Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/308

288 It may naturally be inquired, Why do the Orchideæ exhibit so many perfect contrivances for their fertilisation? From the observations of various botanists and my own, I am sure that many other plants offer analogous adaptations of high perfection; but it seems that they are really more numerous and perfect with the Orchideæ than with most other plants. To a certain extent this inquiry can be answered. As each ovule requires at least one, probably several, pollen-grains, and as the seeds produced by Orchids are so inordinately numerous, we can see that it is necessary that large masses of pollen should be left on the stigma of each flower. Even in the Neotteæ, which have granular pollen, with the grains tied together by weak threads, I have observed that considerable masses of pollen are generally left on the stigmas. This circumstance apparently explains why the grains cohere in packets or large waxy masses, as they do in so many tribes, namely, to prevent waste in the act of transportal. The flowers of most plants produce pollen enough to fertilise several flowers, so as to allow of or to favour cross-fertilisation. But with the many Orchids which produce only two pollen-masses, and with some of the Malaxeæ which produce only one, the pollen from a single flower cannot possibly fertilise more than two flowers or only a single one; and cases of this kind do not occur, as I believe, in any other group of plants. If the Orchideæ had elaborated as much pollen as is produced by other plants, relatively to the number of seeds which they yield, they would have had to produce a most extravagant amount, and this would have caused exhaustion. Such exhaustion is avoided by pollen not being produced in any great