Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/86

Whether we compare together the state of the rostellum in the various Orchid-tribes, or compare the rostellum with the pistil and stigma of ordinary flowers, the differences are wonderfully great. A simple pistil of a common plant consists of a cylinder surmounted by a small viscid surface. Now, see what a contrast the rostellum of Catasetum, when dissected from the other elements of the column, presents; and as I traced all the vessels in this Orchid, the drawing may be trusted as approximately accurate. The whole organ has lost its normal function of fertility. Its shape is most singular, with its upper end thickened, bent over and produced into two long tapering and sensitive antennæ, hollow within like adder's fangs. Behind and between the bases of these antennae we see the large viscid disc, attached to the pedicel, which differs in structure from the underlying portion of the rostellum, and is separated from it by a layer of hyaline tissue, that spontaneously dissolves. The disc, attached to the surrounding parts by membrane which ruptures when excited by a touch, consists of strong upper tissue, with an underlying elastic cushion, coated with viscid matter; and this again in most Orchids is overlaid by a film of a different nature. What an amount of specialisation of parts do we here behold! Yet we have seen in the comparatively few Orchids described in this volume, so many and such plainly-marked gradations in the structure of this organ, and such plain facilities for the original conversion of the upper pistil into the rostellum, that it becomes far from incredible, if we had every Orchid which has ever existed throughout the world, that every gap in the existing chain, and every gap in many lost chains, would be amply filled up by a series of easy transitions.

We now come to the last great peculiarity in Orchids, namely, their pollen-masses. The anther opens early, and often deposits the naked masses of pollen on the back of the rostellum. This action is prefigured in Canna, a member of the family nearest allied to Orchids, in which the pollen is deposited on the pistil, close beneath the stigma. In the state of the pollen there is great diversity: in the anomalous Cypripedium single grains are embedded in a glutinous fluid; in all other Orchids (except the degraded Cephalanthera) each grain consists of generally four united granules.^[1]

Having alluded to the monstrous flowers of Aceras, I will add that I examined several, always the lowest on the spike; in these the labellum was hardly developed, and was pressed close against the stigma. The rostellum was not developed, so that the pollinia had not viscid discs; but the most curious feature was, that the two anther-cells had become, apparently in consequence of the position of the rudimentary labellum, widely separated, and were joined by a connective membrane, almost as broad as that of Habenaria

1. In several cases I have observed four tubes emitted from the four granules. In some semi-monstrous flowers of Malaxis paludosa, and of Aceras anthropophora, and in perfect flowers of Neottia nidus-avis, I have observed pollen-tubes emitted from the pollen-grains, whilst still within the anther and not in contact with the stigma. I have thought this worth mentioning as R. Brown (in 'Linn. Transact.' vol. xvi. p. 729) states, apparently with some surprise, that the pollen-tubes were emitted from the pollen, whilst still within the anther, in a decaying flower of Asclepias. These cases show that the protruding tubes are, at least at first, formed at the expense of the contents of the pollen-grains.