Page:Popular Science Monthly Volume 26.djvu/198

186 habitually fertilizes it is even closer and more lasting than in any of the instances we have yet considered. Everybody knows those large and handsome tropical lilies, the yuccas, with their tall, clustered heads of big white blossoms. Well, Professor Riley, the great American entomologist, has shown that the yuccas are entirely run (to use a favorite expression of his countrymen) by a comparatively small and inconspicuous moth, solely for its own benefit: and so completely is this the case, that the yucca can't manage to exist at all without its little winged intermediary. Professor Riley has, therefore, playfully named the little insect Pronuba yuccasella; freely translated, the yucca's bridesmaid. The moth bores the young capsule of the flower in several places, lays an egg in each hole, and then carefully collects pollen, with which it fertilizes the blossom, of set purpose, thus deliberately producing a store of food for its own future larvæ. The eggs hatch inside the capsule, and the young grubs eat part of the seeds, at the same time prudently leaving enough for the continuation of the yucca family in the future. As soon as the grubs are full-grown, they bore a hole again through the capsule, lower themselves by a thread to the ground, and there spin a cocoon which lies buried in the earth all through the autumn and winter. But in the succeeding summer, just fourteen days before the yuccas begin to flower, the grubs in their cocoons pass into the chrysalis stage; and, by the time the yuccas are in full blossom, they issue forth as perfect moths, and once more commence the fertilization of their chosen food-plant, and the laying of their own eggs. So singular an instance of mutual accommodation between flower and insect is rare indeed in this usually greedy and self-regarding world.

The extremely odd, inside-out, topsy-turvy flowers of the fig owe their fertilization, however, to a still more extraordinary and complicated cross-relationship. Hardly anybody (except a botanist) has ever seen a fig-flower, because it grows inside the stalk, instead of outside, and so can only be observed by cutting it open lengthwise. The fig, in its early youth, in fact, consists of a hollow branch on whose inner surface a number of very small flowers cluster together; and, when they are ripe for fertilization, the eye or hole at the top opens to admit the insect visitor. This visitor is the fig-wasp, who comes, not from other cultivated fig-trees, but from a wild tree called the caprifico. On this tree the mother wasps first lay their eggs in the inedible figs, which thereupon swell out into galls, and become the nurses of the young wasp-grubs. When the wasps are mature, they eat their way out of the wild fig where they were born, and set forth to lay their own eggs in turn, either on a brother caprifico or on its sister, a true fig-tree. Those wasps which enter the wild figs of a caprifico succeed in carrying out their maternal purpose, and lay their eggs on the right spot for more grubs to be duly developed. But those which happen to go into a true fig merely fertilize the flowers without laying