pollination(redirected from Polination)
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The transport of pollen grains from the plant parts that produce them to the ovule-bearing organs, or to the ovules (seed precursors) themselves. In gymnosperms, the pollen, usually dispersed by the wind, is simply caught by a drop of fluid excreted by each freely exposed ovule. In angiosperms, where the ovules are contained in the pistil, the pollen is deposited on the pistil's receptive end (the stigma), where it germinates. See Flower
Without pollination, there would be no fertilization; it is thus of crucial importance for the production of fruit crops and seed crops. Pollination also plays an important part in plant breeding experiments aimed at increasing crop production through the creation of genetically superior types. See Breeding (plant), Reproduction (plant)
Self- and cross-pollination
In most plants, self-pollination is difficult or impossible, and there are various mechanisms which are responsible. For example, in dichogamous flowers, the pistils and stamens reach maturity at different times; in protogyny, the pistils mature first, and in protandry, the stamens mature before the pistils. Selfing is also impossible in dioecious species, where some plants bear flowers that have only pistils (pistillate or female flowers), while other individuals have flowers that produce only pollen (staminate or male flowers). In monoecious species, where pistillate and staminate flowers are found in the same plant, self-breeding is at least reduced. Heterostyly is another device that promotes outbreeding. Here some flowers (pins) possess a long pistil and short stamens, while others (thrums) exhibit the reverse condition; each plant individual bears only pins or only thrums.
As immobile organisms, plants normally need external agents for pollen transport. These can be insects, wind, birds, mammals, or water, roughly in that order of importance. In some plants the pollinators are simply trapped; in the large majority of cases, however, the flowers offer one or more rewards, such as sugary nectar, oil, solid food bodies, perfume, sex, an opportunity to breed, a place to sleep, or some of the pollen itself. For the attraction of pollinators, flowers provide either visual or olfactory signals. Color includes ultraviolet, which is perceived as a color by most insects and at least some hummingbird species. Fragrance is characteristic of flowers pollinated by bees, butterflies, or hawkmoths, while carrion or dung odors are produced by flowers catering to certain beetles and flies. A few orchids, using a combination of olfactory and visual signals, mimic the females of certain bees or wasps so successfully that the corresponding male insects will try to mate with them, thus achieving pollination (pseudocopulation).
While some flowers are “generalists,” catering to a whole array of different animals, others are highly specialized, being pollinated by a single species of insect only. Extreme pollinator specificity is an important factor in maintaining the purity of plant species in the field, even in those cases where hybridization can easily be achieved artificially in a greenhouse or laboratory, as in most orchids. The almost incredible mutual adaptation between pollinating animal and flower which can frequently be observed exemplifies the idea of coevolution. See Pollen
the transfer of pollen from anther to stigma of the pistil in angiosperms or from anther to ovule in gymnosperms. Pollen tubes subsequently develop from the pollen grains and deliver the male sex cells, the spermatozoids, to the ovum in the ovule, where fertilization and development of the embryo occur.
Angiosperms are characterized by two kinds of pollination— cross-pollination and self-pollination (autogamy). In cross-pollination, pollen from one flower is transferred to the stigma of another flower on the same plant (geitonogamy) or on another plant (xenogamy). In self-pollination, the pollen from the stamens is transferred to the stigma of the same flower. An embryo produced by xenogamy has characteristics of both parent plants. In self-pollination, improvement of the daughter organism, as a rule, does not occur. In this way, cross-pollination has an advantage over self-pollination. Many plants, however, reproduce only by self-pollination. Their flowers are always bisexual, the anthers mature simultaneously with the pistils, and the pollen easily makes contact with the stigma.
In the process of evolution, many features associated with pollination appeared in plants. Examples are the development of bisexual flowers and, especially, dioecious plants. The anthers and pistils of bisexual flowers often do not mature simultaneously. This separation in time is called dichogamy. In such cases the anthers usually mature earlier than the pistils (protandry); rarely do the pistils mature first (protogyny). Some plants are marked by heterostyly, by which the stamens and pistils in different flowers are of different lengths. In many cross-pollinating plants the pollen germinates poorly or not at all on the stigma of its own pistil, thus barring self-pollination. Cross-pollination is effected by animals (zoophilia), wind (anemophily), and water.
Most flowering plants are pollinated by animals, the majority by insects (entomophily), fewer by birds (ornithophily), and still fewer by such mammals as bats and rodents. Animals visit plants to obtain nectar or pollen.
Zoophilous plants generally have large, irregular, and sticky pollen grains, which easily adhere to the body of an animal. The structure of their flowers usually enables the visiting animal to touch both the anthers and the stigma. The flowers are generally brightly colored and have a rather strong fragrance to help guide the animal to them. Special adaptations for extracting nectar and/or pollen have been elaborated in animal pollinators. Some plants are pollinated only by those insects whose cycle of development is closely related to the rhythm of development of these plants. Thus, the fig tree is pollinated only by fig wasps, and the yucca is pollinated only by the yucca moth.
Wind pollination is principally a phenomenon of plants in open spaces. It characterizes many trees. Anemophilous plants, as a rule, have multiflorous inflorescences that are easily shaken by the wind. They also have long stamens with movable anthers, heavily branched stigmas, an enormous quantity of light, powdery pollen that can be carried long distances by the wind, and a poorly developed or absent perianth. Flowering occurs before leaf formation, or the inflorescences rise above the leaves (grasses and reeds).
Plants pollinated by the agency of water currents are few. Their anthers open underwater and the pollen is carried by the water to the stigma. In many plants characterized by this type of pollination, such as the common eelgrass, the pollen is filamentous. Sometimes the pollen grains germinate when the anther opens, with the pollen tubes helping them adhere to the stigma.
Some plants are pollinated by various methods. For example, the ribwort and the hoary plantain can be pollinated by both insects and wind. In many cross-pollinating plants, self-pollination occurs at the end of blossoming if cross-pollination did not occur earlier.
REFERENCESDarwin, C. Deistvie perekrestnogo opyleniia i samoopyleniia v rastitel’nom mire. Moscow-Leningrad, 1939. (Translated from English.)
Eames, A. Morfologiia tsvetkovykh rastenii. Moscow, 1964. (Translated from English.)
Takhtadzhian, A. L. Osnovy evoliutsionnoi morfologii pokrytosemennykh. Leningrad, 1964.
Pervukhina, N. V. Problemy morfologii i biologii tsvetka. Leningrad, 1970.
Kugler, H. Blütenökologie, 2nd ed. Jena, 1970.
V. N. VEKHOV