Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/48

28 alight on the opposite sepals and petals. As a consequence of this, when a butterfly inserts its proboscis into the narrow entrance of the nectary, the viscid discs become attached to the lower surface of the proboscis, and the pollinia afterwards move upwards, instead of as in all other orchids downwards. They are then in the proper position for striking the stigma of the next flower which is visited. Dr. Müller remarks that the flowers are frequented by an extraordinary number of butterflies.

I have now described the structure of most of the British and of a few foreign species in the genus Orchis and its close allies. All these species, with the exception of the Neotinea, require the aid of insects for their fertilisation. This is obvious from the fact that the pollinia are so closely embedded in the anther-cells, and the ball of viscid matter in the pouch-formed rostellum, that they cannot be shaken out by violence. We have also seen that the pollinia do not assume the proper position, for striking the stigmatic surface until some time has elapsed; and this indicates that they are adapted to fertilise, not their own flowers, but those on a distinct plant. To prove that insects are necessary for the fertilisation of the flowers, I covered up a plant of Orchis morio under a bell-glass, before any of its pollinia had been removed, leaving three adjoining plants uncovered; I looked at the latter every morning, and daily found some of the pollinia removed, till all were gone with the exception of those in a single flower low down on one spike, and of those in one or two flowers on the summits of all the spikes, which were never removed. But it should be observed that when only a very few flowers remain open on the summits of the spikes, these are no longer conspicuous,