But, as Robert Brown long ago remarked, it is not a new organ. It is impossible to look at the two groups of spiral vessels (Fig. XXXII.) running from the mid-ribs of the two lower sepals to the two, sometimes quite distinct, lower stigmas, and then look at the third group of vessels running from the mid-rib of the upper sepal to the rostellum, which occupies exactly the position of the third stigma, and doubt its homological nature. There is every reason to believe that the whole of this upper stigma, and not merely a part, has been converted into the rostellum; for there are plenty of cases of two stigmas, but not one instance of three stigmatic surfaces being present in those Orchids which have a rostellum. On the other hand, in Cypripedium and Apostasia (the latter ranked by Brown in the Orchidean order), which are destitute of a rostellum, the stigmatic surface is trifid.
As we know only those plants which are now living, it is impossible to follow all the gradations by which the upper stigma has been converted into the rostellum; but let us see what are the facilities and indications of such a change having been effected. The change of function has not been so great as it at first appears. The function of the rostellum is to secrete a quantity of viscid matter; it has lost that of fertility, but this loss is so common with plants that it is hardly worth notice. The stigmas of Orchids, as well as of most other plants, secrete viscid matter, the use of which in all cases is to retain the pollen when brought by any means to its surface. Now, if we look to one of the simplest rostellums,—for instance, to that of Cattleya or Epidendrum,—we find a thick layer of viscid matter, not distinctly separated from the viscid surface of the two confluent stigmas: its action is simply to smear and affix to the back of a retreating insect the pollen-masses, which are thus dragged out of the anther and transported to another flower, where they are retained by the almost equally viscid stigmatic surface. So that the office of the rostellum is still to secure the pollen- masses, but indirectly by means of their attachment to an insect's body.
The viscid matter of the rostellum and of the stigma appears to have nearly the same character: that of the rostellum generally has the peculiar property of quickly drying or setting hard; that of the stigma, when removed from the plant, apparently dried more quickly than gum-water of about equal tenacity. This tendency to dry is the more remarkable, as Gärtner[1] found that drops of the stigmatic secretion from Nicotiana did not dry in two months. The viscid matter of the rostellum in many Orchids when exposed to the air changes colour with remarkable quickness, and becomes brownish-purple; and I have noticed a similar but slow change of colour in the viscid secretion of the stigmas of some Orchids, as of Cephalanthera grandiflora. When the viscid disc of an Orchis, as Bauer and Brown have also observed, is placed in water, minute particles are expelled with violence in a peculiar manner; and I have observed exactly the same fact in the layer of viscid matter covering the stigmatic utriculi in an unopened flower of Mormodes ignea.
In order to compare the minute structure of the rostellum and stigma, I examined young flower-buds of Epidendrum cochleatum and floribundum, which, when mature, have a simple rostellum. The posterior surface was the same in both organs: the rostellum at this early age consisted of a mass of nearly orbicular cells, containing spheres of brown matter, which resolve themselves into the viscid matter: the stigma was covered with a thinner layer of similar cells, and beneath them were the coherent spindle-formed utriculi. The utriculi are believed to be connected with the penetration of the pollen-tubes; and their absence in the rostellum probably accounts for its infertility. As I do not find the exterior layer of nearly orbicular cells on the stigma in the bud-state (which apparently secrete the viscid matter), mentioned by more experienced observers, I cannot help feeling some doubt on the subject; though I have no other reason to doubt my accuracy. If the structure of the rostellum, in one of the simplest Orchids, and of the stigma, be as I have described them, their only difference is, that in the rostellum the layer of cells, which secretes the viscid matter, is thicker, and the utriculi have disappeared.
Hence, during a course of slow modification, it is quite conceivable that the upper stigma, whilst still in some degree fertile or capable of penetration by the pollen-tubes, might secrete a superfluity of viscid matter; and that insects smeared with this might remove and transport the pollen-masses to the stigmas of other flowers. In this case an incipient rostellum would have been formed.
The following details on the rostellum and pollinia will have no interest to any one, unless he cares much about the structure of Orchids, or wishes to see how far very different states of the same organ can be graduated together within the limits of the same order. In the several Tribes, the rostellum presents a marvellous amount of diversity of structure; but most of the differences can be connected without very wide breaks. One of the most striking diferences is, that either the whole anterior surface to some depth or only the central portion becomes viscid, in the latter case the surface retaining, as in Orchis, a membranous condition. But these two states so graduate into each other, that it is scarcely possible to draw any line of separation: thus, in Epipactis, the exterior surface undergoes a vast change from its early cellular condition, and becomes
- ↑ 'Beiträge zur Kenntnis der Befruchtung,' 1844, s. 236.
