Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/29

 The lower part of the stigma is bilobed and projects in front of the column (see s in the side and front views, C, D, Fig. XIII.). On its square summit a single, small, nearly globular rostellum is seated. The anterior face of the rostellum (r, C, D) projects a little beyond the surface of the upper part of the stigma, and this is of importance. In the early bud the rostellum consists of a friable mass of cells, with the exterior surface rough: these superficial cells undergo a great change during development, and become converted into a soft, smooth, highly elastic membrane or tissue, so excessively tender that it can be penetrated by a human hair; when thus penetrated, or when slightly rubbed, the surface becomes milky and in some degree viscid, so that the pollen-grains adhere to it. In some cases, though I observed this more plainly in E. latifolia, the surface of the rostellum apparently becomes milky and viscid without having been touched. This exterior soft elastic membrane forms a cap to the rostellum, and is internally lined with a layer of much more adhesive matter; this matter, when exposed to the air, dries in from five to ten minutes. By a slight upward and backward push with any object, the whole cap, with its viscid lining, is removed with the greatest ease; and a minute square stump, the basis of the rostellum, alone is left on the summit of the stigma.

In the bud-state the anther stands quite free behind the rostellum and stigma; it opens longitudinally whilst the flower is still unexpanded, and exposes the two oval pollen-masses, which are now loose in their cells. The pollen consists of spherical granules, cohering in fours, but not affecting each other's shapes; and these compound grains are tied together by fine elastic threads. These threads are collected into bundles, extending longitudinally along the middle of the front face of each pollinium, where it comes into contact with the back of the uppermost part of the rostellum. From the number of these threads this middle line looks brown, and each pollen-mass here shows a tendency to be divided longitudinally into two halves: in all these respects there is a close general resemblance to the pollinia of the Ophreæ.

The line where the parallel threads are the most numerous is the line of greatest strength; elsewhere the pollinia are extremely friable, so that the masses of pollen can be easily broken off. In the bud-state the rostellum is curved a little backwards, and is pressed against the recently opened anther; and the above-mentioned slightly projecting bundles of threads become firmly attached to the posterior flap of the membranous cap of the rostellum. The point of attachment lies a little beneath the summit of the pollen-masses; but the exact point is somewhat variable, for I have met with specimens in which the attachment was one-fifth of the length of the pollen-masses from their summits. This variability is so far interesting, as it is a step leading to the structure of the Ophrea, in which the confluent threads, or caudicles, spring from the lower ends of the pollen-masses. After the pollinia are firmly attached by their threads to the back of the rostellum, the rostellum bends a little forwards, and this partly draws the pollinia out of their anther-cells. The upper end of the anther consists of a blunt, solid point, not including pollen; this blunt point projects slightly beyond the face of the rostellum, which circumstance, as we shall see, is important.

The flowers stand out (Fig. A) horizontally from the stem. The labellum is curiously shaped, as may be seen in the drawings: the distal half, which projects beyond the other petals and forms an excellent landing-place for insects, is joined to the basal half by a narrow hinge, and naturally (Fig. A) is turned a little upwards, so that its edges pass within the edges of the basal portion. So flexible and elastic is the hinge that the weight of even a fly, as Mr. More informs me, depresses the distal portion; it is represented in Fig. B in this state; but when the weight is removed it instantly springs up to its former (Fig. A) and ordinary position, and with its curious medial ridges partly closes the entrance into the flower. The basal portion of the labellum forms a cup, which at the proper time is filled with nectar.

Now let us see how all the parts, which I have been obliged to describe in detail, act. When I first examined these flowers I was much perplexed: trying in the same way as I should have done with a true Orchis; I slightly pushed the protuberant rostellum downwards, and it was very easily ruptured; some of the viscid matter was withdrawn, but the pollinia remained in their cells. Reflecting on the structure of the flower, it occurred to me that an insect in entering to suck the nectar, from depressing the distal portion of the labellum, would not touch the rostellum; but that, when within the flower, from the springing up of the distal half of the labellum, it would be almost compelled to back out parallel to the stigma by the higher part of the flower. I then brushed the rostellum lightly upwards and backwards with the end of a feather and other such objects; and it was pretty to see how easily the membranous cap of the rostellum came off, and how well, from its great elasticity, it fitted the object, whatever its shape might be, and how firmly it clung to it from the viscidity of its under surface. Together with the cap large masses of pollen, adhering by the threads, were necessarily withdrawn.

Nevertheless the pollen-masses were not nearly so cleanly removed as those which had been naturally removed by insects. I tried dozens of flowers, always with the same imperfect results. It then occurred to me that an insect in backing out of the flower would naturally push with some part of its body against the blunt and projecting upper end of the anther which overhangs the stigmatic surface. Accordingly I so held the