Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/192

172 into it only with extreme difficulty. In the case of Cirrhæa, he found that this could be effected more easily, after they had shrunk a little from being left to dry for half an hour or an hour. He observed two flowers with pollen-masses naturally inserted by some means into their stigmas. On several occasions after forcing the end of a pollen-mass into the mouth of the stigma, he witnessed a most curious process of deglutition. The extremity of the pollen-mass swells from imbibing moisture, and as the chamber gradually widens downwards, the swelling part is forced downwards; so that the whole is at last drawn inwards and disappears. In the case of Notylia, Fritz Müller observed that the entrance into the stigma became a little larger after the flower had remained expanded for about a week. In whatever manner this latter plant is fertilised, it is certain that it must be impregnated with pollen from a distinct plant; as it offers one of those extraordinary cases in which its own pollen acts like poison on the stigma.

In the last edition of this work it was shown that the ovaria of mature flowers of Acropera do not contain any ovules. But I erred greatly in the interpretation of this fact, for I concluded that the sexes were separate. I was however soon convinced of my error by Mr. Scott, who succeeded in artificially fertilising the flowers with their own pollen. A remarkable discovery by Hildebrand, namely, that in many Orchids the ovules are not developed unless the stigma is penetrated by the pollen-tubes, and that their development occurs only after an interval of several weeks or even months, explains the state of the ovarium in Acropera, as observed by me. According also to