If we compare the flowers of different plants, we shall find almost infinite variety in structure, and this variation at first appears to follow no fixed laws; but as we study the matter more thoroughly, we find that these variations have a deep significance, and almost without exception have to do with the fertilization of the flower.

In the simpler flowers, such as those of a grass, sedge, or rush among the monocotyledons, or an oak, hazel, or plantain, among dicotyledons, the flowers are extremely inconspicuous and often reduced to the simplest form. In such plants, the pollen is conveyed from the male flowers to the female by the wind, and to this end the former are usually placed above the latter so that these are dusted with the pollen whenever the plant is shaken by the wind. In these plants, the male flowers often outnumber the female enormously, and the pollen is produced in great quantities, and the stigmas are long and often feathery, so as to catch the pollen readily. This is very beautifully shown in many grasses.

If, however, we examine the higher groups of flowering plants, we see that the outer leaves of the flower become more conspicuous, and that this is often correlated with the development of a sweet fluid (nectar) in certain parts of the flower, while the wind-fertilized flowers are destitute of this as well as of odor.

If we watch any bright-colored or sweet-scented flower for any length of time, we shall hardly fail to observe the visits of insects to it, in search of pollen or honey, and attracted to the flower by its bright color or sweet perfume. In its visits from flower to flower, the insect is almost certain to transfer part of the pollen carried off from one flower to the stigma of another of the same kind, thus effecting pollination.

That the fertilization of a flower by pollen from another is beneficial has been shown by many careful experiments which show that nearly always—at least in flowers where there are special contrivances for cross-fertilization—the number of seeds is greater and the quality better where cross-fertilization has taken place, than where the flower is fertilized by its own pollen. From these experiments, as well as from very numerous studies on the structure of the flower with reference to insect aid in fertilization, we are justified in the conclusion that all bright-colored flowers are, to a great extent, dependent upon insect aid for transferring the pollen from one flower to another, and that many, especially those with tubular or zygomorphic (bilateral) flowers are perfectly incapable of self-fertilization. In a few cases snails have been known to be the conveyers of pollen, and the humming-birds are known in some cases, as for instance the trumpet-creeper (Fig.121, A), to take the place of insects.[14]

At first sight it would appear that most flowers are especially adapted for self-fertilization; but in fact, although stamens and pistils are in the same flower, there are usually effective preventives for avoiding self-fertilization. In a few cases investigated, it has been found that the pollen from the flower will not germinate upon its own stigma, and in others it seems to act injuriously. One of the commonest means of avoiding self-fertilization is the maturing of stamens and pistils at different times. Usually the stamens ripen first, discharging the pollen and withering before the stigma is ready to receive it, e.g. willow-herb (Fig.113, D), campanula (Fig.123, A, D), and pea; in the two latter, the pollen is often shed before the flower opens. Not so frequently the stigmas mature first, as in the plantain (Fig.121, G).

In many flowers, the stamens, as they ripen, move so as to place themselves directly before the entrance to the nectary, where they are necessarily struck by any insect searching for honey; after the pollen is shed, they move aside or bend downward, and their place is taken by the pistil, so that an insect which has come from a younger flower will strike the part of the body previously dusted with pollen against the stigma, and deposit the pollen upon it. This arrangement is very beautifully seen in the nasturtium and larkspur (Fig.99, J).

The tubular flowers of the SympetalÆ are especially adapted for pollination by insects with long tongues, like the bees and butterflies, and in most of these flowers the relative position of the stamens and pistil is such as to ensure cross-fertilization, which in the majority of them appears to be absolutely dependent upon insect aid.

The great orchid family is well known on account of the singular form and brilliant colors of the flowers which have no equals in these respects in the whole vegetable kingdom. As might be expected, there are numerous contrivances for cross-fertilization among them, some of which are so extraordinary as to be scarcely credible. With few exceptions the pollen is so placed as to render its removal by insects necessary. One of the simpler contrivances is readily studied in the little spring-orchis (Fig.89) or one of the Habenarias (Fig.90, G). In the first, the two pollen masses taper below where each is attached to a viscid disc which is covered by a delicate membrane. These discs are so placed that when an insect enters the flower and thrusts its tongue into the spur of the flower, its head is brought against the membrane covering the discs, rupturing it so as to expose the disc which adheres firmly to the head or tongue of the insect, the substance composing the disc hardening like cement on exposure to the air. As the insect withdraws its tongue, one or both of the pollen masses are dragged out and carried away. The action of the insect may be imitated by thrusting a small grass-stalk or some similar body into the spur of the flower, when on withdrawing it, the two pollen masses will be removed from the flower. If we now examine these carefully, we shall see that they change position, being nearly upright at first, but quickly bending downward and forward (Fig.89, D, ii, iii), so that on thrusting the stem into another flower the pollen masses strike against the sticky stigmatic surfaces, and a part of the pollen is left adhering to them.

The last arrangement that will be mentioned here is one discovered by Darwin in a number of very widely separated plants, and to which he gave the name “heterostylism.” Examples of this are the primroses (Primula), loosestrife (Lythrum), partridge-berry (Mitchella), pickerel-weed (Pontederia), (Fig.84, I), and others. In these there are two, sometimes three, sets of flowers differing very much in the relative lengths of stamens and pistil, those with long pistils having short stamens and vice versa. When an insect visits a flower with short stamens, that part is covered with pollen which in the short-styled (but long-stamened) flower will strike the stigma, as the pistil in one flower is almost exactly of the length of the stamens in the other form. In such flowers as have three forms, e.g. Pontederia, each flower has two different lengths of stamens, both differing from the style of the same flower. Microscopic examination has shown that there is great variation in the size of the pollen spores in these plants, the large pollen from the long stamens being adapted to the long style of the proper flower.

It will be found that the character of the color of the flower is related to the insects visiting it. Brilliantly colored flowers are usually visited by butterflies, bees, and similar day-flying insects. Flowers opening at night are usually white or pale yellow, colors best seen at night, and in addition usually are very strongly scented so as to attract the night-flying moths which usually fertilize them. Sometimes dull-colored flowers, which frequently have a very offensive odor, are visited by flies and other carrion-loving insects, which serve to convey pollen to them.

Occasionally, flowers in themselves inconspicuous are surrounded by showy leaves or bracts which take the place of the petals of the showier flowers in attracting insect visitors. The large dogwood (Fig.110, J), the calla, and Jack-in-the-pulpit (Fig.86, A) are illustrations of this.