Insects are the chief marriage priests of the plant world, but in the tropics they are aided and abetted by Humming-Birds, Sun-Birds and Lories, which are all provided with long, tubular tongues.
Most insects act as if they were unaware of the important place they occupy in plant hymeneals. So intent are they on their honey-gathering that they become covered from head to foot with pollen without appearing to notice it. Yet in a few instances, the Bees not only recognize that they have been pressed into the plant’s messenger service, but by underhand methods seek the rewards of labour without giving adequate return. They have learned how to cut a hole in the calyx tube of the Bean and the Scarlet Runner, and get at the precious honey by short cut. If all Bees and other fertilizing insects should master this trick, the flowers would have to wear defensive armour or perish.
Pollen to be effective must remain dry. The plants have perfected many devices to shield it from moisture. Frequently, the flowers hang so that their petals act as tiny umbrellas for it. Others wear rainy day hoods, and practically all close when the night mists are abroad.
The necessity for dry pollen obtains even among the water plants. If they are surface-floaters like the Pond Lily or the Victoria Regia, it is easy enough for them to thrust their blossoms up into the air, where they may be as dry as though they were on land. The sub-aqueous plants have a harder problem and are sometimes driven to developing their flowers in leaf air-chambers below the surface. The Water Chestnut (Trapa Natans) makes itself buoyant at its flowering period with generated air and rises en masse to the surface. After fertilization, it sinks again to its sub-aqueous quiet.
Self-fertilization in its strictest sense occurs within the individual flower. Plants only resort to it as an extreme measure and commonly make use of many devices to prevent it. In the Iris, the petal-like stamens are in direct contact with the pistil and yet self-fertilization does not result, because the pollen surface is always carefully turned away from the ovary.
By bringing their pistils and stamens to maturity at different times, many flowers make sure that they will not fertilize themselves. Such is the case in the Bulbous Buttercup and the Arrowhead.
Flowers of the same tree or bush might be called distant cousins. Their union results in healthy offspring, though the marriage of still more divergent individuals is preferable. Plants like the Begonia, which bear single-sex flowers, often grow in somewhat isolated positions and so must intermarry a great deal among themselves. Staminate flowers at the top of a stalk can shower pollen over many female flowers growing below them.
The exception always proves the rule, which explains why we find a few flowers which deliberately choose to fertilize themselves. In the Fuchsia, the flower droops, throwing the long pistil below the stamens, which can readily drop pollen onto it. Minute hooks hold the petals of the Indigo and Lucerne partly closed until the flower is completely developed. When they give way, the petals fly back, so shaking the whole flower that the anthers shower pollen on the pistil. The single-sex flowers of the Aloe bend near each other at mating time.
The Violets and Polygalas are also largely self-fertilizing. They are, therefore, borne under the leaves or close to the ground, where they attract little attention.
The love and marriages in plantdom may seem to be largely instinctive and mechanical, but that is probably because we have not investigated them sufficiently. The Persian poet Osmai believed that the plants had affairs of the heart as real as those recorded in the human world. Here is his account of one:—