Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/235

. VII. to the edges of the stigmatic cavity where it is lodged, ruptured, and the pollinium was shot upwards and fell on the crest of the labellum as just described.

Now let us suppose an insect to alight on the folded crest of the labellum, and no other convenient landing-place is afforded, and then to lean over the front of the column so as to gnaw or suck the bases of the petals swollen with sweet fluid. The weight and movements of the insect would disturb the labellum and the bent underlying summit of the column; and the latter, pressing on the hinge in the angle, would cause the ejection of the pollinium, which would infallibly strike the head of the insect and adhere to it. I tried by placing my gloved finger on the summit of the labellum, with the tip just projecting beyond its margin, and then gently moving my finger it was really beautiful to see how instantly the pollinium was projected upwards, and how accurately the viscid surface of the disc struck my finger and firmly adhered to it. Nevertheless, I doubt whether the weight and movements of an insect would suffice to thus act indirectly on the sensitive point; but look at the drawing and see how probable it is that an insect leaning over would place its front legs over the edge of the labellum on the summit of the anther-case, and thus touch the sensitive point. The pollinium would then be ejected, and the viscid disc would certainly strike and adhere to the insect's head.

Before proceeding, it may be worth while to mention some of the early trials which I made. I pricked deeply the column in different parts, including the stigma, and cut off the petals, and even the labellum, without causing the ejection of the pollinium; this, however, once happened when I cut rather roughly through the thick footstalk of the labellum, the fila-