Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/172

152 and until stretched are rarely visible, for the pollen-masses lie close to the pedicel of the rostellum. These caudicles answer both in position and function to the elastic threads, by which the packets of pollen are tied together in the Ophreæ, at the point where they become confluent; for the function of the true caudicle in the Vandeæ is to break when the masses of pollen, transported by insects, adhere to the stigmatic surface.

In many Vandeæ the caudicles are easily ruptured, and the fertilisation of the flower, as far as this point is concerned, is a simple affair; but in other cases their strength, and the length to which they can be stretched before they break, are surprising. I was at first perplexed to understand what purpose these qualities could serve. The explanation probably is that the pollen-masses in this tribe are very precious objects; in most of the genera a flower produces only two, and judging from the size of the stigma both are generally left adhering to it. In other genera, however, the orifice leading into the stigma is so small that probably only one pollen-mass is left on it, and in this case the pollen from one flower would suffice to fertilise two flowers, but never a greater number. From the large size of the flowers of many of the Vandeæ, they no doubt are fertilised by large insects, and these whilst flying about would be likely to brush away and lose the pollinia attached to them, unless the caudicles were very strong and highly elastic. So again, when an insect thus provided visited a flower either too young, with its stigma not yet sufficiently adhesive, or one already impregnated, with its stigma beginning to dry, the strength of the caudicle would prevent the pollen-masses from being uselessly removed and lost.

Although the stigmatic surface is astonishingly adhesive at the proper period in many of these Orchids,