Sprengel, who at the close of the 18th century was the first to study closely the relation of flowers to insects, pointed out that the various rows of dots or lines with which flowers are marked lead to the places where the nectar was concealed. He called these nectar guides and proposed the theory, still generally accepted, that they indicate to the insects visiting the flower the position of the nectar and thus lead them to assume positions best suited for the process of cross-pollination. He also pointed out that the nectar is almost always well pro tected from rain. Nearly 90 years later, the Austrian botanist, A. Kerner von Marilaun, drew attention to the fact that not only was the nectar carefully protected from washing by rain but even more completely so from robbery by insects not suited to effect pollination. Thus ants are kept away from the flowers of many species of catchfly (Silene) by bands of a sticky secretion at or near the nodes, and a great many flowers are protected from these same unwelcome guests by sticky hairs on the stem or on the flower stalk or outside of the calyx. In the teasel (Dipsacus) the leaves are joined together at their bases to form cups which become filled by rain water and thus effectually bar the way for small creeping insects. Many flowers have tufts of soft or sometimes stiff hairs in such positions that small insects that reach the flower by flight cannot pass down to the nectar although the larger and stronger insects are able to push their tongues past the hairs to the nectaries, and in so doing to pol lenize the flower. Night-blooming flowers usu ally dispense no attractive odors during the day and frequently are without nectar until night, so that they are not liable to visits from the undesired day insects. Some flowers main tain such positions that only certain insects are able to enter them or to extract the nectar. Nevertheless many flowers are subject to rob bery by short-tongued bees which, instead of entering the flower by the regular way, a method impossible for them on account of the shortness of their tongues, go directly to the exterior of the flower just opposite the nectary and gnaw a hole through the base of the co rolla and in this manner gain possession of the nectar, without, of course, pollenizing the flower. Thus some species of monkshood (Aconitum) show a high percentage of their blossoms rifled in this manner. Even the flowers of red clover are frequently robbed by short-tongued bees that cannot reach the nec tar from the top of the flower.
Heterostyly.— Sprengel noticed that in cer tain plants the stamens and pistils were of dif ferent length in the flowers of some plants than in those of others of the same species. It was not, however, until Darwin studied the subject that the matter was cleared up. He showed that the common yellow cowslip or primrose of England (Primula officinalis) had two forms of flowers, the long-styled and the short-styled. (See Fig. 2). In the former the stigma is on a level with the top of the tube of the salverform corolla and the stamens are attached to the inside of the tube about half way from the base to the top. In the other form the stamens are attached at the top of the corolla tube and the short style brings the stigma only about half way to the top. The two forms are always on separate
plants. When visited by insects, chiefly bumble bees, the base of the proboscis becomes covered with pollen in the case of the short-styled flowers, and the middle portion with the long styled flowers. Upon visiting a flower of the opposite type the pollen upon the base of the proboscis is brushed off upon the stigma of the long-styled flower while that upon the mid dle of the proboscis pollenizes the stigma of the short-styled form. Darwin's further ex periments showed that while pollenization of a flower with pollen from another flower of that same type would result in a certain amount of good seed, yet these "illegitimate' unions were not nearly so productive of good seed as when short-styled flowers were pollenized with pollen from the long-styled flowers and vice versa. The ratio was as 35 to 54. When the legitimate union took place the seeds produced would give rise to both kinds of plants.
.Even more complicated than the case of the primrose is that of the loosestrife (Lythrum sahcaria) in which three types of flowers exist, those with (1) long styles and medium and short stamens, (2) medium styles And long and short stamens, and (3) short styles and long and medium stamens. (See Fig. 3). When pollenized by pollen from stamens differing in length from the style (illegitimate unions) the flowers are absolutely sterile in some cases and only partially fertile in others, while when the pollen comes from stamens of the same length as the style (legitimate union) the fertility is very high.
Spanish Bayonet.— The relation between insect and flower in the various species of Spanish Bayonet (Yucca) is one that has led to a great amount of speculation in the attempt to explain the evolution of such habits. The plants have large white or cream-colored, bell shaped flowers which hang with the opening directed downward. The blossoms are free from nectar or nearly so. The stamens are much shorter than the pistil and are curved away from it. The stigmatic surface is not at the summit of the pistil, as is the usual arrangement, but on the interior of the hollow style, where there is no possible chance of the pollen being brought by the wind or by ordi nary insect visitors. Fertilization is effected by the females of moths (see Fig. 4) of the genus Pronuba, the species common east of the Rocky Mountains being Pronuba yuccasella, which visit the flowers at dusk and go to each stamen, scraping off the rather sticky pollen with specially modified mandibles, massing it into a large ball held beneath the head. After collecting the pollen the moth usually flies to another flower and deposits one to several eggs in the ovary, after each act rushing to the top of the pistil and pushing down into the hollow style part of the pollen from the ball, putting it as far into the hollow as she can reach. She then flies to another flower, lays more eggs, and pollenizes that flower, and so on until the eggs are all deposited. Sometimes in the mean time she stops and collects more pollen. The eggs hatch into small larva which destroy a few of the very numerous seeds. However, as no seeds are set without the activities of this in sect the plant can well afford to spare the few seeds consumed.