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Thursday, August 2, 2007

"Pollination- The Foremost ecological discipline "


Why pollination is an ecological discipline??

Pollination is the transfer of pollen from the male reproductive organ (stamen or staminate cone) to the female reproductive organ (pistil or pistillate cone) of the same or of another flower or cone. Pollination is not to be confused with fertilization, which it may precede by some time—a full season in many conifers. The most common agents of pollination are flying insects (as in most flowering plants) and the wind (as in many trees and all grasses and conifers), but crawling and hopping insects, snails, bats, primates, rodents, and hummingbirds may also serve. The devices that operate to ensure cross-pollination and prevent self-pollination are varied and sometimes extremely intricate. Among them are different maturation times for the pollen and eggs of the same flower or plant, separate staminate and pistillate flowers on the same or on different plants, chemical properties that make the pollen and eggs of the same plant sterile to each other, and specialized mechanisms or structural arrangements that prevent the pollinating agent from transferring the pollen of a flower to its own stigma. In the lady's-slipper the bee enters the nectar-filled pouch by one opening and must leave by another; in so doing it brushes first past the stigma, which scrapes pollen off its back, and then past the stamens, which deposit another load of pollen. The stamens of the mountain laurel are bent back and held like springs by notches in the petals; when the bee alights it contacts the tall pistil and then, in probing deeper for nectar, triggers the stamens. Pollen is catapulted onto the insect's underside, ready for contact with the next pistil. Other examples of floral adaptations to their pollinating agents are the fig and its wasp and the yucca and its moth. Wind pollination, depending as it does on statistical chance for successful pollination, requires vast quantities of pollen, which may be forcefully ejected by the anther sac (as in grasses and ragweed) or may be exposed (as in cones and catkins) to the slightest breeze.




Breeding systems in plants refer to the variety of ways plants answer the general question of "Who mates with whom" by answering specific questions such as whether flowers mature at the same time, whether a plant has more than one kind of flower or differs from other plants in types of flowers, and whether there are chemicals that keep certain plants from mating with each other.




Most vertebrate species consist of separate male and female individuals. In contrast, the majority of flowering plants are hermaphroditic, with both pollen and ovules produced by the same plant. As a consequence, many flowering plants are capable of self-fertilization (selfing), with seeds resulting from pollen and eggs produced by the same plant. The self pollen that fertilizes the egg may be produced by the same flower (called autogamy) or by different flowers on the same plant (geitonogamy). Selfing or mating among close relatives (inbreeding) often results in offspring that have reduced vigor and produce fewer offspring compared to offspring from matings between unrelated plants (outcrossing). This reduction in fitness of selfed offspring relative to outcrossed offspring is referred to as inbreeding depression. If both selfing levels and levels of inbreeding depression are high, natural selection may favor mechanisms that promote outcrossing. High levels of inbreeding depression are likely to be found in populations that have been outcrossing for a long time. In contrast, in populations that have been inbreeding (high selfing rates) for many generations, inbreeding depression levels may now be low because harmful genes have already been eliminated from the population by natural selection.
Selfing may also have direct advantages. Selfing plants may have an automatic selection advantage and contribute more genes to the next generation because they contribute both maternal genes (through the egg) and paternal genes (through pollen) to selfed seeds, and they also contribute pollen (and thus paternal genes) to other plants, spreading their genes further. In contrast, outcrossing plants contribute pollen to other plants, but only maternal genes to their own seeds. This automatic selection advantage will lead to selection for selfing if selfing does not decrease outcrossing (thereby limiting the spread of genes), and if inbreeding depression is not too severe. In addition, selfing tends to produce offspring more similar to the parent plant than outcrossing. If seeds are dispersed locally into habitat similar to that of the parent, these selfed offspring may do better than outcrossed offspring. In habitats where pollen is limited because of low population density or because there are few pollinators, selfing may also provide reproductive assurance with a guaranteed source of pollen. Some plants, such as touch-me-not (Impatiens) and some violet species (Viola) have evolved flowers that are pollinated autogamously and never open (called cleistogamy), as well as the more showy open flowers (chasmogamy).




Pollination is a process involving flowers (the pollen source and stigma receiver) and a pollination vector (an agent carrying pollen to the stigma). The study of pollination is an interdisciplinary field involving botany (the plant side) and some other field (the vector side). In some ways, then, it is an ecological discipline.

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