Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/132

112 proboscis causes the rostellum to split in front and behind, and frees the long, narrow, boat-formed disc, which is filled with extremely viscid matter, and is sure to adhere longitudinally to the proboscis. When the bee flies away, so surely will it carry away the pollinia. As the pollinia are attached parallel to the disc, they adhere parallel to the proboscis. When the flower first opens and is best adapted for the removal of the pollinia, the labellum lies so close to the rostellum, that the pollinia attached to the proboscis of an insect cannot possibly be forced into the passage so as to reach the stigma; they would be either upturned or broken off: but we have seen that after two or three days the column becomes more reflexed and moves from the labellum,—a wider passage being thus left. When I inserted the pollinia attached to a fine bristle into the nectar-receptacle of a flower in this condition (n, fig. B), it was pretty to see how surely the sheets of pollen were left adhering to the viscid stigma. It may be observed in the diagram, B, that owing to the projection of the stigma, the orifice into the nectar-receptacle (n) lies close to the lower side of the flower; insects would therefore insert their proboscides along this lower side, and an open space above is thus left for the attached pollinia to be carried down to the stigma, without being brushed off. The stigma evidently projects so that the ends of the pollinia may strike against it.

Hence, in Spiranthes, a recently expanded flower, which has its pollinia in the best state for removal, cannot be fertilised; and mature flowers will be fertilised by pollen from younger flowers, borne, as we shall presently see, on a separate plant. In conformity with this fact the stigmatic surfaces of the older flowers are far more viscid than those of the