Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/35

 The transparent sides of the rostellum, on each side of the disc, consist of membrane attached behind to the edges of the boat, and folded over in front, so as to form the anterior face of the rostellum. This folded membrane, therefore, covers, almost like a deck, the cargo of viscid matter within the boat.

The anterior face of the rostellum is slightly furrowed in a longitudinal line over the middle of the boat, and is endowed with a remarkable vital property; for, if the furrow be touched very gently by a needle, or if a bristle be laid along the furrow, it instantly splits along its whole length, and a little milky adhesive fluid exudes. This action is not mechanical, or due to simple violence. The fissure runs up the whole length of the rostellum, from the stigma beneath to the summit: at the summit the fissure bifurcates, and runs down the back of the rostellum on each side and round the stem of the boat-formed disc. Hence after this splitting action the boat-formed disc lies quite free, but embedded in a fork in the rostellum. The act of splitting apparently never takes place spontaneously. I covered up under a net a plant with unexpanded flowers, and five of these flowers remained fully expanded for exactly a week under the net: I then examined their rostella, and not one had split; whereas almost every flower on the surrounded and uncovered spikes, which had been visited and touched by insects, after remaining expanded for only twenty-four hours, had their rostella fissured. Exposure for two minutes to the vapour of a little chloroform causes the rostellum to split; and this we shall hereafter see is likewise the case with some other Orchids.

When a bristle is laid for two or three seconds in the furrow of the rostellum, and the membrane has consequently become fissured, the viscid matter within the boat-formed disc lies so close to the surface, and, indeed, slightly exudes, that the disc is almost sure to be glued longitudinally to the bristle and to be withdrawn with it. When the disc is withdrawn the two sides of the rostellum (Fig. D), which have been described by some botanists as two distinct foliaceous projections, are left sticking up like a fork. This is the common condition of the flowers after they have been open for two or three days, and have been visited by insects. The fork soon withers.

When the flower is in bud the back of the boat-formed disc is covered with a layer of large rounded cells, so that the disc does not strictly form the exterior surface of the back of the rostellum. These cells contain slightly viscid matter: they remain unaltered (as may be seen at Fig. E) towards the apex of the disc, but at the point where the pollinia are attached they disappear. Hence I concluded that the viscid matter contained in these cells, when they burst, served to fasten the threads of the pollinia to the disc; but, as in several large exotic Orchids I could see no trace of such cells, I presume that this view is erroneous.

The stigma lies beneath the rostellum, and projects with a sloping surface (see side-view, B); its lower margin is rounded and fringed with hairs. On each side a membrane (cl, B) extends from the edges of the stigma to the filament of the anther, thus forming a membranous cup or clinandrum, in which the lower ends of the pollen-masses lie protected.

Each pollinium consists of two leaves of pollen, quite disconnected at their lower ends, and with their summits distinct, but they are united by elastic threads for about half of their length: a very slight modification would convert the two pollinia into four leaves of pollen, as occurs in the genus Malaxis and in many foreign Orchids. Each of the four leaves, moreover, consists of a double layer of pollen-grains, united only along their edges. The pollen-grains (each consisting of four granules) are united by elastic threads, which are more numerous along the edges of the leaves, and converge at the summit of each pollinium. The leaves of pollen are very brittle, and, when placed on the viscid stigma of a flower, large sheets are easily broken off.

Long before the flower expands, the anther-cells, which are pressed against the back of the rostellum, open in their upper part, so that the included pollinia come exactly into contact with the back of the boat-formed disc; the projecting threads then become firmly attached to rather above the middle part of the back of the disc. The anther-cells afterwards open lower down, and their membranous walls contract and become brown; so that by the time the flower has fully expanded the upper parts of the pollinia lie quite naked, their bases rest in little cups formed by the withered anther-cells, and they are laterally protected by the clinandrum. As the pollinia thus lie loose, they are easily removed.

The tubular flowers are elegantly arranged in a spire round the spike, extending horizontally (Fig. A) from it. The labellum is channelled down the middle, and is furnished with a reflexed and fringed lip, on which bees alight; its basal internal angles are produced into two globular processes, which secrete an abundance of nectar. The nectar is collected ( n, Fig. B) in a small receptacle beneath. Owing to the protuberance of the lower margin of the stigma, and of the two lateral inflexed nectaries, the orifice into the nectar-receptacle is much contracted, and is central.