Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/71

 that it instantaneously rebounds from the protuberant face of the rostellum. As the two pollen-masses adhere, at first, rather firmly to the anther-case, the latter is torn off by its base to be rebound; and as the thin hinge at the summit of the anther-case does not at first break, the pollinium with the anther-case is instantly swung upwards like a pendulum: but in the course of the upwards swing the hinge yields and the whole body is projected perpendicularly up in the air, an inch or two above and close in front of the terminal part of the labellum. When no object is in the way, and the pollinium falls down, it generally alights and sticks, though not firmly, in the fold on the crest of the labellure, directly over the column. I witnessed repeatedly all that has been here described.

The curtain of the disc, which, after the pollinium has formed itself into a hoop, lies on the anther-case, is of considerable service in preventing the viscid matter from the disc adhering to the anther, and thus permanently retaining the pollinium in the form of a hoop. This would have been fatal, as we shall presently see, to a subsequent movement in the pollinium necessary to the fertilisation of the flower. In some of my experiments, when the free action of the parts was checked, this did occur, and the pollinium, with the anther, remained glued together in the shape of an irregular hoop.

I have already stated that the minute hinge by which the anther-case is articulated to the column, a little way beneath its bent filamentary apex, is sensitive to the touch. I tried four times and found that I could touch with some force any other part; but when I gently touched this point with the finest needle, instantaneously the membrane which unites the disc to the edges of the cavity in which it is lodged, ruptured, and the pollinium was shot upwards and alighted on the crest of the labellum as just described.

Now let us supposse an insect to alight on the crest of the labellum (and no other convenient landing-place is afforded), and then to lean over in front of the column to gnaw or suck the bases of the petals swollen with sweet fluid. The weight and movements of the insect would press and move the labellum together with the bent underlying summit of the column; and the latter, pressing on the hinge in the angle, would cause the ejection of the pollinJure, 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 labellure, with the tip just projecting beyond its margin, and then gently moving my finger it was really beautiful to see how instantly the pollinum was projected upwards, and how accurately the whole viscid surface of the disc struck my finger and firmly adhered to it. Nevertheless, I doubt whether the weight and movement of an insect would suffice to thus act directly 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 would thus touch the sensitive point; the pollinum would then be ejected, and the viscid disc would certainly strike and adhere to the insect's head.

Before proceeding, it will be worth while to mention some of the early trials which I made. I pricked deeply the column in different parts and the stigma, and cut off the petals, and even the labellum, without causing the ejection of the pollinium; once, however, in cutting rather roughly through the thick footstalk of the labellum, the ejection ensued, no doubt caused by the filamentary summit of the column being disturbed. When I gently prised up the anther-case at its base or at one side, the pollinium was ejected, but then the sensitive hinge must necessarily have been bent. When the flower has long remained expanded and is nearly ready for spontaneous ejection, a slight jar on any part of the flower causes the action. Pressure on the thin pedicel, and therefore on the underlying protuberant rostellum at the basal edge of the anther-case, is followed by the ejection of the pollinium; but this is not surprising, as the stimulus from a touch on the sensitive hinge has to be conveyed through this part of the rostellum to the disc. In Catasetum slight pressure on this point does not cause the ejection; but in this genus the protuberant part of the rostellum does not lie in the line of conveyed stimulus between the antenna and the disc. A drop of chloroform, of spirits of wine, and of boiling water placed on this part of the rostellum produced no effect; nor, to my surprise, did exposure of the whole flower to vapour of chloroform.

Seeing that this part of the rostellum was sensitive to pressure, and that the flower was laterally widely open, and being pre-occupied with the case of Catasetum, I at first felt convinced that insects entered the lower part of the flower and touched the rostellum. Accordingly I pressed the rostellum with variously-shaped objects, but the viscid disc never once well adhered to the object. If I used a thick needle, the pollinium, when ejected, formed a hoop round it with the viscid surface outside; if I used a broad flat object, the pollinium struggled against it and sometimes coiled itself up spirally, but the disc either did not adhere at all or very imperfectly. At the close of the twelfth trial I was in despair. The strange position of the labellum, perched on the summit of the column, ought to have shown me that here was the place for experiment. I ought to have scorned the notion that the labellum was thus placed for no good purpose: I neglected this plain guide, and for a long time completely failed to understand the structure of the flower.