Page:Popular Science Monthly Volume 41.djvu/185

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173
THE YUCCA MOTH AND YUCCA POLLINATION.

are at rest, would protect the flower from spoliation by useless raiders. This belief is also strengthened by the fact that anemophilous flowers, or those fertilized by the wind, never sleep, and that flowers which attract insects by smell emit their odor at particular hours.

But the most interesting fact not commonly understood, that has now been very fully established by the most thorough researches, is, that a very large number of plants, even where the sexes are united in the same flower, absolutely depend on insect aid for pollination, and that the contrivances to induce cross-fertilization are infinite in diversity, while the modifications in structure which these insects have undergone the better to fit them to perform this service, are equally remarkable. Yet in most cases we have adaptation of the plant only, and except in a few instances, as, for instance, in that Madagascar orchid, Angrœcum sesquipedale, where the nectary is so deep that its nectar can be reached only by a moth (like Macrosila cluentius) with a very long tongue, our orchids are not dependent for pollination on any one Lepidopterous species, but may be aided by many which have tongues of sufficient length.

There are, in fact, few plants which are dependent on a single species for pollination. So far as I know, the yuccas furnish the PSM V41 D185 243 Pronuba yuccasella.jpgFig. 2.—Pronuba yuccasella: a, larva; b, ♀ moth with closed wing; c, ♀ moth with wings expanded—natural sue; d, side view of larval joint; e, head of larva, beneath; f, head of larva, above; g, thoracic leg of same; h, maxilla; i, mandible; j, spinneret and labial palpi; k, antenna—enlarged. only instance of this kind, for they actually depend on some particular species of little white moths belonging to the Tineina and to the genus Pronuba. The yuccas are a very interesting genus of lily-like plants, so familiar to every one in our public and private gardens that I need not say very much about them (Fig. 1). There are numerous species and even sub-genera, but they are all characterized by anthers not reaching anywhere near the stigma, so that fertilization unaided can take place only by the merest accident. In other words, the stigmatic tube is nowhere within reach of the stamens, and the pollen either remains attached to the open and withered anthers or falls and remains in different-sized lumps on the inside of the perianth, and can not be introduced into the stigmatic tube without artificial aid.

Our commoner garden yuccas, forms of filamentosa, depend on the commoner yucca moth, Pronuba yuccasella (Fig. 2, b, c), and so do all the different species found east of the Rocky Mountains, so far as we yet know. During the daytime we may, by knowing