Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/273

. IX. after protruding through a small opening in the membranous case in a semi-developed state, they adhere to the rostellum, against which the anther is pressed. So that the adhesion of the caudicle to the back of the rostellum seems to depend solely on the early rupturing of the anther-case, and on a slight protrusion of the caudicles, before they have become fully developed and hardened.

In all the Orchideæ a portion of the rostellum is removed by insects when the pollinia are removed; for the viscid matter, though conveniently spoken of as a secretion, is in fact part of the rostellum in a modified condition. But in those species which have their caudicles attached at an early period to the rostellum, a membranous or solid portion of its exterior surface in an unmodified condition is likewise removed. In the Vandeæ this portion is sometimes of considerable size (forming the disc and pedicel of the pollinium), and gives to their pollinia their remarkable character; but the differences in the shape and size of the removed portions of the rostellum can be finely graduated together, even within the single tribe of the Vandeæ; and still more closely by commencing with the minute oval atom of membrane to which the caudicle of Orchis adheres, passing thence to that of Habenaria bifolia, to that of H. chlorantha with its drum-like pedicel, and thence through many forms to the great disc and pedicel of Catasetum.

In all the cases in which a portion of the exterior surface of the rostellum is removed together with the caudicles of the pollen-masses, definite and often complicated lines of separation are formed, so as to allow of the easy separation of the removed portions. But the formation of these lines of separation does not differ much from the process by which certain portions