed germinates readily under a favourable moisture and temperature regime, but the upper one fails to do so unless the seed coat is punctured or removed or the intact seed is placed under very high oxygen concentrations.
The most difficult cases of dormancy to overcome are those in which the embryos, although not underdeveloped, remain dormant even when the seed coats are removed and conditions are favourable for growth. Germination in these takes place only after a series of little-understood changes, usually called afterripening , have taken place in the embryo.
In some species, one winter suffices for afterripening.
In others, the process is drawn out over several years, with some germination occurring each year. This can be viewed as an insurance of the species against flash catastrophes that might completely wipe out certain year classes. Many species require moisture and low temperatures; for example, in apples, when the cold requirement is insufficiently met, abnormal seedlings result.
Others cereals, dogwood afterripen during dry storage. The seeds of certain legumes —for example, the seeds of the tree lupin, the coats of which are extremely hard and impermeable—possess a hilum with an ingenious valve mechanism that allows water loss in dry air but prevents reuptake of moisture in humid air. Of great practical importance is stratification, a procedure aimed at promoting a more uniform and faster germination of cold-requiring, afterripening seeds.
Here, two successive cold treatments separated by a warm period are needed for complete seedling development. The first cold treatment eliminates the dormancy of the root; the warm period permits its outgrowth; and the second cold period eliminates epicotyl or leaf dormancy. Thus, almost two years may be required to obtain the complete plant. Seeds of Scotch broom and some Medicago species can be boiled briefly without losing viability.
Ecologically, such heat resistance is important in vegetation types periodically ravaged by fire, such as in the California chaparral , where the germination of Ceanothus seeds may even be stimulated. The major stimulus after a fire is a butenolide called karrikin that occurs in smoke. Karrikin is derived from the burning of cellulose. Also important ecologically is a germination requirement calling for a modest daily alternation between a higher and a lower temperature.
Especially in the desert, extreme temperature fluctuations are an unavoidable feature of the surface, whereas with increasing depth these fluctuations are gradually damped out. A requirement for a modest fluctuation—e. This is advantageous because a seed germinating in soil has to strike a balance between two conflicting demands that depend on depth. On one hand, germination in deeper layers is advantageous because a dependable moisture supply simply is not available near the surface, but, on the other hand, closeness to the surface is desirable because it allows the seedling to reach air and light rapidly and become self-supporting.
Many seeds are insensitive to light, but in a number of species, germination is stimulated or inhibited by exposure to continuous or short periods of illumination. So stimulated are many grasses, lettuce , fireweed , peppergrass Lepidium , mullein , evening primrose , yellow dock, loosestrife , and Chinese lantern plant. Corn maize , the smaller cereals, and many legumes, such as beans and clover , germinate as well in light as in darkness.
Inhibition by light is found in chive , garlic , and several other species of the lily family , jimsonweed , fennel flower Nigella , Phacelia , Nemophila , and pigweed Amaranthus. Sometimes, imbibed wet seeds that do not germinate at all in darkness may be fully promoted by only a few seconds or minutes of exposure to white light or to karrikin.
Alternations of the two treatments to almost any extent indicate that the last treatment received is the decisive one in determining whether the seeds will germinate. This response involves the phytochrome system, a mechanism that involves a pigment called phytochrome, which allows green plants to absorb red light. Red light inhibits stem elongation and lateral root formation but stimulates leaf expansion, chloroplast development, red flower coloration, and spore germination. Such stimulation by red light can be reversed by exposure to far-red light.
Laboratory experiments and field observations indicate that light is a main controller of seed dormancy in a wide array of species. The absence of light, for example, was found in one study to be responsible for the nongermination of seeds of 20 out of 23 weed species commonly found in arable soil. In regions of shifting sands, seeds of Russian thistle germinate only when the fruits are uncovered, often after a burial period of several years. Conversely, the seeds of Calligonum comosum and the melon Citrullus colocynthis , inhabiting coarse sandy soils in the Negev Desert , are strongly inhibited by light.
The survival value of this response, which restricts germination to buried seeds, lies in the fact that at the surface fluctuating environmental conditions may rapidly create a very hostile microenvironment. The seeds of Artemisia monosperma have an absolute light requirement but respond to extremely low intensities, such as is transmitted by a 2-mm- 0.
In seeds buried too deeply, germination is prevented. The responsiveness to light, however, increases with the duration of water imbibition. Even when full responsiveness to light has been reached, maximal germination occurs only after several light-exposures are given at intervals.
In the field, this combined response mechanism acts as an integrating cumulative rain gauge, because the seeds as indicated become increasingly responsive to light, and thus increasingly germinable, the longer the sand remains moistened. Certain Juncus seeds have an absolute light requirement over a wide range of temperatures; consequently, they do not germinate under dense vegetation or in overly deep water.
Beneath dense canopies, seed germination is inhibited because the green leaves above intercept and absorb red light. In combination with temperature, light in the sense of day length may also restrict germination to the most suitable time of year. In birch, for example, seeds that have not gone through a cold period after imbibing water remain dormant after release from the mother plant in the fall and will germinate only when the days begin to lengthen the next spring.
A number of chemicals potassium nitrate, thiourea , ethylene chlorhydrin, and karrikin and plant hormones gibberellins and kinetin have been used experimentally to trigger germination. Their mode of action is obscure, but it is known that in some instances thiourea, gibberellin, kinetin, and karrikin can substitute for light. Natural inhibitors that completely suppress germination coumarin , parasorbic acid, ferulic acid, phenols , protoanemonin, transcinnamic acid, alkaloids , essential oils , and the plant hormone abscisic acid may be present in the pulp or juice of fruits or in various parts of the seed.
The effect of seed-coat phenols , for example, may be indirect; being highly oxidizable, they may screen out much-needed oxygen. Ecologically, such inhibitors are important in at least three ways. Their slow disappearance with time may spread germination out over several years a protection against catastrophes. Furthermore, when leached out by rainwater, they often serve as agents inhibiting the germination of other competitive plants nearby.
Finally, the gradual leaching out of water-soluble inhibitors serves as an excellent integrating rain gauge. Indeed, it has been shown that the germination of certain desert plants is not related to moisture as such but to soil water movement, i. Seed plant reproductive part. Written By: Hans Lambers. See Article History. Read More on This Topic. In the two great groups of seed plants, gymnosperms and angiosperms, the sporophyte is the dominant phase in the life cycle, as it is also…. Start Your Free Trial Today.
Learn More in these related Britannica articles:. In the two great groups of seed plants, gymnosperms and angiosperms, the sporophyte is the dominant phase in the life cycle, as it is also in the vascular cryptogams; the gametophytes are microscopic parasites on the sporophytes. This type of vegetable farming requires special skills and techniques. The crop is not ready for harvest when the edible portion of the plant reaches the stage of maturity; it must be carried through further stages of growth. Production under isolated conditions ensures…. Plants, usually herbaceous, that live for only one growing season and produce flowers and seeds in that time are called annuals.
They may be represented by such plants as corn and marigolds, which spend a period of a few weeks to a…. History at your fingertips. Sign up here to see what happened On This Day , every day in your inbox! By signing up, you agree to our Privacy Notice. Be on the lookout for your Britannica newsletter to get trusted stories delivered right to your inbox.
More About Seed 30 references found in Britannica articles Assorted References domestication In domestication: Origins of domestication effect of predators In community ecology: Specialization in predation formation of fats In fat: Synthesis and metabolism in living organisms life span In life span: Growing season of seed plants North American Indian customs In Native American: Desert Archaic cultures structure In plant: Seed plants In plant: Variations involving seed plants commercial uses cereal crop production In cereal farming: Breeding forest management In forestry: Gymnosperms vegetable farming In vegetable farming: Vegetables raised for seed production View More.
Introduction The nature of seeds Angiosperm seeds Gymnosperm seeds Form and function Seed size Seed size and predation Seed size and germination The shape of dispersal units Polymorphism of seeds and fruits Agents of dispersal Dispersal by animals Dispersal by birds Dispersal by ants Dispersal by wind Dispersal by water Self-dispersal Germination Dormancy and life span of seeds Lack of dormancy Immature embryos Role of the seed coat Afterripening, stratification, and temperature effects Light and seed germination Ecological role of light Stimulators and inhibitors of germination.