Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/91

. II. fig. A), above and some way on each side of the stigmatic surface; if attached in this position to the head of an insect, the insect might visit any number of flowers, and no pollen would be left on the stigma. But observe what takes place: in a few seconds after the inner end of the drum-like pedicel has been removed from its embedded position and exposed to the air, one side of the drum contracts, and this contraction draws the thick end of the pollinium inwards, so that the caudicle and the viscid surface of the disc are no longer parallel, as they were at first, and as they are represented in the section, fig. C. At the same time the drum rotates through nearly a quarter of a circle, and this moves the caudicle downwards, like the hand of a clock, depressing the thick end of the pollinium or mass of pollen-grains. Let us suppose the right-hand disc to be affixed to the right side of an insect's face, and by the time required for the insect to visit another flower on another plant, the pollen-bearing end of the pollinium will have moved downwards and inwards, and will now infallibly strike the viscid surface of the stigma, situated in the middle of the flower beneath and between the two anther-cells.

The little rudimentary tail of the caudicle projecting beyond the drum-like pedicel is an interesting point to those who believe in the modification of species; for it shows us that the disc has been carried a little inwards, and that primordially the two discs stood even still further in advance of the stigma than they do at present. We thus learn that the parent-form approached in this respect the structure of that extraordinary Orchid, the Bonatea speciosa of the Cape of Good Hope.

The remarkable length of the nectary, containing much free nectar, the white colour of the conspicuous