Water-fertilized, or hydrophilous, plants, are those in which the pollen is transferred through the agency of water, and are not very common. Following Delphino, they may be divided into two types: the first type includes Zostera, Posi donia, etc., and the Florideee, where the pollen is of the same specific gravity as water, and is carried here and there by water currents; the second type includes such plants as Ruppia and Vallisneria, in which the pollen is lighter than water, or is borne on a floating raft formed by the loosened flower. The peduncles of the fe male flowers elongate and bring them to the surface of the water where their position allows the stigma to be pollinated by the floating pollen.
By far the greatest number of plants that require cross-fertilization depend upon insects as pollen carriers, and these plants have been termed entomophilous. Such plants are character ized by the large size, showy colors and mark ings of their flowers and their odor, which serve to advertise the nectar and nourishing pollen that the flower contains and thus attract insects to the flower. The insects in passing from flower to flower in search of nectar and pollen become useful to the plant by incidentally trans ferring pollen from one flower to the stigmas of other flowers, thus causing cross-fertilization. Flowers have in many cases become adapted to certain insects and have stamens and pistils de veloped in the position best suited to ensure pollination when these insects visit the flowers for nectar or pollen. Many insects depend wholly, or in large part, on the nectar and pollen of flowers as food, and such insects usually visit only one kind of flower during the day and therefore carry but one kind of pollen. They work systematically, passing from one flower to another, and clearly do an enormous amotmt of crossing and waste less pollen than the wind or water. Some insects, like certain beetles, have smooth bodies and carry but little pollen, but many beetles, and all bees, moths, butter flies, etc., have their bodies, wings and limbs roughened with hairs and scales, and these col lect and retain a large quantity of pollen ready to be left on the sticky stigmatic surface of the pistil of the flower when the insect rubs against it. The nectar glands of the flower are usually located in such a position that the insect in get ting to them to suck the nectar must enter the flower in the best way to ensure cross-pollina tion. Insects are greatly attracted by odors, and the flowers of many plants have a strong scent which serves to make them more attractive.
The evening primrose ((Enothera), which opens early in the evening and is fertilized by night flying moths, has a very strong odor, and the same is true of many night-fertilized flowers, such as the honeysuckle (Lonicera caprifolium), night-blooming Cereus, etc. Night-blooming flowers which are fertilized by insects are usu ally white and generally more sweet-scented than day-blooming flowers. Some flowers, such as the Stapelias, are purplish or brownish, re semble decayed flesh in appearance and are car rion-scented to attract carrion flies. Certain in sects are attracted by certain colors more than others. The favorite color of the honey-bee, for instance, is said to be deep bluish-violet, while pure blue and violet are pleasing. Yellow is less sought, but is not avoided, while red is dis liked and shunned. Kerner states that in the Vienna Botanical Gardens the honeytbees in great numbers visit the bluish-violet flowers of Monorda fistulosa, and the blue flowers of the hyssop (Hysopus oificinalis), but avoid the scar let flowers of the Monarda didyma. Experi ments, however, tend to show that color does not necessarily attract insects; flowers rich in nectar are the most attractive. In some plants the involucre is highly colored and takes the place of the colored parts of the flower. Such is the case in some Euphorbias such as snow-on-the-mountain, which is nearly white, and the poinsettia (E. pulcherrima), in which the involucre is red. In a large number of cases the petals of large flowered species show spots or lines on the main part of the flower of different color from the main ground color of the petals. These are usually known as nectar guides and are believed to be of serv ice in aiding insects to find the nectar.
One of the most interesting and instructive modifications to secure cross-fertilization is the formation on different plants of flowers with different lengths of stamens and styles. Spren gel noted that some plants in Hottonia bear only flowers whose anthers are included in the tube, but whose style is exserted; while other plants bear only flowers having short styles and long stamens, longer than the flowing tube. He was, however, unable to suggest any reason for such variations. Some plants, such as Lythrum, reg ularly show these different lengths of stamen and style. Darwin subjected the peculiar sexual relations of these plants which he called di morphic and trimorphic, to most careful and extensive research, crossing the different forms back and forth in various ways. The results of these researches are summarized below.