Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/141

. IV. and to the tips of the pollinia. The insect was not so large as one of the pollinia, and after causing the explosion had not strength enough to remove them; it was thus punished for attempting a work beyond its strength, and perished miserably.

In Spiranthes the young flowers, which have their pollinia in the best state for removal, cannot possibly be fertilised; they must remain in a virgin condition until they are a little older and the column has moved away from the labellum. Here the same end is gained by widely different means. The stigmas of the older flowers are more adhesive than those of the younger flowers. These latter have their pollinia ready for removal; but immediately after the rostellum has exploded, it curls forwards and downwards, thus protecting the stigma for a time; but it slowly becomes straight again, and now the mature stigma is left freely exposed, ready to be fertilised.

I wished to know whether the rostellum would explode, if never touched; but I have found it difficult to ascertain this point, as the flowers are highly attractive to insects, and it is scarcely possible to exclude very minute ones, the touch of which suffices to cause the explosion. Several plants were covered by a net and left till the surrounding plants had set their capsules; and the rostella in most of the covered-up flowers were found not to have exploded, though their stigmas were withered, and the pollen mouldy and incapable of removal. Some few of the very old flowers, however, when roughly touched, were still capable of a feeble explosion. Other flowers under the nets had exploded, and they had the tips of their pollinia fixed to the crest of the rostellum; but whether these had been touched by some minute insect, or had exploded spontaneously, it was impossible to deter-