For the proper transmission of pollen it is necessary that the pollinia should adhere very firmly to the body of the insects, and that the viscous substance which holds them together should have time to solidify. It is thus of great advantage to the plant if the insects remain for a considerable time on the flower. In several orchids the nectar is not easily accessible, and frequently the insect has to search for a long time before finding what it desires, and sometimes it even has to pierce a membranous covering before reaching the fluid. The operation takes a certain time, and this is long enough to allow the mucus by which the pollinia adhere to the insect to set firmly. In the case of orchids the mucus of which sets instantaneously, there is no reason for the visit of the insect to be prolonged. In such cases the nectar is easy to extract, and the insect finds it without loss of time. Darwin, after describing these facts, proceeds to say:[32] “In these five species” (in which the viscid matter “is so adhesive that it serves to attach the pollinia firmly to the insects without getting hard”), “and in these alone, we find copious nectar ready stored for rapid suction in open nectaries. On the other hand, whenever the viscid matter gets hard by exposure for a short time to the air, it would manifestly be advantageous to the plant if insects were delayed in obtaining the nectar; and in all such species the nectar is lodged within intercellular spaces, so that it can be obtained only by the inner membrane being penetrated at several points, and this will require time. If this double relation is accidental, it is a fortunate accident for the plants; but I cannot believe it to be so, and it appears to me one of the most wonderful cases of adaptation which has ever been recorded.” Some orchids secrete instead of nectar a clear liquid like water. This fluid is collected in a petal inserted at the lower part of the flower and shaped into a deep cup-shaped receptacle. It does not attract insects, but by wetting their wings compels them to leave the flower by a different exit which passes close to the reproductive organs (_i.e._, the anther and the stigma). The soft linings of the cup are greedily devoured by certain insects, particularly by bees. Dr. Cruger, who observed this, has often seen bees fall into the cup whereupon their wings became so wet as to prevent their flying away, and they have been obliged to get out by the channel that carries off the waste from the reservoir. As the saturated bees creep along the narrow passage after their involuntary immersion, they come inevitably in contact with the stigma and the masses of pollen. The latter adhere to the bodies of the bees and can be conveyed to the sticky stigma of a neighbouring flower. [Illustration: FIG. 1.—_Catasetum saccatum_ (from “La Lindenia,” Gand, 1890). ]