Friday, March 28, 2008



Pollination can occur in two ways: self-pollination-the transfer of pollen from an anther to a stigma in the same flower or to a flower on the same plant-and cross-pollination-the transfer of pollen from one plant to a stigma on a different plant, but of the same species. Self-pollination can be an effective method of pollination, but it has the disadvantage of excluding genetic variability. Cross-pollination, on the other hand, increases genetic variability within species and is promoted by many plants in a variety of ways. In some species, unisexual (male or female) flowers are produced on separate plants. This condition is called dioecy and guarantees cross-pollination. Chemical reactions between the pollen and stigmas of the same plant prevent self-pollination in many species, usually by hindering germination of the pollen grains or preventing the growth of pollen tubes. Another, perhaps less effective method for ensuring cross-pollination involves the timing of maturation of stamens and pistils: in some species the anthers release pollen before the stigmas of the same plant are ready to receive it, or the stigma may be isolated from the mature pollen produced by the same plant because its styles have not yet elongated. Pollen is usually able to germinate for only a short period after being released from the anthers, and so these species are much more likely to be pollinated by another plant.

Wind and animals are the major agents of pollination. The flowers of wind-pollinated plants are usually small, greenish, lack fragrance, and have small petals or no petals. Oaks and grasses are examples of wind-pollinated plants. The wind generally does not carry pollen far, and so individual plants frequently grow close to one another to help guarantee pollination. Nevertheless, wind delivery of pollen to stigma is not a particularly precise procedure, and some plants have evolved more accurate pollination mechanisms-they employ animals as pollinators.

Flowers must be able to attract animals if animals are to be efficient pollinators. This has been accomplished through the evolution of flower colors, shapes, sizes, and fragrances that appeal to different animals. Plants use other strategies to lure pollinators to their flowers as well, including the production of sweet nectar and nutritious pollen. However, once an animal visits a flower and comes in contact with its pollen, it needs to visit another flower of the same species for pollination to occur. Inflorescences, more or less organized clusters of two or more flowers, may have evolved in many plants to ensure visits to different flowers. In some plant groups, such as the aster family, flowers have undergone a great deal of modification and are grouped together in "head-like" inflorescences that frequently look like a single flower. The sunflower, for example, is actually made up of hundreds of tiny, separate flowers. Animal pollination has advantages for both the plants and their pollinators. The plants are able to disperse their pollen accurately and widely. The animal pollinators benefit by having a reliable and easily recognizable food source. The development of this relationship between plants and animals has been one of the most important factors in of the evolution of both groups. This coevolution between plants and their animal pollinators has meant that flowers have become adapted to attract specific animals. For example, yellow flowers are primarily attractive to bees, while red flowers attract birds. Some plants produce nectar deep inside their flowers that can be reached only by hummingbirds (with their long bills) or long-tongued insects such as moths and butterflies. Once flowers have evolved to attract a specific pollinator, they must be able to continue to attract that particular pollinator or the chances for successful pollination will be drastically reduced.

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