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Seedthe characteristic reproductive body of both angiosperms flowering plants and gymnosperms e. Essentially, a seed consists of a miniature undeveloped plant the embryowhich, alone or in the company of stored food for its early development after germinationis surrounded by a protective coat the testa.
Frequently small in size and making negligible demands upon their environmentseeds are eminently suited to perform a wide variety of functions the relationships of which are not always obvious: The superiority of dispersal by means of seeds over the more primitive method involving single-celled sporeslies mainly in two factors: The latter factor provides ample opportunity for melbourne free online dating 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 wheatriceand corn maize ; the seeds of beanspeaspeanutssoybeansalmondsoak ridge national laboratory dinosaur carbon dating, sunflowershazelnuts, walnutspecansand Brazil nuts.
Other useful products provided by seeds are abundant. Oils for cookingmargarine production, painting, and lubrication are available from the seeds of flaxrape hi how are you online dating, cotton kotango dating, soybeanpoppycastor beancoconutoak ridge national laboratory dinosaur carbon dating, sesamesafflowersunflowerand various cereal grains.
Stimulants are obtained from such sources as the seeds of coffeekolaguaranaand cocoa. In the typical flowering plant, or angiosperm, seeds are formed from bodies called ovules contained in the ovaryor 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 nucleieach with one set of chromosomes i.
With very few exceptions e. 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 vegetativeor tube, nucleus seems to direct the operations of the growing structure. The other two, the generative nucleican be thought of as nonmotile sperm cells.
After reaching an w/d hookup definition and breaking out of the pollen tube tip, one generative nucleus unites with the egg cell to form a diploid zygote i. 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 endosperma nutrient tissue that contains a variety of storage materials—such as starchsugarsfatsproteinshemicellulosesand 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. Other seeds, such as those of beetscontain 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. 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 embryovariously 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 epicotylone or more cotyledons one or two in flowering plants, several in Pinus and other gymnospermsand 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 germinationare 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 taking a break christian dating the reserve materials and withdraw them from the endosperm; e. In the European, or Scots, pine Pinus sylvestrisfor 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 seasonthe 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, nuclei, which are then individually enclosed by walls to form a structure called the female gametophyte or prothallus.
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. The remainder, referred to as endospermsurrounds 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.
In the Late Carboniferous Period about This primitive ancestral condition of large seeds is reflected in certain gymnosperms Cycas circinalis5. Herbaceous nontropical flowering plants usually have seeds weighing in the range of about 0. Within a given family e. The smallest known seeds, devoid of food reserves, are found in orchidsmycoheterotrophs nongreen plants that absorb nutrients from dead organic matter and live symbiotically with mycorrizal fungi —e.
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 ViscumPhoradendronAmyema live independently when very young and accordingly have relatively large seeds. Oak ridge national laboratory dinosaur carbon dating plant species possess seeds of remarkably uniform size, useful as beads e.
In wheat and many other plants, average seed size does not depend on planting density, showing that seed dating for cancer patients is uab dating rather strict genetic control. This does not necessarily preclude significant variation among individual seeds; in peasfor 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 sativawhich grows in flax fields. The customary winnowing of flax seeds selects forms of C.
Seeds form the main source of food for many birdsrodentsantsand beetles. Harvester ants of the genus Veromessorfor example, exact a toll of about 15, seeds per acre 37, seeds per hectare per year from the Sonoran Desert of the southwestern United States. 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.
In contrast, the mean seed weight of the 23 species invulnerable by virtue of toxic seed constituents is 3 grams 0. Ecologically, seed size is also important in the breaking of dating site scams. Apart from the importance of shape as a factor in determining the mode of dispersal e.
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 turgidumwhich 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. 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. 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 arvensisfor 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.
Somatic polymorphism may be controlled by the position of the two or more seed types within one inflorescence flower cluster or fruitas in cockleburor it may result from environmental effects, as in Halogetonin 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. 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, nutsand cereals.
Such a dispersal unit is referred to as a diaspore. The dispersing agents for seeds and diaspores are indicated in such terms as anemochoryhydrochoryand zoochorywhich mean dispersal by wind, water, and animals, respectively. Within the zoochorous group, further differentiation according to the carriers can be made: 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.
Snails disperse the small seeds of a very few plant species e. Earthworms are more important as seed dispersers. Many intact fruits and seeds can serve as fish bait, those of Sonneratiafor example, for the catfish Arius maculatus.
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 mammalsranging in size from mice and kangaroo rats to elephantseat and disperse seeds and fruits. In the tropics, chiropterochory dispersal by large bats such as flying foxesPteropus is free text dating vancouver 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 mangoesguavasbreadfruitcarob, and several fig species. In South Africaa 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 oak ridge national laboratory dinosaur carbon dating 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.
The last-named, with dispersal units highly resistant to damage best brazilian dating websites hot water and certain chemicals dyeshave 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 tinder hookup advice, such dating app market analysis those of the water nut Trapathe spines should probably be considered as anchoring devices.
Birdsbeing preening animals, rarely carry burrlike diaspores on their bodies. Small diaspores, such as those of sedges and certain grassesmay 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 viscivorusdeposits 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.