Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/131

. IV. passage is left for insects to deposit pollen on the stigmatic surface. On this slight movement of the column the fertilisation of the flower absolutely depends.

With most Orchids the flowers remain open for some time before they are visited by insects; but with Spiranthes I have generally found the boat-formed discs removed very soon after their expansion. For example, in the two last spikes which I happened to examine there were numerous buds on the summit of one, with only the seven lowest flowers expanded, of which six had their discs and pollinia removed; the other spike had eight expanded flowers, and the pollinia of all were removed. We have seen that when the flowers first open they would be attractive to insects, for the receptacle already contains nectar; and at this period the rostellum lies so close to the channelled labellum that a bee could not pass down its proboscis without touching the medial furrow of the rostellum. This I know to be the case by repeated trials with a bristle.

We thus see how beautifully everything is contrived, that the pollinia should be withdrawn by insects visiting the flowers. They are already attached to the disc by their threads, and, from the early withering of the anther-cells, they hang loosely suspended but protected within the clinandrum. The touch of the