, In some cases the flower appears to secrete no nectar although the odor is sweet. Other flowers emit a very 'disgusting odor. So, for example, the flowers . of a South African genus of the milkweed family, Stapelia, give out the odor of carrion. The large star-shaped blooms are marked with a brown or purple mottling, resembling in some species the belly of a toad. The carrion scent is very intense and attracts various carrion-moving flies which wander over the flower in an attempt to find a suitable place to deposit their eggs. In this process they lead to the pollination of the flower.
The actual benefit to the flower of this inti mate relation between insects and flowers lies in the transfer of the pollen from the stamens to the stigma of the pistil, upon which is de pendent the production of the seeds. In flowers that are dependent upon insects (ento mophilou flowers) the stigma and stamens are not in sufficiently close juxtaposition to permit of the direct transfer of the pollen or this transfer is prevented by other means. The pollen, moreover, is often of such a character that it is not easily transported by the wind, either because of its size, on account of its sticky nature, its location, etc. Insects (more rarely birds, snails and other animals) that visit the flower in search of the pollen or nec tar or for any other purpose become more or less covered with pollen. Their bodies and appendages are to a considerable extent cov ered with hairs to which the pollen adheres or with which they become entangled. In addi tion, some of the bees and wasps have special structures for holding pollen. It is a familiar sight to many to see bees on their return to the hive laden with nectar and with the two posterior legs bearing large masses of pollen. In moving about in the flower some of the pollen may be transferred directly from the stamens to the stigmas; when the insect visits another flower some of the pollen from the first flower is carried to the next one. Soon after pollination has occurred we find that the emission of odors and the secretion of nectar cease, and in most cases the petals fall or shrivel so that the flower no longer exerts a lure for insects.
Floral noted in the article FLOWER (q.v.) some plants do not set seed well and others not at all when pollenized by pollen from the same flower. In many cases the floral structures are so arranged that self pollination is impossible, even when the flower is visited by insects. In the simplest cases the stamens set free their pollen at a time other than that when the stigma is receptive. Such flowers may shed all their pollen first (pro terandry) or they may wait until after the stigma has passed the receptive stage before the pollen begins to be set (proterogyny). In
such cases the period during which perfume and nectar are produced may be considerably longer than in those flowers where stamens and stigma are mature at the same time. In more specialized cases the flower assumes a shape such that the visiting insect must enter it at a definite place, thus insuring that any pol len obtained from a flower previously visited shall be brought into immediate contact with the stigma. In the sage (Salvia), for example, the flower is strongly two lipped. The bee settles on the lower lip and can enter the flower only one way. (See Fig. 1). In inserting the head and tongue into the flower in search for the nectary the tongue comes into contact with the peculiar extensions of the hinged stamens and in pushing these up so as to get at the nectar causes the anterior, pollen-bearing portion to curve downward until in contact with the back of the insect which is thus dusted with the pollen. In flowers at this stage the style and stigma lie concealed under the arching upper lip of the corolla. As the flower gets older the style curves somewhat so that the stigma now comes in contact with the back of a visiting bee, thus receiving the pollen deposited there by the hinged stamens of another flower. The stamens having discharged their pollen twist to the sides so as to be out of the way. In the Lady's Slipper (Cypripedium) the lower lip is a large sack with a median opening through which in sects, bees chiefly, may easily enter, attracted by the odor. Downward pointing hairs and the incurving of the border of the opening prevent the insects escaping by the opening through which they entered. There are, however, two other openings, leading up by the stigma and stamens. After the bee has fed sufficiently on the sugary secretions of the hairs it climbs up to one of the small openings and squeezes out. In doing this it comes into close contact first with the stigma and then against a stamen, be coming covered with pollen from the latter. On visiting the next flower this pollen is brushed off upon the stigma and perhaps a second load of pollen acquired. Charles Darwin studied the relation of insects to this and other orchids and found that many orchids are incapable of being pollenized except by certain insects, so fully are the flowers adapted in structure to fit the structure and dimensions of these particu lar insects. Kronfeld noted that all of the species of monkshood (Aconitum) are confined to thoseparts of the world where bumble bees are native. Because of their rather deep-seated nectaries their nectar is not avail able to short-tongued bees and flies which are not built in such a way as to be able to pollenize the flowers.