Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/105

. III. uncovered plants weighed 1.5 grain; whilst those from an equal number of capsules on the covered plant weighed under 1 grain; but this does not give a fair idea of the relative difference of their fertility, for I observed that a great number of the seeds from the covered plant consisted of minute and shrivelled husks. Accordingly I mixed the seeds well together, and took four little lots from one heap and four little lots from the other heap, and, having soaked them in water, compared them under the microscope: out of forty seeds from the uncovered plants there were only four bad ones, whereas out of forty seeds from the covered-up plants there were at least twenty-seven bad; so that there were nearly seven times as many bad seeds from the covered plants, as from those left free to the access of insects.

We may therefore conclude that this orchid is constantly self-fertilised, although in a very imperfect manner; but this would be highly useful to the plant, if insects failed to visit the flowers. The penetration of the pollen-tubes, however, is apparently even more serviceable by retaining the pillars of pollen in their proper places, so that insects, in crawling into the flowers, may get dusted with pollen. Self-fertilisation also may, perhaps, be aided by insects, carrying pollen from the same flower on to the stigma; but an insect thus smeared with pollen could hardly fail likewise to cross the flowers on other plants. From the relative position of the parts, it seems indeed probable (but I omitted to prove this by the early removal of the anthers, so as to observe whether pollen was brought to the stigma from other flowers) that an insect would more frequently get dusted by crawling out of a flower than by crawling into one; and this would of course facilitate a cross between distinct individuals. Hence