Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/52

 caudicles, though they were stretched to a great length, did not rupture, nor did the viscid disc separate from the scalpel; consequently the balls of pollen were not left on the stigma. It then occurred to me that an insect in flying away would not pull the pollinia straight out of the chamber, but would pull at nearly right angles to its orifice. When I thus acted, the stretched caudicles were necessarily dragged over the margin of the chamber, and the friction thus caused, together with the vicidity of the stigmatic surface, generally ruptured them and left the pollen-masses on the stigma. Thus, it seemed that the great strength and extensibility of the caudicles, which, until stretched, lie embedded within the pollen-masses, serve to protect the pollen-masses from being wasted, and yet, by friction being brought into play, allow them, at the proper time and by means of insects, to be left adhering to the stigmatic surface, and the fertilisation of the flower to be safely effected.

The discs and the pedicel of the rostellum in the Vandee present great diversities in shape, and an apparently exhaustless number of adaptations. Even in species of the same genus, as in Oncidium, these parts differ greatly. I have here given a few figures (Fig. XXIII.), taken almost at hazard. The pedicel generally consists (as far as I have seen) of a piece of thin ribbon-shaped membrane (Fig. A), long or short; but sometimes it is almost (Fig. C) cylindrical and often of all sorts of shapes. The pedicel is generally nearly straight, but in Miltonia Clowesii it is naturally curved; and in some other cases, as we shall immediately see, it assumes, after removal, various shapes. The extensible and elastic caudicles, by which the pollen-masses are attached to the pedicel, are here not visible, being embedded in a cleft or hollow within each pollen-mass. The disc, which is viscid on the under side, consists of a piece of thin or thick membrane of the most diversified shapes. In Acropera it is like a pointed cap; in some cases it is tongue-shaped, or heart-shaped (Fig. C), or saddle-shaped, as in some Maxillarias, or like a thick cushion (Fig. A), as in many species of Oncidium, with the pedicel attached at one end, instead of, as is more usual, nearly to its centre. In Angræcum distichum and sesquipedale the rostellum is notched, and two separate, thin, membranous discs can be removed, each carrying by a short pedicel its pollen-mass. In Sarcanthus teretifolius the disc (Fig. D) is very oddly shaped; and as the stigmatic chamber is very deep, and likewise curiously shaped, one is tempted to believe that the disc has to be fastened with great precision on to the square projecting head of some insect.

In most cases there is a plain relation between the length of the pedicel and the depth of the stigmatic chamber, into which the pollen-masses have to be inserted; in some few cases, however, in which there is a long pedicel and a shallow stigma, we shall meet with curious compensating actions. After the disc and pedicel have been removed, the shape of the rostellum is altered, being generally only slightly shortened and made thinner: sometimes it becomes notched: in Stanhopea, the entire circumference of the extremity of the rostellum is removed, and a thin, pointed, needle-like process alone is left, which originally ran up its centre.

If we turn to the former imaginary diagram (Fig. XXII), and suppose the rectangularly bent rostellum to be thinner and the stigma to lie closer under it, we shall see that, if an insect with the pollinium attached to its head were to fly to another flower and occupy almost exactly the same position which it held when the attachment of the disc was effected, the pollen-masses would strike the stigma, especially if, from their