POLLINATION

transfer of pollen grains from the stamens, the flower parts that produce them, to the ovule-bearing organs or to the ovules (seed precursors) themselves. In plants such as conifers and cycads, in which the ovules are exposed, the pollen is simply caught in a drop of fluid secreted by the ovule. In flowering plants, however, the ovules are contained within a hollow organ called the pistil, and the pollen is deposited on the pistil's receptive surface, the stigma. There the pollen germinates and gives rise to a pollen tube, which grows down through the pistil toward one of the ovules in its base. In an act of double fertilization, one of the two sperm cells within the pollen tube fuses with the egg cell of the ovule, making possible the development of an embryo, and the other cell combines with the two subsidiary sexual nuclei of the ovule, which initiates formation of a reserve food tissue, the endosperm. The growing ovule then transforms itself into a seed. As a prerequisite for fertilization, pollination is essential to the production of fruit and seed crops and plays an important part in programs designed to improve plants by breeding. Furthermore, studies of pollination are invaluable for understanding the evolution of flowering plants and their distribution in the world today. As sedentary organisms, plants usually must enlist the services of external agents for pollen transport. In flowering plants, these are (roughly in order of diminishing importance): insects, wind, birds, mammals, and water. (Top) The contrasting yellow nectar guides of the blue iris (Iris) that indicate the 1/4 Pollination by insects probably occurred in primitive seed plants, reliance on other means being a relatively recent evolutionary development. Reasonable evidence indicates that flowering plants first appeared in tropical rain forests during the Mesozoic Era (about 65,000,000 to 225,000,000 years ago). The most prevalent insect forms of the period were primitive beetles; no bees and butterflies were present. Some Mesozoic beetles, already adapted to a diet of spores from primitive plants, apparently became pollen eaters, capable of effecting chance pollination with grains accidentally spared. The visits of such beetles to primitive flowering plants may have been encouraged by insect attractants, such as odours of carrion, dung, or fruit, or by sex attractants. In addition, visits of the insects to the plants could be made to last longer and thus potentially be more valuable to the plant as far as fertilization was concerned, if the flower had a functional, traplike structure. Nowadays, such flowers are found predominantly, although not exclusively, in tropical families regarded as ancient; e.g., the water lily (Nymphaeaceae) and the arum lily (Araceae) families. At the same time, other plants apparently began to exploit the fact that primitive gall-forming insects visited the flowers to deposit eggs. In the ancient genus Ficus (figs and banyan trees), pollination still depends on gall wasps. In general, Mesozoic flowering plants could not fully rely on their pollinators, whose presence also depended on the existence of a complete, well-functioning ecological web with dung, cadavers, and food plants always available. More advanced flowers escaped from such dependence on chance by no longer relying on deceit, trapping, and tasty pollen alone; nectar became increasingly important as a reward for the pollinators. Essentially a concentrated, aqueous sugar solution, nectar existed in certain ancestors of the flowering plants. In bracken fern even nowadays, nectar glands (nectaries) are found at the base of young leaves. In the course of evolutionary change, certain nectaries were incorporated into the modern flower (floral nectaries), although extrafloral nectaries also persist. Flower colours thus seem to have been introduced as "advertisements" of the presence of nectar, and more specific nectar guides (such as patterns of dots or lines, contrasting colour patches, or special odour patterns [see photographs]) were introduced near the entrance to the flower, pointing the way to the nectar hidden within. At the same time, in a complex pattern of parallel evolution, groups of insects appeared with sucking mouthparts capable of feeding on nectar. In extreme cases, there arose a complete mutual dependence. For example, a Madagascar orchid, Angraecum sesquipedale, with a nectar receptacle 20 to 35 centimetres (eight to 14 inches) long, depends for its pollination exclusively on the local race of a hawkmoth, Xanthopan morganii, which has a proboscis of 221/2 centimetres (nine inches). Interestingly enough, the existence of the hawkmoth was predicted by Charles Darwin and Alfred Russel Wallace, codiscoverers of evolution, about 40 years before its actual discovery. the transfer of pollen grains in seed plants from the stamens, where they are formed, to the ovules or ovule-bearing organs. Pollination is a prerequisite for fertilization and the production of seeds. In flowering plants the ovules are contained at the base of a hollow pistil, and the pollen is deposited on the pistil's surface, the stigma. There the pollen grains germinate and form pollen tubes that grow downward toward the ovules. Fertilization takes place as a sperm cell in a pollen tube fuses with the egg cell of an ovule, giving rise to the plant embryo. The ovule then grows and becomes a seed. Since the pollen-bearing parts of the stamens are rarely in contact with the stigma, plants commonly rely on external agents for pollen transport. Insects and wind are the most important pollinators; other agents include birds and a few mammals. Water transport of pollen is rare. Very ancient flowering plants apparently developed odours and mechanical traps to attract and hold beetles, which would scatter stray grains after they fed on the pollen. More advanced species began to produce nectar, a concentrated sugar solution found in modern flowers, and developed patterns of colour as guides for insects to the presence of nectar. Insects acquired sucking mouthparts capable of feeding on the nectar. Fertilization of an egg cell may take place by self-pollination, when the sperm cell derives from pollen produced by the same flower or by another flower on the same plant; or by cross-pollination, when the sperm comes from the pollen of a different plant. Cross-pollination, producing seeds that contain the hereditary traits of two parent plants, tends to result in a species that is sturdier and more adaptable to environmental changes. Many plants have developed mechanisms that prevent self-pollination. Some species produce only male (staminate) flowers on some plants and only female (pistillate) flowers on other plants. Species with staminate and pistillate flowers on the same plant and those with flowers having both stamens and pistils often avoid self-pollination by shedding their pollen either before or after the stigmas on the same plant are receptive to it. Another preventive device is chemical self-incompatibility; the pollen either fails to grow on a stigma of the same plant or fails to produce pollen tubes that reach the ovules. Self-pollination is sometimes advantageous, however, and may take place when external pollinators are scarce, when plants are widely scattered, or when cross-pollination has not occurred at the end of a flower's life span. It is common among annual plants, which often produce an abundance of seed to ensure species survival. A blue iris (Iris) with contrasting yellow nectar guides indicating the location of the 1/4 Prominent nectar guides on the lower spurred petal of the viola (Viola). The most important insect pollinators are the bees, which live on nectar and feed their larvae on pollen and honey, a modified nectar. Bees have long tongues that can reach the nectar; hairy bodies that pick up sticky pollen grains, facilitating their transport to another flower; and a sensitivity to colours and aromas that helps guide them to nectar-producing flowers. Bee flowers are brightly coloured, fragrant, and open in the daytime. Many have a broad lower lip on which the bee can land before pushing its way inside the flower (see photograph). Spikes of sedge (Carex pendula) showing reduced floral parts adapted to wind pollination. 1/4 Pollination by wind, although prevalent in the conifers and cycads, is less common in flowering plants. Plants that depend on wind pollination produce copious quantities of light, powdery pollen that can be blown a considerable distance; but even so, an individual stigma is likely to be hit by only one or two grains. To facilitate exposure to the wind, the flowers often bloom before the leaves come out in the spring, or they may grow high up on the tree or plant. Stigmas tend to be long and lobed to provide a large area for catching pollen grains, and plants of one species frequently grow in dense populations. Wind flowers are usually inconspicuous and devoid of the fragrance, colours, and nectar that attract insects. Costa's hummingbird (Calypte costae) foraging for nectar in the bright red tubular flowers 1/4 Birds are important as pollinators, especially in tropical regions. Many species have adapted to feeding on nectar, pollen, or flower-inhabiting insects or spiders by developing slender beaks and brushlike or tubelike tongues. Birds rely on their powerful vision and colour sense in their quest for food, and bird-pollinated flowers often display vivid colours. They generally lack the fragrance of most insect flowers, are bigger and more sturdily built, and produce greater quantities of nectar. The stamens are usually numerous and protruding so that they touch the bird as it feeds. Some bird flowers provide a landing platform or a perch of twigs near the flower. A few species of small mammals, notably certain bats, also help to pollinate plants as they feed on the flowers. Additional reading Types and agents of pollination are analyzed in K. Faegri and L. van der Pijl, The Principles of Pollination Ecology, 3rd rev. ed. (1979), a well-balanced work based on extensive experience in both tropical and temperate regions; Michael Proctor and Peter Yeo, The Pollination of Flowers (1973), an excellent work on the process of pollination; and Bastiaan Meeuse and Sean Morris, The Sex Life of Flowers (1984). Bastiaan J.D. Meeuse The Editors of the Encyclopdia Britannica