Two of the very largest plant families, not so far mentioned in this account, depend almost absolutely upon insects. In the daisy family, with over eleven thousand members, the large heads of flowers, often containing scores of individual flowers, are constantly brushed, over and over again, by the pollen-coated bodies of insect visitors. And in all or nearly all orchids (Figures 73-75), comprising over five thousand kinds, the same process is accomplished. In these plants, in fact, the act is, if possible, more [Illustration: FIG. 73.--PINK LADY’S SLIPPER (_Cypripedium acaule_) A native orchid in northeastern North America.] complicated than in any so far noted. Darwin’s book, “On the Various Contrivances by Which British and Foreign Orchids Are Fertilized by Insects,” reads like a fairy tale. Yet it is the result of years of patient observation by incomparably the greatest naturalist of recent times. To it the reader must go for the details of a drama of absorbing interest, but too long to sketch even briefly here. Perhaps one illustration may be mentioned of how far the principle of cross-fertilization has been carried, and to the deadly effects of its failure in at least one case. In a certain orchid from Brazil, known as butterfly orchid, the pollen is nearly always carried out of the flower by an insect visitor, but, if by mischance it is not, and falls on the stigma, not only does it fail to fertilize the ovules, but it kills the pistil forthwith. There may be a few other cases of such drastic results of self-fertilization, but in any case, and disregarding these apparently suicidal fanatics, cross-fertilization is so very nearly universal that nature must find it of enormous advantage. Only in this way is it possible to explain the intricate adjustments of insects and flowers, which work together in such wonderful harmony that cross-fertilization hardly ever fails in those flowers where it appears to be necessary. [Illustration: FIG. 74.--SHOWY ORCHID (_Orchis spectabilis_) Native of eastern North America, with showy magenta-pink or white flowers in a loose raceme.] It must have struck many thoughtful readers to ask a rather obvious question at this point. Why, if untold millions of insects are constantly flitting from flower to flower, does not the pollen get mixed, as it is quite certain that they will not fly from a [Illustration: FIG. 75.--RATTLESNAKE PLANTAIN (_Peramium pubescens_) One of the few orchids native in eastern North America, with white variegated leaves. It grows in dense patches and bears free-blooming spikes of whitish flowers.]