The New Student's Reference Work/Pollination

Pol′lina′tion, the transfer of the pollen of plants from the stamen which produces it to the stigma which is prepared to receive it. Pollination is frequently confused with fertilization. Fertilization is the union of the male cell and egg, which may or may not follow pollination. In general this transfer of pollen is effected by one of two agents, the wind or insects. The plants which use wind as a carrier of pollen are called anemophilous plants; those which use insects for this purpose are called entomophilous. The gymnosperms and many of the more primitive angiosperms, as grasses and many of the common trees, are anemophilous. The great majority of angiosperms are entomophilous, and their flowers have been modified in every way to be adapted to insect-visits, and the insects have been variously adapted, to flowers. For anemophilous pollination see. Entomophilous pollination is of two kinds: namely, self-pollination, in which the pollen is transferred to the stigma of its own flower; and cross-pollination, in iwhich the pollen is transferred to the stigrna of some other flower. In the case of cross-pollination the two flowers concerned may be upon different plants which may be quite distant from one another. The advantage of this relation to the insects is to secure food. This the flower provides either in the form of nectar or pollen, and the insects which pollinate flowers may be roughly divided into the two groups of nectar-feeding insects, represented by butterflies and moths, and the pollen-feeding insects, represented by bees, wasps etc. The presence of these food-supplies is made known to the insect by odor, form and, perhaps, color of flowers. Those insects which are suitable for this work are necessarily flying forms, as creeping insects would brush off the pollen from their bodies in passing from one flower to another. Plants have many devices by which such unsuitable creeping insects as ants may be warded off. Just what the advantage of cross-pollination over self-pollination may be is not clear, but that it is of great advantage is evident from the fact that the angiosperms have made most elaborate attempts to secure it. In most insect-pollinating flowers there are three problems: To hinder self-pollination; to secure the visits of suitable insects; and to ward off the visits of unsuitable insects. Most of the insect-pollinating flowers may fall into three great divisions on the basis of the methods they use to hinder self-pollination. The first division includes those flowers in which the pollen and stigma are so related to each other in position that the pollen is not likely to fall upon the stigma. This method results in much of the irregularity of flowers, and conspicuous among these forms are the sweet peas, irises and orchids. The story of cross-pollination in orchards has been told by Darwin in a wonderfully interesting book. The second division includes those flowers in which self-pollination is prevented, not by the relative positions of the parts, but by the fact that the pollen and stigma are not ready at the same time. When the stigma is ready to receive, the pollen may not be ready for shedding; or when the pollen is ready for shedding, the stigma may not be ready to receive. These are the so-called dichogamous flowers (which see). It is evident that there may be two groups upon this basis: Those flowers in which the pollen is ready first and those flowers in which the stigma is ready first. The former are called protandrous the latter protogynous flowers. The third division includes those flowers in which there are at least two forms, and the two forms differ from one another in the relative lengths of their stamens and styles and in the nature of their pollen. For example, in the common houstonia, a small tubular flower, one kind of flower has the stamens included in the tube and the stigmas protruding; while the other kind has the stigmas included in the tube and the stamens protruding. Each kind of pollen is most effective upon the stigma of the same level. Accordingly, as an insect passes from one such flower to another, crowding its way into the tube, its thorax is likely to receive a band of pollen from the short stamens and its abdomen another band from the long stamens. In this way the pollen from the thorax is rubbed upon the included stigmas and the pollen from the abdomen is rubbed upon the protruding stigmas. The whole story of the relation between insects and flowers is an exceedingly intricate one and full of surprises.