Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/178

158 manner shortened, it is hardy possible that the flower could have been fertilised. After this movement, the pollinium attached to any small object can be inserted into the flower, and the balls of pollen readily adhere to the stigmatic surface. Here we have an instance of one of those compensating actions in the pollinia, before alluded to, in relation to the shallowness of the stigma.

In some cases, besides hygrometric movements, elasticity comes into play. In Aerides odorata and virens, and in an Oncidium (roseum?), the pedicel of the rostellum is fastened down in a straight line, at one extremity by the disc, and at the other by the anther; it has, however, a strong elastic tendency to spring up at right angles to the disc. Consequently, if the pollinium, attached by its viscid disc to some object, is removed from the anther, the pedicel instantly springs up and stands at nearly right angles to its former position, with the pollen-masses carried aloft. This has been noticed by other observers; and I agree with them that the object gained is to free the pollen-masses from the anther-cells. After this upward elastic spring, the downward hygrometric movement immediately commences, which, oddly enough, carries the pedicel back again into almost exactly the same position, relatively to the disc, which it held whilst forming part of the rostellum. In Aerides the end of the pedicel, to which the pollen-masses are attached by short dangling caudicles, after springing up, remains a little curved upwards; and this curvature seems well adapted to drop the pollen-masses into the deep stigmatic cavity over the ledge in front. The difference between the first elastic and the second or reversed hygrometric movement, was well shown by placing the pollinium of the above Oncidium into