One of the Rosin-weeds, Silphium perfolia tum, has large sheathing leaves which form a cup around the stem. The water in the cup prevents insects from crawling up to the flow ers. Only flying insects, which could carry the • pollen to other plants, can reach the flowers. Many plants prevent self-pollination in this way.
In most of the cases mentioned, it is true the pollen of a flower is dusted upon its own stigma, but it is practically certain that when pollen from its own anthers and pollen from the anthers of another flower fall together upon the stigma, the foreign pollen is more likely to germinate.
A couple of instances of insect pollination, no more remarkable than scores of others, may be given as illustrations of the complicated relations between certain insects and certain plants. In the Fig, pollination is effected by small wasps. There are two kinds of urn shaped inflorescences which we call figs, borne on different trees. One, which is called Cap rificus, has male flowers near the neck of the urn and short-styled female flowers farther down; the other, called Ficus, has only female flowers with long styles. The wasp thrusts its ovipositor down through the short style of the female flower Caprificus and lays an egg in the ovary and a larva begins to develop, caus ing the flower to become distorted into a gall. As the wasps mature, the males escape first and fertilize the females which are still in the gall flowers. The males then leave the urn before the pollen is ripe, but the females stay awhile— just long enough for the pollen to ripen —and then escape, becoming dusted with pollen as they pass through the narrow opening at the top of the urn. Then they visit the younger urns, dusting the stigma with pollen and laying eggs. In case of the short-styled flowers of Caprificur, galls are produced; but the wasp's ovipositor is not long enough to reach the ovary of the long-styled flowers of Ficus and, conselauently, pollen tubes develop normally, fertilization follows and normal seeds are matured.
Another remarkable instance of insect pol lination is seen in the Yucca, an arborescent member of the lily family, with rigid, sword like leaves, which has given it the name of °Spanish Bayonet') or "Spanish Dagger.* Pol lination is effected by the moth, Prortuba yuc casella. The female visits the flowers for the pollen, which she .does not eat, but molds into a ball and carries to another flower. Here she bores through the wall of the ovary and de posits her eggs in the young ovules and then crowds the ball of pollen into the funnel-shaped top of the stigma, thus effecting abundant pol lination, which causes the young ovules to grow rapidly. The developing larvae eat many of the young ovules, but soon bore their way out and escape, while the ovules which are left develop into good seeds.
In the case of yucca and pronuba, the moth has been considerably modified so that it is adapted to rolling up the ball of pollen. Many flowers are adapted to pollination by certain in sects and many insects are adapted to certain flowers, so that a scarcity of its particular flower means hardship or death to a certain species of insect ; while a scarcity of some par ticular insect may mean the extinction of the particular flower which it pollinates. These mutual adaptations are very numerous among the orchids.
In all the foregoing instances, pollination results between flowers of the same species. When effective pollination occurs between plants of different varieties, species or genera, and fertilization follows, the resulting individ ual is called a hybrid. It often happens that hybrids are more vigorous than either of the parents and it is often possible to combine desirable characteristics of different species by artificial pollination. An anther is plucked out from a flower and its mature pollen is dusted upon the stigma of the other species. This method has been of immense value in improv ing various fruits and crops. It is well known that the male and female are equally poterit in transmitting hereditary characters, but cross pollination has shown that, in some cases at least, some particular characters are regularly determined by the pollen. In corn, if self pollination be prevented and field corn, which has a starchy kernel, be pollinated with sweet corn, which has a sugary kernel, the field corn stalks will bear an ear with sugary kernels; and, similarly, starchy kernels can be grown upon sweet corn stalks. In corn, pollination by a red-eared parent will produce ears with red kernels or with kernels showing various mix tures of red and yellow. By pollinating differ ent stigmas of an ear with different pollen, it is possible to produce on the same ear both red and yellow and also starchy and sugary grains.
There are scores of books on pollination and a vast number of articles in magazines. Dr. Knuth, in his exhaustive treatise, lists 3,748 titles, most of them dealing with pollina tion in connection with other features of the life history but many of them devoted entirely to this subject.
Bibliography.— Knuth, Dr. Paul, 'Hand book of Flower Pollination' (Oxford 1906). This work in three large volumes gives citations to practically all the literature up to 1906) ; Darwin, Charles, 'The Effects of Cross and Self-Fertilization of Plants) (London 1888), and 'The Various Contrivances by which Orchids are Fertilized by Insects) (New York 1892).