Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/58

 Extraordinary difference in the male, female, and hermaphrodite forms of Catasetum tridentatum.—Mormodes ignea, curious structure of flower; ejection of its pollinia—Cypripedium, importance of the slipper-like form of the labellum—Secretion of nectar—Advantage derived from insects being delayed in sucking the nectar—Singular excrescences on the labellum, apparently attractive to insects.

I have reserved for separate description one sub-family of the Vandeæ, namely the Catasetidæ, which may, I think, be considered as the most remarkable of all Orchids.

I will begin with the most complex genus, Catasetum. A brief inspection of the flower shows that here, as with other Orchids, some mechanical aid is requisite to remove the pollen-masses from their receptacles, and to carry them to the stigmatic surface. We shall, moreover, presently see that the three following species of Catasetum are male plants; hence it is certain that their pollen-masses must be transported to female plants, in order that seed may be produced. The pollinium is furnished with a viscid disc, in this genus of huge size; but the disc, instead of being placed, as in other Orchids, in a position likely to touch and adhere to an insect visiting the flower, is turned inwards and lies close to the upper and back surface of a chamber, which must be called the stigmatic chamber, though functionless as a stigma. There is nothing in this chamber to attract insects; and even if they did enter it, it is hardly possible that the disc should adhere to them, for its viscid surface lies in contact with the roof of the chamber.

How then does nature act? She has endowed these plants with, what must be called for want of a better term, sensitiveness, and with the remarkable power of forcibly ejecting their pollinia to a distance. Hence, when certain definite points of the flower are touched by an insect, the pollinia are shot out like an arrow which is not barbed, but has a blunt and excessively adhesive point. The insect, disturbed by so sharp a blow, or after having eaten its fill, flies sooner or later to a female plant, and, whilst standing in the same position as it did when struck, the pollen-bearing end of the arrow is inserted into the stigmatic cavity, and a mass of pollen is left on its viscid surface. Thus, and thus alone, at least three species of the genus Catasetum are fertilised.

In many Orchids, as in Listera, Spiranthes, Orchis, we have seen that the surface of the rostellum is so far sensitive, that, when touched or when exposed to the vapour of chloroform, it ruptures in certain defined lines. So it is in the tribe of the Catasetide, but with this remarkable difference, that in Catasetum the rostellum is prolonged into two curved tapering horns, or, as I shall call them, antenine, which stand over the labellum where insects alight, and the excitement of a touch is conveyed along these antenine to the membrane which has to be ruptured; and when this is effected, the disc of the pollinium is suddenly set free. We have also seen that in several Vandee the pedicels of the pollinia are fastened down flat, but are elastic and tend to spring up, so that, as soon as they are freed, they suddenly curl upwards, apparently for the purpose of detaching the pollen-masses from their anther-cells. In the genus Catasetum, on the other hand, the pedicels are fastened down in a curved position; and when freed by the rupture of the attached edges of the disc, they straighten themselves with such force, that not only do they drag the balls of pollen and anther-cells from their places of attachment, but the whole pollinium is jerked forward, over and beyond the tips of the so-called antenine, to the distance of two or three feet. Thus, as throughout nature, pre-existing structures and capacities are utilised for new purposes.

Catasetum saccatum