Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/62

 The labellum stands at right angles to the column, or hangs a little downwards; its lateral and basal lobes are turned under the middle portion, so that an insect can stand only in front of the column. In the middle of the labellum there is a deep cavity, bordered by crests: this cavity does not secrete nectar, but its walls are thick and fleshy, and have a slightly sweet nutritious taste. I believe, as we shall hereafter see, that insects visit the flowers to gnaw these fleshy walls and crests. The extremity of the left-hand antenna stands immediately over the cavity, and would almost certainly be touched by an insect visiting this part of the labellum for any purpose.

The antennæ are the most singular organs of the flower, and occur in no other genus. They form rigid, curved horns, tapering to a point. They consist of a narrow ribbon of membrane, with the edges curled inwards so as to touch, but the edges are not united; hence each horn is tubular, like an adder's fang, with a slit down one side. They are composed of numerous, much elongated, generally hexagonal cells, pointed at both ends; and these cells (like those in most of the other tissues of the flower) have nuclei with nucleoli. The antenine are prolongations of the sides of the anterior face of the rostellum. As the viscid disc is continuous with the little fringe of membrane on each side, and as this fringe is continuous with the bases of the antenine, these latter organs are put in direct connection with the disc. The pedicel of the pollinium passes between the bases of the two antenine. The antenine are not free for their whole length; but their exterior edges, for a considerable space, are firmly united to and blended with the margins of the stigmatic chamber.

In all the flowers which I have examined, gathered from three plants, both antennæ occupied the same position; but though otherwise alike, they do not stand symmetrically. The extreme part of the left-hand antenna bends upwards (see Fig. B, in which the position is shown plainer than in A), and at the same time a little inwards, so that its tip is medial and guards the entrance into the pit of the labellum. The right-hand antenna hangs downwards, with its tip turned a little outwards; owing to this position, the crease or furrow formed by its inflexed edges is externally visible in this antenna; whilst in the other it is hidden along the under side. As we shall immediately see, the depending right-hand antenna is almost paralysed, and is apparently functionless.

Now for the action of the parts. When the lehand antenna of this species (or either antenna of the two following species) is touched, the edges of the upper membrane of the disc, which are continuously united to the surrounding surface, instantaneously rupture, and the disc is set free. The highly elastic pedicel then instantly flirts the heavy disc out of the stigmatic chamber with such force, that the whole pollinium is ejected, bringing away with it the two balls of pollen, and tearing the loosely attached spike-like anther from the top of the column. The pollinium is always ejected with its viscid disc foremost. I imitated this action with a minute strip of whalebone, slightly weighted at one end to represent the disc; and by bending it round a cylindrical object, gently holding at the same time the upper end under the smooth head of a pin, to represent the retarding action of the anther, I then let the lower end suddenly free, and the whalebone was pitched forward, like the pollinium of the Catasetum, with the weighted end foremost.

That the disc is first jerked out, I ascertained by pressing with a scalpel on the middle of the pedicel and by then touching the antenna; instantaneously out came the disc, but, owing to the pressure on the pedicel, the pollinium was not ejected. Besides the spring from the straightening of the pedicel, elasticity in a transverse direction comes into play: if a quill be split lengthways, and the half be forced longitudinally on a too thick pencil, immediately the pressure is removed the quill jumps off; and an analogous action takes place with the pedicel of the pollinium, owing to the sudden inward curling of its edges. These combined forces suffice to eject the pollinium with considerable force to the distance of two or three feet. Several persons have told me that, when touching the flowers of this genus in their hot-houses, the pollinia have struck their faces. I touched the antenine of C. callosum whilst holding the flower at about a yard's distance from the window, and the pollinium hit the pane of glass, and adhered to the smooth vertical surface by its adhesive disc.

The following observations on the nature of the excitement which causes the disc to separate from the surrounding parts, include some made on the two succeeding species. Several flowers were sent me by post and railroad, and must have been much jarred, but had not exploded. I let two flowers fall from a height of two or three inches on the table, but the pollinia were not ejected. I cut off the ovarium close under the flower, and the sepals, and in some cases the thick labellum, with a crash by a pair of scissors; but this violence produced no effect. Nor did deep pricks in various parts of the column even within the stigmatic chamber. A blow, sufficiently hard to knock off the anther suddenly, caused the ejection of the pollinium, as occurred to me once by accident. Twice I pressed rather hard on the pedicel, and consequently on the underlying rostellum, without any effect. Whilst pressing on the pedicel, I gently removed the anther, and then the pollen-bearing end of the pollinium sprang up from its elasticity, and this movement caused the disc to separate. M. Ménirè, however, has stated that the anther-case sometimes detaches itself, or can be gently detached, without the disc separating, and that then the pedicel swings downwards in front of the stigmatic