Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/57

 utriculi of very many Orchids, and have hitherto found no exception to the rule that spirits of wine causes a considerable quantity of yellowish-brown matter to coagulate within them. I have taken fresh Orchids and placed them in spirits, and in twenty-four hours have found the contents coagulated and the nuclei much darkened. More numerous observations would be requisite for much stress to be laid on this fact, but I must at present infer that the utriculi of Acropera are in a different condition from those of all other Orchids. The state of the ovarium offers better evidence.

When a thin transverse slice of the ovarium is taken and examined under a quite weak power, small projections are seen on the three proper ovule-bearing cords or segments, which at first seem like true ovules. But when these are more closely examined, they are seen to consist of sub-branched, quite thin and transparent fringes of membrane, which in some specimens exhibited cellular structure far more plainly than in others. If these fringes are placentae, they are more largely developed than in other Orchids; if they are ovules (or rather the testae of ovules) in an atrophied condition, as I believe to be the case, they are more firmly fixed to the placentae than in other cases; they do not exhibit the proper opening at their free ends, and no nucleus is visible; nor were any of them inverted. I examined six ovaria of young and old flowers of the Acropera, some fresh and some which had been kept in spirits of wine, and all the ovule-bearing cords were nearly in the same condition. I examined, for comparison, the ovaria of Orchids belonging to nearly all the main Tribes, of young and old (but not fertilised) flowers, some of which were fresh, and some kept in spirits, and invariably the ovules presented a widely different appearance.

From these several facts—namely, the narrowness of the mouth of the stigmatic chamber, into which the pollen-masses can hardly be forced, whereas the length and thinness of the pedicel of the rostellum, the smallness of the viscid disc, and the movement of depression, all indicate the necessity of a large stigmatic cavity seated low down the slight viscidity of the stigmatic surface the empty condition of the stigmatic utriculi and especially the condition of the ovule-bearing cords lead me to infer that the plant at Kew, from which the many flowers of the Acropera luteola were at different times gathered, is a male plant. From having examined many Orchids grown in hot-houses, I have no reason to believe that cultivation affects the female organs in the manner described. It is scarcely possible to believe that cultivation could contract the solid edges of the stigmatic chamber. Therefore I see no reason to doubt my conclusion on the male sex of this plant.

What the female of hermaphrodite form of the Acropera luteola may prove to be whether resembling in most respects the male, or whether it be at present named and masked as some distinct genus it is impossible to say. In Acropera Loddigesii, which closely resembles, in all respects except in colour, A. luteola, I found the same almost insuperable difficulty in inserting the pollen masses into the stigmatic cavity, but I did not at that time suspect the masculine nature of the genus, and did not examine the ovarium.

I have now described, perhaps in too much detail, a few of the many contrivances by which the Vandeæ are fertilised. The relative position of the parts—friction, viscidity, elastic and hygrometric movements, all nicely related to each—other come into play. But all these appliances are subordinate to the action of insects. Without their aid, not a plant in this tribe, in the twenty-four genera examined, would set a seed. It is also evident that, in a vast majority of cases, insects would withdraw the pollinia only when retreating from a flower, and, carrying them away, would thus effect a union between two distinct flowers. This fact is conclusively shown in all those many cases in which the pollinia undergo a change of position, after removal from the rostellum, in order to stand in a proper direction to strike the stigma; for this could only be effected after the insect had left one flower, which would serve as the male, and before it visited a second flower, which would serve as the female. CHAPTER VI. Catasetidæ, the most remarkable of all Orchids—The mechanism by which the pollinia of Catasetum are ejected to a distance, and are transported by insects—Sensitiveness of the horns of the rostellum—