In case of wind pollination and water pol lination the male and female flowers are usually upon different plants or, at least, upon different parts of the same plant. In these cases cross pollination — the pollination of the stigma by pollen from another plant or from another flower on the same plant — results necessarily. Botanists agree that cross-pollination is the most efficient method and that close pollination, which results in close fertilization, is almost always bad for the species. However, there are some cases of efficient close pollination, chiefly in flowers which do not open, but always re main closed and, therefore, are called cleistog amous flowers. The common blue violet, in addition to its showy flowers, has small, nearly colorless flowers, looking like unpromising buds near the bases of the leaves. These flowers, entirely self-pollinated, produce good seed. But in most cases where the male and female struct ures, the anther and the pistil, are in the same flower, plants have worked out innumerable methods with innumerable variations in details to avoid close pollination. In nearly all of the cases, the pollen is carried from the anther to the stigma by insects. There are scores of books on insect pollination.
Most plants which are pollinated by insects have conspicuous flowers or a distinct odor or both. Some observers claim that the corolla does not attract insects, but none can deny the constant association of conspicuous flowers and insect pollination. All flowers pollinated by insects have nectar glands which secrete the nectar, which is the most usual cause of insect visits.
In many plants with stamens and pistils in the same flower, self-pollination is prevented by the maturing of the pollen and stigma at different times. The pollen matures and is shed before the stigma exudes the sticky sub stance which makes the pollen adhere to it and at the same time affords a nutritive medium in which the pollen tube begins to form; or the stigma and dries up before the pollen is ripe; consequently, the stigma could be pollinated only by pollen from another flower.
In many cases, however, the pollen and the stigma of a flower mature at the same time. In such flowers the devices to prevent self pollination are innumerable. Practically all such flowers are pollinated by insects. Some of the devices afford a general protection against self-pollination, but others are so complicated and specialized that only certain insects with certain peculiarities in structure can reach the nectar, and so none but these favored guests are tempted to visit the flower.
A common contrivance for the prevention of self-pollination is seen in species in which some flowers have long anthers and short pistils, while others have short anthers of the same length as the pistil in the other flower and long pistils of the same length as the anthers of the short-styled flower. The Bluet (Houstonia)
is a familiar example (Figs. 3 and 4). As the insect, usually a butterfly, thrusts its proboscis down the tube of a flower with short anthers (Fig. 3) to reach the nectar at the base of the ovary, the pollen adheres to the proboscis; then, as the butterfly visits a flower with a short style (Fig. 4) the pollen on the proboscis is at the right level to brush against the stigma and, at the same time, pollen from the long anthers adheres to the proboscis at the higher level, so that this pollen will be deposited on the stigma of any long-styled flower the butter fly may visit.
In the Snapdragon and similar flowers the two lips of the corolla are so tightly closed that small insects cannot get to the pollen or nectar, but the lips open when some heavy insect, like the bumble bee, alights upon the flower. Thus the pollen and nectar are not wasted upon visitors which would be of little or no use to the plant in effecting cross-pollina tion.
The common milkweeds secrete a sticky, milky substance which hardens rapidly when exposed to air. The epidermis of those plants is very delicate in the region of the flower duster, so that the sharp claws of ants or other creeping insects, which would be of no use to the plant in effecting cross-pollination, break through and the sticky substance holds them fast and prevents them from reaching the nectar or pollen. On the other hand, butterflies, which are very effective in accomplishing pollination in milkweeds, alight directly upon the flowers, securing the nectar and carrying the pollen to other flowers on the same plant or on different plants. In the milkweeds and in most of the orchids, the pollen grains stick together in large masses, called upollinie and, consequently, when pollination takes place at all it is very abundant.
The Catchfly (Silene antirrhina) as the flowers bloom exudes a sticky brownish sub stance which forms a girdle around the stem below the flowers, preventing any creeping in sects from reaching the nectar or pollen. Con sequently, pollination is effected only by flying insects. This hint from the Catchfly is used artificially on a large scale in the sticky girdles put around tree trunks to prevent the ascent of various pests.
Many plants, like the Evening Primrose, blossom only at night when there is not much danger from creeping insects, but when moths and other flying insects, particularly useful in effecting cross-pollination, are abroad in great numbers.