seed
, the characteristic reproductive body of both angiosperms (flowering plants) and gymnosperms (e.g., conifers, cycads, and ginkgos). Essentially, a seed consists of a miniature undeveloped plant (the embryo), which, alone or in the company of stored food for its early development after germination, is surrounded by a protective coat (the testa). Frequently small in size and making negligible demands upon their environment, seeds are eminently suited to perform a wide variety of functions the relationships of which are not always obvious: multiplication, perennation (surviving seasons of stress such as winter), dormancy (a state of arrested development), and dispersal. Pollination and the “seed habit” are considered the most important factors responsible for the overwhelming evolutionary success of the flowering plants, which number more than 300,000 species.
The superiority of dispersal by means of seeds over the more primitive method involving single-celled spores, lies mainly in two factors: the stored reserve of nutrient material that gives the new generation an excellent growing start and the seed’s multicellular structure. The latter factor provides ample opportunity for the development of adaptations for dispersal, such as plumes for wind dispersal, barbs, and others.
Economically, seeds are important primarily because they are sources of a variety of foods—for example, the cereal grains, such as wheat, rice, and corn (maize); the seeds of beans, peas, peanuts, soybeans, almonds, sunflowers, hazelnuts, walnuts, pecans, and Brazil nuts. Other useful products provided by seeds are abundant. Oils for cooking, margarine production, painting, and lubrication are available from the seeds of flax, rape, cotton, soybean, poppy, castor bean, coconut, sesame, safflower, sunflower, and various cereal grains. Essential oils are obtained from such sources as juniper “berries,” used in gin manufacture. Stimulants are obtained from such sources as the seeds of coffee, kola, guarana, and cocoa. Spices—from mustard and nutmeg seeds; from the aril (“mace”) covering the nutmeg seed; from the seeds and fruits of anise, cumin, caraway, dill, vanilla, black pepper, allspice, and others—form a large group of economic products.
The nature of seeds
Angiosperm seeds
In the typical flowering plant, or angiosperm, seeds are formed from bodies called ovules contained in the ovary, or basal part of the female plant structure, the pistil. The mature ovule contains in its central part a region called the nucellus that in turn contains an embryo sac with eight nuclei, each with one set of chromosomes (i.e., they are haploid nuclei). The two nuclei near the centre are referred to as polar nuclei; the egg cell, or oosphere, is situated near the micropylar (“open”) end of the ovule.
With very few exceptions (e.g., the dandelion), development of the ovule into a seed is dependent upon fertilization, which in turn follows pollination. Pollen grains that land on the receptive upper surface (stigma) of the pistil will germinate, if they are of the same species, and produce pollen tubes, each of which grows down within the style (the upper part of the pistil) toward an ovule. The pollen tube has three haploid nuclei, one of them, the so-called vegetative, or tube, nucleus seems to direct the operations of the growing structure. The other two, the generative nuclei, can be thought of as nonmotile sperm cells. After reaching an ovule and breaking out of the pollen tube tip, one generative nucleus unites with the egg cell to form a diploid zygote (i.e., a fertilized egg with two complete sets of chromosomes, one from each parent). The zygote undergoes a limited number of divisions and gives rise to an embryo. The other generative nucleus fuses with the two polar nuclei to produce a triploid (three sets of chromosomes) nucleus, which divides repeatedly before cell-wall formation occurs. This process gives rise to the triploid endosperm, a nutrient tissue that contains a variety of storage materials—such as starch, sugars, fats, proteins, hemicelluloses, and phytate (a phosphate reserve).
The events just described constitute what is called the double-fertilization process, one of the characteristic features of all flowering plants. In the orchids and in some other plants with minute seeds that contain no reserve materials, endosperm formation is completely suppressed. In other cases it is greatly reduced, but the reserve materials are present elsewhere—e.g., in the cotyledons, or seed leaves, of the embryo, as in beans, lettuce, and peanuts, or in a tissue derived from the nucellus, the perisperm, as in coffee. Other seeds, such as those of beets, contain both perisperm and endosperm. The seed coat, or testa, is derived from the one or two protective integuments of the ovule. The ovary, in the simplest case, develops into a fruit. In many plants, such as grasses and lettuce, the outer integument and ovary wall are completely fused, so seed and fruit form one entity; such seeds and fruits can logically be described together as “dispersal units,” or diaspores. More often, however, the seeds are discrete units attached to the placenta on the inside of the fruit wall through a stalk, or funiculus.
The hilum of a liberated seed is a small scar marking its former place of attachment. The short ridge (raphe) that sometimes leads away from the hilum is formed by the fusion of seed stalk and testa. In many seeds, the micropyle of the ovule also persists as a small opening in the seed coat. The embryo, variously located in the seed, may be very small (as in buttercups) or may fill the seed almost completely (as in roses and plants of the mustard family). It consists of a root part, or radicle, a prospective shoot (plumule or epicotyl), one or more cotyledons (one or two in flowering plants, several in Pinus and other gymnosperms), and a hypocotyl, which is a region that connects radicle and plumule. A classification of seeds can be based on size and position of the embryo and on the proportion of embryo to storage tissue; the possession of either one or two cotyledons is considered crucial in recognizing two main groups of flowering plants, the monocotyledons and the eudicotyledons.
Seedlings, arising from embryos in the process of germination, are classified as epigeal (cotyledons aboveground, usually green and capable of photosynthesis) and hypogeal (cotyledons belowground). Particularly in the monocots, special absorbing organs may develop that mobilize the reserve materials and withdraw them from the endosperm; e.g., in grasses, the cotyledon has been modified into an enzyme-secreting scutellum (“shield”) between embryo and endosperm.
Gymnosperm seeds
In gymnosperms (plants with “naked seeds”—such as conifers, cycads, and ginkgo), the ovules are not enclosed in an ovary but lie exposed on leaflike structures, the megasporophylls. A long time span usually separates pollination and fertilization, and the ovules begin to develop into seeds long before fertilization has been accomplished; in some cases, in fact, fertilization does not occur until the ovules (“seeds”) have been shed from the tree. In the European, or Scots, pine (Pinus sylvestris), for example, the female cones (essentially collections of megasporophylls) begin to develop in winter and are ready to receive pollen from the male cones in spring. During the first growing season, the pollen tube grows slowly through the nucellus, while within the ovule the megaspore nucleus, through a series of divisions, gives rise to a collection of some 2,000 nuclei, which are then individually enclosed by walls to form a structure called the female gametophyte or prothallus. At the micropylar end of the ovule, several archegonia (bottle-shaped female organs) develop, each containing an oosphere (“egg”). The pollen tube ultimately penetrates the neck of one of the archegonia. Not until the second growing season, however, does the nucleus of one of the male cells in the tube unite with the oosphere nucleus. Although more than one archegonium may be fertilized, only one gives rise to a viable embryo. During the latter’s development, part of the prothallus is broken down and used. The remainder, referred to as endosperm, surrounds the embryo; it is mobilized later, during germination of the seed, a process that occurs without delay when the seeds are liberated from the female cone during the third year after their initiation.
Form and function
Seed size
In the Late Carboniferous Period (about 315.2 million to 298.9 million years ago), some seed ferns produced large seeds (12 × 6 cm [5 × 2 inches] in Pachytesta incrassata). This primitive ancestral condition of large seeds is reflected in certain gymnosperms (Cycas circinalis, 5.5 × 4 cm [2.2 × 1.6 inches]; Araucaria bidwillii, 4.5 × 3.5 cm [1.8 × 1.4 inches]) and also in some tropical rainforest trees with nondormant water-rich seeds (Mora excelsa, 12 × 7 cm [4.7 × 2.8 inches]). The “double coconut” palm Lodoicea maldivica represents the extreme, with seeds weighing up to 27 kg (about 60 pounds). Herbaceous nontropical flowering plants usually have seeds weighing in the range of about 0.0001 to 0.01 gram. Within a given family (e.g., the pea family, Fabaceae), seed size may vary greatly; in others it is consistently large or small, justifying the recognition of “megaspermous” families (e.g., beech, nutmeg, palm, and soursop families) and “microspermous” ones (e.g., milkweed, daisy, heather, nettle, and willow families).
The smallest known seeds, devoid of food reserves, are found in orchids, mycoheterotrophs (nongreen plants that absorb nutrients from dead organic matter and live symbiotically with mycorrizal fungi—e.g., Indian pipe, Monotropa; coral root, Corallorhiza), carnivorous plants (sundews, pitcher plants), and total parasites (members of the families Rafflesiaceae and Orobanchaceae, or broomrapes, which have seeds weighing as little as 0.001 mg—about 3.5 hundred-millionths of an ounce). Clearly, seed size is related to lifestyle. Total parasites obtain food from their host, even in their early growth stages, and young orchids are mycoheterotrophs that receive assistance in absorbing nutrients from mycorrhizal fungi that are associated closely with their roots. In both cases only very small seeds that lack endosperm are produced. Dodders (Cuscuta) and mistletoes (Viscum, Phoradendron, Amyema) live independently when very young and accordingly have relatively large seeds.
Many plant species possess seeds of remarkably uniform size, useful as beads (e.g., Abrus precatorius) or units of weight—one carat of weight once corresponded with one seed of the carob tree, Ceratonia siliqua. In wheat and many other plants, average seed size does not depend on planting density, showing that seed size is under rather strict genetic control. This does not necessarily preclude significant variation among individual seeds; in peas, for example, the seeds occupying the central region of the pod are the largest, probably as the result of competition for nutrients between developing ovules on the placenta. Striking evolutionary changes in seed size, inadvertently created by humans, have occurred in the weed known as gold-of-pleasure (Camelina sativa), which grows in flax fields. The customary winnowing of flax seeds selects forms of C. sativa whose seeds are blown over the same distance as flax seeds in the operation, thus staying with their “models.” Consequently, C. sativa seeds in the south of Russia now mimic the relatively thick, heavy seeds of the oil flax that is grown there, whereas in the northwest they resemble the flat, thin seeds of the predominant fibre flax.
Seed size and predation
Seeds form the main source of food for many birds, rodents, ants, and beetles. Harvester ants of the genus Veromessor, for example, exact a toll of about 15,000,000 seeds per acre (37,050,000 seeds per hectare) per year from the Sonoran Desert of the southwestern United States. In view of the enormous size range of the predators, which include minute weevil and bruchid-beetle larvae that attack the seeds internally, evolutionary “manipulation” of seed size by a plant species cannot in itself be effective in completely avoiding seed attack. With predation inescapable, however, it must be advantageous for a plant species to invest the total reproductive effort in a large number of very small units (seeds) rather than in a few big ones. The mean seed weight of those 13 species of Central American woody legumes vulnerable to bruchid attack is 0.26 gram (0.009 ounce). In contrast, the mean seed weight of the 23 species invulnerable by virtue of toxic seed constituents is 3 grams (0.1 ounce).
Seed size and germination
Ecologically, seed size is also important in the breaking of dormancy. Being small, a seed can only “sample” that part of the environment immediately adjacent to it, which is not necessarily representative of the generally prevailing conditions. For successful seedling establishment, there is clearly a risk in “venturing out” in adverse conditions. The development in seeds of mechanisms acting as “integrating rain gauges” should be considered in that light (see below).
The shape of dispersal units
Apart from the importance of shape as a factor in determining the mode of dispersal (e.g., wind dispersal of winged seeds, animal dispersal of spiny fruits), shape also counts when the seed or diaspore is seen as a landing device. The flatness of the enormous tropical Mora seeds prevents rolling and effectively restricts germination to the spot where they land. In contrast, Eusideroxylon zwageri does not grow on steep slopes, because its heavy fruits roll downhill. The grains of the grass Panicum turgidum, which have a flat and a round side, germinate much better when the flat rather than the convex side lies in contact with wet soil. 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. The rounded seeds of cabbage species, for example, tend to roll into crevices, whereas the reticulate ones of lamb’s quarters (Chenopodium album) often stay in the positions in which they first fall. Several seeds have appendages (awns, bristles) that promote germination by aiding in orientation and self-burial. In one study, for example, during a six-month period, awned grains of Danthonia penicillata gave rise to 12 times as many established seedlings as de-awned ones.
Polymorphism of seeds and fruits
Some plant species produce two or more sharply defined types of seeds that differ in appearance, colour, shape, size, internal structure, or dormancy. In common spurry (Spergula arvensis), for example, the seed coat (part of the mother plant) may be either smooth or papillate (covered with tiny nipple-like projections). Here the phenomenon is genetically controlled by a single factor, so all the seeds of a given plant are either papillate or smooth. More common is somatic polymorphism, the production by individual plants of different seed types, or “morphs.” Somatic polymorphism occurs regularly in saltbush (Atriplex) and goosefoot (Chenopodium), in which a single plant may produce both large brown seeds capable of immediate germination and small black ones with some innate dormancy. Somatic polymorphism may be controlled by the position of the two (or more) seed types within one inflorescence (flower cluster) or fruit, as in cocklebur, or it may result from environmental effects, as in Halogeton, in which imposition of long or short days leads to production of brown or black seeds, respectively. Since the different morphs in seed (and fruit) polymorphism usually have different dispersal mechanisms and dormancies, so germination is spread out both in space and in time, the phenomenon can be seen as an insurance against catastrophe.
Agents of dispersal
While some seeds are dispersed independently of the fruits they matured in, others are dispersed together with the fruit, as is common in many edible fruits, nuts, and cereals. Such a dispersal unit is referred to as a diaspore. The dispersing agents for seeds and diaspores are indicated in such terms as anemochory, hydrochory, and zoochory, which mean dispersal by wind, water, and animals, respectively. Within the zoochorous group, further differentiation according to the carriers can be made: saurochory, dispersal by reptiles; ornithochory, by birds; myrmecochory, by ants. Or the manner in which the seeds or diaspores are carried can be emphasized, distinguishing endozoochory, seeds or diaspores carried within an animal; epizoochory, seeds or diaspores accidentally carried on the outside; and synzoochory, seeds or diaspores intentionally carried, mostly in the mouth, as in birds and ants.
Dispersal by animals
Snails disperse the small seeds of a very few plant species (e.g., Adoxa). Earthworms are more important as seed dispersers. Many intact fruits and seeds can serve as fish bait, those of Sonneratia, for example, for the catfish Arius maculatus. Certain Amazon River fishes react positively to the audible “explosions” of the ripe fruits of Eperua rubiginosa. Fossil evidence indicates that saurochory is very ancient. The giant Galapagos tortoise is important for the dispersal of local cacti and tomatoes. The name alligator apple for Annona glabra refers to its method of dispersal, an example of saurochory. Many birds and mammals, ranging in size from mice and kangaroo rats to elephants, eat and disperse seeds and fruits. In the tropics, chiropterochory (dispersal by large bats such as flying foxes, Pteropus) is particularly important. Fruits adapted to these animals are relatively large and drab in colour, with large seeds and a striking (often rank) odour. Such fruits are accessible to bats because of the pagoda-like structure of the tree canopy, fruit placement on the main trunk, or suspension from long stalks that hang free of the foliage. Examples include mangoes, guavas, breadfruit, carob, and several fig species. In South Africa, a desert melon (Cucumis humifructus) participates in a symbiotic relationship with aardvarks—the animals eat the fruit for its water content and bury their own dung, which contains the seeds, near their burrows.
Furry terrestrial mammals are the agents most frequently involved in epizoochory, the inadvertent carrying by animals of dispersal units. Burrlike seeds and fruits, or those diaspores provided with spines, hooks, claws, bristles, barbs, grapples, and prickles, are genuine hitchhikers, clinging tenaciously to their carriers. Their functional shape is achieved in various ways—in cleavers, or bedstraw (Galium aparine), and enchanter’s nightshade (Circaea lutetiana), the hooks are part of the fruit itself; in common agrimony (Agrimonia eupatoria), the fruit is covered by a persistent calyx (the sepals, parts of the flower, which remain attached beyond the usual period) equipped with hooks; in wood avens (Geum urbanum), the persistent styles have hooked tips. Other examples are bur marigolds, or beggar’s-ticks (Bidens species); buffalo bur (Solanum rostratum); burdock (Arctium); Acaena; and many Medicago species. The last-named, with dispersal units highly resistant to damage from hot water and certain chemicals (dyes), have achieved wide global distribution through the wool trade.
A somewhat different principle is employed by the so-called trample burrs, said to lodge themselves between the hooves of large grazing mammals. Examples are mule grab (Proboscidea) and the African grapple plant (Harpagophytum). In water burrs, such as those of the water nut Trapa, the spines should probably be considered as anchoring devices.
Dispersal by birds
Birds, being preening animals, rarely carry burrlike diaspores on their bodies. They do, however, transport the very sticky (viscid) fruits of Pisonia, a tropical tree of the four-o’clock family, to distant Pacific islands in this way. Small diaspores, such as those of sedges and certain grasses, may also be carried in the mud sticking to waterfowl and terrestrial birds.
Synzoochory, deliberate carrying of diaspores by animals, is practiced when birds carry seeds and diaspores in their beaks. The European mistle thrush, Turdus viscivorus, deposits the viscid seeds of European mistletoe (Viscum album) on potential host plants when, after a meal of the berries, it whets its bill on branches or simply regurgitates the seeds. The North American (Phoradendron) and Australian mistletoes (Ameyema) are dispersed by various birds, and the comparable tropical species of the plant family Loranthaceae by flowerpeckers (of the bird family Dicaeidae), which have a highly specialized gizzard that allows seeds to pass through but retains insects. Plants may also profit from the forgetfulness and sloppy habits of certain nut-eating birds that cache part of their food but neglect to recover everything or drop units on their way to the hiding place. Best known in this respect are the nutcrackers (Nucifraga), which feed largely on the “nuts” of beech, oak, walnut, chestnut, and hazel; the jays (Garrulus), which hide hazelnuts and acorns; the nuthatches; and the California woodpecker (Balanosphyra), which may embed literally thousands of acorns, almonds, and pecan nuts in bark fissures or holes of trees. Secondarily, rodents may aid in dispersal by stealing the embedded diaspores and burying them. In Germany an average jay may transport about 4,600 acorns per season, over distances of up to 4 km (2.5 miles). Woodpeckers, nutcrackers, and squirrels are responsible for a similar dispersal of Pinus cembra in the Alps near the tree line.
Most ornithochores (plants with bird-dispersed seeds) have conspicuous diaspores attractive to such fruit-eating birds as thrushes, pigeons, barbets (members of the bird family Capitonidae), toucans, and hornbills (family Bucerotidae), all of which either excrete or regurgitate the hard embryo-containing part undamaged. Such diaspores have a fleshy, sweet, or oil-containing edible part; a striking colour (often red or orange); no pronounced smell; a protection against being eaten prematurely in the form of acids and tannins that are present only in the green fruit; a protection of the seed against digestion—bitterness, hardness, or the presence of poisonous compounds; permanent attachment; and, finally, absence of a hard outer cover. In contrast to bat-dispersed diaspores, they occupy no special position on the plant. Examples are rose hips, plums, dogwood fruits, barberry, red currant, mulberry, nutmeg fruits, figs, blackberries, and others. The natural and abundant occurrence of Euonymus, which is a largely tropical genus, in temperate Europe and Asia, can be understood only in connection with the activities of birds. Birds also contributed substantially to the repopulation with plants of the island Krakatoa after the catastrophic eruption of 1883. Birds have made Lantana (originally American) a pest in Indonesia and Australia; the same is true of wild plums (Prunus serotina) in parts of Europe, Rubus species in Brazil and New Zealand, and olives (Olea europaea) in Australia.
Mimicry—the protection-affording imitation of a dangerous or toxic species by an edible, harmless one—is shown in reverse by certain bird-dispersed “coral seeds” such as those of many species in the genera Abrus, Ormosia, Rhynchosia, Adenanthera, and Erythrina. Hard and often shiny red or black and red, many such seeds deceptively suggest the presence of a fleshy red aril and thus invite the attention of hungry birds.
Dispersal by ants
Mediterranean and North American harvester ants (Messor, Atta, Tetramorium, and Pheidole) are essentially destructive, storing and fermenting many seeds and eating them completely. Other ants (Lasius, Myrmica, and Formica species) eat the fleshy, edible appendage (the fat body or elaiosome) of certain specialized seeds, which they disperse. Most myrmecochorous plants (species of violet, primrose, hepatica, cyclamen, anemone, corydalis, Trillium, and bloodroot) belong to the herbaceous spring flora of northern forests. Tree poppy (Dendromecon), however, is found in the dry California chaparral; Melica and Centaurea species, in arid Mediterranean regions. The so-called ant epiphytes of the tropics (i.e., species of Hoya, Dischidia, Aeschynanthus, and Myrmecodia—plants that live in “ant gardens” on trees or offer the ants shelter in their own body cavities) constitute a special group of myrmecochores that provide oil in seed hairs. The ancestral forms of these hairs must have served in wind dispersal. The primary ant attractant of myrmecochorous seeds is not necessarily oil; instead, an unsaturated, somewhat volatile fatty acid is suspected in some cases. The myrmecochorous plant as a whole may also have specific adaptations; for example, cyclamen brings fruits and seeds within reach of ants by conspicuous coiling (shortening) of the flower stalk as soon as flowering is over.
Dispersal by wind
In the modern world, wind dispersal (although numerically important) reflects the climatic and biotic poverty of certain regions; it is essentially a feature of pioneer vegetations. The flora of the Alps is 60 percent anemochorous; that of the Mediterranean garrigue (a scrubland region) is 50 percent. By making certain assumptions (e.g., for average wind velocity and turbulence), the “average limits of dispersal”—that is, the distance that 1 percent of the seeds or diaspores can reach—can be calculated for dispersal units of various construction and weight. This calculation yields values of 10 km (6 miles) for dandelion (Taraxacum officinale) and 0.5 km (0.3 mile) for European pine (Pinus sylvestris). Storms result in higher values—30 km (20 miles) for poplar and 200 km (125 miles) for Senecio congestus.
Too much success in dispersal may be ecologically futile, as exemplified by certain Florida orchids that arise from windblown West Indian seeds but do not multiply because of the lack of specific pollinators, usually certain bees or wasps. Anemochorous diaspores can be subdivided into flyers, dust diaspores, balloons, and plumed or winged diaspores; rollers, chamaechores or tumbleweeds; and throwers, ballistic anemochores. Dispersal by means of minute dust diaspores produced in huge quantities is comparable to spore dispersal in lower plants—a “saturation bombing” is required to find the very limited number of targets, or favourable growth habitats, that exist. Not surprisingly, it is practiced mostly by total parasites, such as broomrapes (in which the finding of the specific host is a problem), and mycoheterotrophs. 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 600 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.g., fireweed, milkweeds, dogbane. In woolly fruits and seeds, the pericarp or the seed coat is covered with cottonlike hairs—e.g., willow, poplar or cottonwood, kapok, cotton, and balsa. In some cases, the hairs may serve double duty, in that they function in water dispersal as well as in wind dispersal. In tumbleweeds, the whole plant or its fruiting portion breaks off and is blown across open country, scattering seeds as it goes; examples include Russian thistle, pigweed, tumbling mustard, perhaps rose of Jericho, and “windballs” of the grass Spinifex of Indonesian shores and Australian deserts. 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.
Dispersal by water
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. Once the nuts are ashore, the mesocarp also aids in the aboveground germination process by collecting rainwater; in addition, the endosperm has in its “milk” a provision for seedling establishment on beaches without much fresh water. 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. A “splashcup mechanism,” common in fungi for spore dispersal, is suggested by the open fruit capsule with exposed small seeds in the pearlwort (Sagina) and mitrewort (Mitella). Hygrochasy, the opening of fruits in moist weather, is displayed by species of Mesembryanthemum, Sedum, and other plants of dry environments.
Self-dispersal
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.
Atelechory, the dispersal over a very limited distance only, represents a waste-avoiding defensive “strategy” that functions in further exploitation of an already occupied favourable site. 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.
Germination
Dormancy and life span of seeds
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.g., winter), which is sure to follow; (2) the unfavourable period has to be survived; and (3) the various dispersing agents must be given time to act. 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. Mimosa glomerata seeds in the herbarium of the Muséum National d’Histoire Naturelle in Paris were found viable after 221 years. 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. Seeds of the sacred lotus (Nelumbo nucifera) found in a peat deposit in Manchuria and estimated by radioactive-carbon dating to be 1,400 ± 400 years old rapidly germinated (and subsequently produced flowering plants) when the seeds were filed to permit water entry. In 1967, seeds of the arctic tundra lupine (Lupinus arcticus) found in a frozen lemming burrow with animal remains established to be at least 10,000 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.
Lack of dormancy
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.
Immature embryos
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.
Role of the seed coat
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.
Afterripening, stratification, and temperature effects
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 this group are many forest trees and shrubs such as pines, hemlocks, and other conifers; some flowering woody plants such as dogwood, hawthorn, linden, holly, and viburnum; fruit trees such as apples, pears, peaches, plums, and cherries; and flowering herbaceous plants such as iris, Solomon’s seal, and lily of the valley. 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. In this procedure, seeds are placed for one to six months, depending on the species, between layers of sand, sawdust, sphagnum, or peat and kept moist as well as reasonably cold (usually 0–10 °C [32–50 °F]). A remarkable “double dormancy” has thus been uncovered in lily of the valley and false Solomon’s seal. 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. The optimal temperature for germination, ranging from 1 °C (34 °F) for bitterroot to 42 °C (108 °F) for pigweed, may also shift slightly as a result of stratification.
Many dry seeds are remarkably resistant to extreme temperatures, some even cooled to that of liquid air (−140 °C or −220 °F). 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.g., from 20 °C (68 °F) at night to 30 °C (86 °F) in the daytime (as displayed by the grass Oryzopsis miliacea)—practically ensures germination at fair depths. 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.
Light and seed germination
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. The best-studied case of this type, and one that is a milestone in plant physiology, concerns seeds of the Grand Rapids variety of lettuce, which is stimulated to germination by red light (wavelength about 660 nanometres) but inhibited by “far-red” light (wavelength about 730 nanometres). 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.
Ecological role of 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- (0.08-inch-) thick sand filter. 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.
Stimulators and inhibitors of germination
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.e., to the amount and duration of rain received.
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canola oil
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- vegetable oil rapeseed
canola oil, vegetable oil made from the pressed seeds of rapeseed (Brassica napus variety napus), a relative of turnips and cabbage in the mustard family.
Rapeseed oil has long been used in industry as a lubricant for engines and other machine parts, but, because of its high level of potentially harmful erucic acid, it was not considered safe for human consumption (though it was sometimes added to animal feed). It was not until the 1970s, when Canadian scientists developed a hybrid that contained a very low level of erucic acid, that experimentation began to place rapeseed oil among the vegetable oils available for use in cooking. Because of the unfortunate association of the element rape, which comes from the Latin word for “turnip,” with the criminal act, whose name comes from an unrelated Latin term meaning “to seize,” the company developing the hybrid brought it to market in 1979 under the name Canola, an acronym for Canadian oil, low acid. Outside of North America, however, it is usually sold as colza oil, which derives from the Dutch word for cabbage seed, or as rapeseed oil.
Widely licensed for manufacture, canola oil—its name now usually lowercase—is produced in many other places besides Canada, including Britain, China, and Pakistan. It is widely used as a cooking oil, valued for its neutral flavour. With a high smoking point (400 °F [204 °C]), canola oil lends itself to sautéing, frying, and baking, and it keeps well in storage.
Canola oil is considered safe for consumption not just because of its lowered erucic acid level, but also because it contains very little saturated fat and high levels of polyunsaturated omega-3 fat and phytosterols, which reduce the absorption of cholesterol in the body. Some health concerns have been voiced because of canola oil’s relatively high level of trans-fatty acids, which is comparable to walnut oil’s but approximately 10 times lower than soybean oil’s. The level of trans-fatty acids increases if the oil is not changed frequently in, say, a deep-fat fryer, but it will remain at the lower end of the scale in most home-cooking settings. In 2018 the U.S. Food and Drug Administration determined that canola oil may reduce the risk of coronary heart disease.
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annual
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- ephemeral self-pollination
annual, any plant that completes its life cycle in a single growing season. The term is usually applied to herbaceous flowering plants in which the dormant seed is the only part of an annual that survives from one growing season to the next. A growing season does not necessarily correspond to the four traditional seasons of a year, though many annuals are categorized into summer annuals or winter annuals. Summer annuals germinate in the spring or early summer and go to seed in the late summer or autumn of the same year. Winter annuals typically germinate in the late summer or autumn and produce seed and die the following spring or summer. A growing season can also be as short as a few weeks, as is the case for some desert annuals that sprout and go to seed rapidly following a rain and spend most of their life cycle as seeds in a soil seed bank. Annuals include many weeds, wildflowers, garden flowers, and vegetables. See also biennial; perennial.
Many economically important food crops are annuals, including all major cereal grains (corn, wheat, oats, barley, etc.), most gourds and melons, peas and other legumes, and lettuce. Many other crops are not true annuals but are typically grown and harvested in one season, including biennials such as carrot, celery, and parsley and tender perennials such as bell peppers and tomatoes. Common garden annuals include marigold, sweet alyssum, nasturtium, and zinnia.
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flaxseed
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- flax linseed oil oilseed
flaxseed, also called linseed, edible seeds harvested from flax (Linum usitatissimum) plants, used as a health food and as a source of linseed, or flaxseed, oil. Consumed as food by the ancient Greeks and Romans, flaxseed has reemerged as a possible “superfood” because of its high dietary fibre and omega-3 fatty acid content. Previously, its main food use was as livestock feed. Linseed oil is also consumed for its health benefits and has a number of industrial applications.
Flax plants grown for their seeds are generally shorter, have more branches, and produce more seeds than those that are grown primarily for linen fibre. Flaxseed is borne in dry globular capsules, each with 10 long flat elliptical seeds with slight projections at one end. The seeds are typically about 3 to 4 mm (0.1 to 0.15 inch) long. They are usually brown and are smooth and shiny, with a mucilaginous substance in their outer layer that makes them sticky when wet. A whole air-dried seed usually contains from 33 to 43 percent oil by weight.
Flaxseed is a rich source of the omega-3 fatty acid alpha-linolenic acid (ALA). The seeds are also high in a class of phytoestrogens known as lignans. Additionally, flaxseeds are high in dietary fibre, protein, iron, calcium, manganese, thiamin, magnesium, phosphorus, and copper. The seeds can be eaten raw or toasted and can be ground or added whole to salads, morning cereals, and smoothies or incorporated into baked goods.
Linseed oil is golden yellow, brown, or amber in colour and has the highest level of ALA of any vegetable oil. Food-grade linseed oil is sometimes taken as a nutritional supplement and can be used in cooking, though it is somewhat unstable and goes rancid quickly. Industrially, it is classified as a drying oil because it thickens and becomes hard on exposure to air. It is slightly more viscous than most vegetable oils and is used in the production of paints, printing inks, linoleum, varnish, and oilcloth. Linseed oil was formerly a common vehicle in exterior house paints, but its chief remaining use in this field is in artists’ oil paints, which are made by grinding raw pigment into the oil.
The chief commercial grades of linseed oil are raw, refined, boiled, and blown. Raw oil is the slowest-drying. Refined oil is raw oil with the free fatty acids, gums, and other extraneous materials removed. The boiled and blown grades dry most quickly and form the hardest films. After the oil has been removed from flaxseed by compression, the remaining meal, high in protein and minerals, is heated and pressed into cakes for livestock.
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germination
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- epigeous germination cryptogeal germination hypogeous germination germ tube appressorium
germination, the sprouting of a seed, spore, or other reproductive body, usually after a period of dormancy. The absorption of water, the passage of time, chilling, warming, oxygen availability, and light exposure may all operate in initiating the process.
In the process of seed germination, water is absorbed by the embryo, which results in the rehydration and expansion of the cells. Shortly after the beginning of water uptake, or imbibition, the rate of respiration increases, and various metabolic processes, suspended or much reduced during dormancy, resume. These events are associated with structural changes in the organelles (membranous bodies concerned with metabolism), in the cells of the embryo.
Germination sometimes occurs early in the development process; the mangrove (Rhizophora) embryo develops within the ovule, pushing out a swollen rudimentary root through the still-attached flower. In peas and corn (maize) the cotyledons (seed leaves) remain underground (e.g., hypogeal germination), while in other species (beans, sunflowers, etc.) the hypocotyl (embryonic stem) grows several inches above the ground, carrying the cotyledons into the light, in which they become green and often leaflike (e.g., epigeal germination).
Seed dormancy
Dormancy is brief for some seeds—for example, those of certain short-lived annual plants. After dispersal and under appropriate environmental conditions, such as suitable temperature and access to water and oxygen, the seed germinates, and the embryo resumes growth.
The seeds of many species do not germinate immediately after exposure to conditions generally favourable for plant growth but require a “breaking” of dormancy, which may be associated with change in the seed coats or with the state of the embryo itself. Commonly, the embryo has no innate dormancy and will develop after the seed coat is removed or sufficiently damaged to allow water to enter. Germination in such cases depends upon rotting or abrasion of the seed coat in the gut of an animal or in the soil. Inhibitors of germination must be either leached away by water or the tissues containing them destroyed before germination can occur. Mechanical restriction of the growth of the embryo is common only in species that have thick, tough seed coats. Germination then depends upon weakening of the coat by abrasion or decomposition.
In many seeds the embryo cannot germinate even under suitable conditions until a certain period of time has lapsed. The time may be required for continued embryonic development in the seed or for some necessary finishing process—known as afterripening—the nature of which remains obscure.
The seeds of many plants that endure cold winters will not germinate unless they experience a period of low temperature, usually somewhat above freezing. Otherwise, germination fails or is much delayed, with the early growth of the seedling often abnormal. (This response of seeds to chilling has a parallel in the temperature control of dormancy in buds.) In some species, germination is promoted by exposure to light of appropriate wavelengths. In others, light inhibits germination. For the seeds of certain plants, germination is promoted by red light and inhibited by light of longer wavelength, in the “far red” range of the spectrum. The precise significance of this response is as yet unknown, but it may be a means of adjusting germination time to the season of the year or of detecting the depth of the seed in the soil. Light sensitivity and temperature requirements often interact, the light requirement being entirely lost at certain temperatures.
Seedling emergence
Active growth in the embryo, other than swelling resulting from imbibition, usually begins with the emergence of the primary root, known as the radicle, from the seed, although in some species (e.g., the coconut) the shoot, or plumule, emerges first. Early growth is dependent mainly upon cell expansion, but within a short time cell division begins in the radicle and young shoot, and thereafter growth and further organ formation (organogenesis) are based upon the usual combination of increase in cell number and enlargement of individual cells.
Until it becomes nutritionally self-supporting, the seedling depends upon reserves provided by the parent sporophyte. In angiosperms these reserves are found in the endosperm, in residual tissues of the ovule, or in the body of the embryo, usually in the cotyledons. In gymnosperms food materials are contained mainly in the female gametophyte. Since reserve materials are partly in insoluble form—as starch grains, protein granules, lipid droplets, and the like—much of the early metabolism of the seedling is concerned with mobilizing these materials and delivering, or translocating, the products to active areas. Reserves outside the embryo are digested by enzymes secreted by the embryo and, in some instances, also by special cells of the endosperm.
In some seeds (e.g., castor beans) absorption of nutrients from reserves is through the cotyledons, which later expand in the light to become the first organs active in photosynthesis. When the reserves are stored in the cotyledons themselves, these organs may shrink after germination and die or develop chlorophyll and become photosynthetic.
Environmental factors play an important part not only in determining the orientation of the seedling during its establishment as a rooted plant but also in controlling some aspects of its development. The response of the seedling to gravity is important. The radicle, which normally grows downward into the soil, is said to be positively geotropic. The young shoot, or plumule, is said to be negatively geotropic because it moves away from the soil; it rises by the extension of either the hypocotyl, the region between the radicle and the cotyledons, or the epicotyl, the segment above the level of the cotyledons. If the hypocotyl is extended, the cotyledons are carried out of the soil. If the epicotyl elongates, the cotyledons remain in the soil.
Light affects both the orientation of the seedling and its form. When a seed germinates below the soil surface, the plumule may emerge bent over, thus protecting its delicate tip, only to straighten out when exposed to light (the curvature is retained if the shoot emerges into darkness). Correspondingly, the young leaves of the plumule in such plants as the bean do not expand and become green except after exposure to light. These adaptative responses are known to be governed by reactions in which the light-sensitive pigment phytochrome plays a part. In most seedlings, the shoot shows a strong attraction to light, or a positive phototropism, which is most evident when the source of light is from one direction. Combined with the response to gravity, this positive phototropism maximizes the likelihood that the aerial parts of the plant will reach the environment most favourable for photosynthesis.
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alternation of generations
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- Frederick Orpen Bower
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- gametophyte sporophyte haploid phase diploid phase foot
alternation of generations, also called metagenesis or heterogenesis, in biology, the alternation of a sexual phase and an asexual phase in the life cycle of an organism. The two phases, or generations, are often morphologically, and sometimes chromosomally, distinct.
In algae, fungi, and plants, alternation of generations is common. It is not always easy to observe, however, since one or the other of the generations is often very small, even microscopic. The sexual phase, called the gametophyte generation, produces gametes, or sex cells, and the asexual phase, or sporophyte generation, produces spores asexually. In terms of chromosomes, the gametophyte is haploid (has a single set of chromosomes), and the sporophyte is diploid (has a double set). In bryophytes, such as mosses and liverworts, the gametophyte is the dominant life phase, whereas in angiosperms and gymnosperms the sporophyte is dominant. The haploid phase is also dominant among fungi. Although some algae have determinate life cycle stages, many species alternate between the sexual and asexual phases in response to environmental conditions.
Among animals, many invertebrates have an alternation of sexual and asexual generations (e.g., protozoans, jellyfish, flatworms), but the alternation of haploid and diploid generations is unknown.
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peanut
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- Billy Carter George Washington Carver
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- legume mafé peanut oil geocarpy peanut butter
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peanut, (Arachis hypogaea), also called groundnut, earthnut, or goober, legume of the pea family (Fabaceae), grown for its edible seeds. Native to tropical South America, the peanut was at an early time introduced to the Old World tropics. The seeds are a nutritionally dense food, rich in protein and fat. Despite its several common names, the peanut is not a true nut. As with other legumes, the plant adds nitrogen to the soil by means of nitrogen-fixing bacteria and is thus particularly valuable as a soil-enriching crop.
The peanut is an annual and can either be an erect shrubby plant, 45–60 cm (18–24 inches) high with short branches, or have a spreading form, 30–45 cm (12–18 inches) high with long branches that lie close to the soil. The stems are sturdy and hairy and bear pinnately compound leaves with two pairs of leaflets. The flowers are borne in the axils of the leaves and feature golden-yellow petals about 10 mm (0.4 inch) across. The oblong pods have rounded ends and are most commonly 25–50 mm (1–2 inches) long with two or three seeds; the pods are contracted between the seeds and have a thin, netted, spongy shell. The seeds vary from oblong to nearly round and have a papery seed coat that ranges in colour from whitish to dark purple.
Peanut legumes have the peculiar habit of ripening underground, a phenomenon known as geocarpy. After pollination and the withering of the flower, an unusual stalklike structure called a peg grows from the base of the flower toward the soil. The fertilized ovules are carried downward in the sturdy tip of the peg until the tip is well below the soil surface, at which point the peg tip starts to develop into the characteristic pod. The pegs sometimes reach down 10 cm (4 inches) or more before their tips can develop fruits. These unusual fruits appear to function as roots to some degree, absorbing mineral nutrients directly from the soil. The pods may not develop properly unless the soil around them is well supplied with available calcium, regardless of the nutrients available to the roots.
Peanut growing requires at least five months of warm weather with rainfall (or irrigation equivalent) of 60 cm (24 inches) or more during the growing season. The best soils are well-drained sandy loams underlain by deep friable (easily crumbled) loam subsoils. At harvest the entire plant, except the deeper roots, is removed from the soil. The pods are often cured by allowing the harvested plants to wilt for a day, then placing them for four to six weeks in stacks built around a sturdy stake driven upright into the soil. The pods are placed toward the inside of each stack to protect them from weather.
Peanuts are sold boiled or roasted and are commonly used to produce an edible oil with a high smoke point. In the United States the seeds are also ground into peanut butter and widely used in candy and bakery products. The peanut is used extensively as feed for livestock in some places; the tops of the plants, after the pods are removed, usually are fed as hay, although the entire plant may be so used. The development of some 300 derivative products from peanuts—including flour, soaps, and plastics—stems mainly from research conducted in the early 20th century by George Washington Carver.
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aril
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aril, accessory covering of certain seeds that commonly develops from the seed stalk, found in both angiosperms and gymnosperms. It is often a bright-coloured fleshy envelope, as in such woody plants as the yews and nutmeg, but smaller seed appendages may also be considered arils, such as the spongy outgrowths on castor beans. Animals are attracted to arils and usually eat them with the seeds, dispersing the undigested seeds in their wastes. The fatty arils on castor beans are a type of elaiosome (oil body) that serve to entice ants for dispersal.
Arils are common in members of the arrowroot family (Marantaceae) and plants of the genus Oxalis. The red flesh surrounding each pomegranate seed is an edible aril, as is the white flesh of ackee, lychee, and rambutan (all three of which are members of the family Sapindaceae). The aril of nutmeg is the source of the spice known as mace.
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perennial
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perennial, any plant that persists for several years, usually with new herbaceous growth from a part that survives from growing season to growing season. Trees and shrubs, including all gymnosperms (cone-bearing plants), are perennials, as are some herbaceous (nonwoody) flowering plants and vegetative ground covers. Herbaceous perennials in cold climates typically survive winter by means of underground root or stem modifications, such as bulbs, corms, tubers, or rhizomes; the aboveground portions of these plants often die back in autumn. See also annual; biennial.
Most garden perennials have only a limited flowering period, but, with maintenance throughout the growing season, they provide a leafy presence and shape to the garden landscape. Popular herbaceous perennials include bellflowers, chrysanthemums, columbines, dahlias, larkspurs, hollyhocks, phlox, pinks, poppies, and primroses. In agriculture, a number of economically important crops are perennials and produce a harvest for a number of years. These include all tree crops (such as apples, citrus, nuts, coffee, chocolate, oil palm, etc.), blueberries, cranberries, asparagus, grapes, alfalfa, rhubarb, chives, mint, and others.
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biennial
biennial, any herbaceous flowering plant that completes its life cycle in two growing seasons. During the first growing season, biennials produce roots, stems, and leaves. During the second growing season, they produce flowers, fruits, and seeds, and then they die. Biennial plants are typically native to temperate climates and frequently overwinter underground. See also annual; perennial.
Many members of the parsley family (Apiaceae) have a biennial life cycle, including parsley, coriander, dill, and Queen Anne’s lace. One of the most economically important biennials is the carrot, which is harvested after a single growing season when the food storage organ (the edible root) has achieved its greatest size. If left to go to seed, the food stored in the taproot will be used by the plant to fuel the growth of its reproductive structures during the second growing season. Certain bellflowers and some forget-me-nots also live for only two growing seasons.
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