Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/70

 anther is torn off during the ejection of the pollinium. This gorged condition may perhaps facilitate the ultimate rupture of the hinge.

The pollinium does not differ much from that of Catasetum (see Fig. XXVI. D): it lies curved round the rostellum, which is less protuberant than in that genus. The upper and broad end of the pedicel, however, extends beneath the pollen-masses, and they are attached by rather weak caudicles to a medial crest on its upper surface.

The viscid surface of the large disc lies in contact with the roof of the stigmatic cavity, so that it cannot be touched by insects. The anterior end of the disc is furnished with a small dependent curtain (dimly shown in Fig. XXIX.); and this, before the act of ejection, is continuously joined on each side to the upper margins of the stigmatic depression. The pedicel is united to the posterior end of the disc; but when the disc is freed, the lower part of the pedicel becomes doubly bent, so that it then appears as if attached to the centre of the disc by a hinge.

The labellum is truly remarkable: it is narrowed at its base into a nearly cylindrical footstalk, and its sides are so much reflexed as almost to meet at the back. After rising up perpendicularly it forms an arch, over and behind the summit of the column, against which it is firmly pressed. The labellum at this point (even in the bud) is depressed into a slight cavity, which receives the ultimately bent summit of the column. This slight depression manifestly represents the large hollow, with thick fleshy walls, in the labellum of the several species of Catasetum and other Vande, which serves to attract insects; here, by a singular change of function, it merely keeps the labellum in its proper position on the summit of the column. In the drawing (Fig. XXIX.) the labellum is represented as forcibly raised so as to show the depression and the bent filament. The labellum in its natural position may almost be compared to a huge cocked-hat, supported by a footstalk and placed on the head of the column.

The twisting of the column, which I have seen in no other Orchid, causes all the important organs of fructification to face to the left in the flowers on the left side of the spike, and to face to the right in all those on the right side. So that two flowers taken from opposite sides of the spike and held in the same relative position are seen to be twisted in opposite directions. One single flower, which was crowded by the others, was barely twisted, so that its column faced the labellum. The labellum is also slightly twisted: for instance, in the flower figured which faces to the left, the midrib of the labellum first bends to the right hand, and then back again, but in a lesser degree, to the left, so as to press against the posterior surface of the summit of the column. The twisting of all the parts of the flowers commences in the bud.

The position thus acquired by the several organs is of the highest importance; for if the column and labellum had not been twisted laterally, the pollinia, when shot forth, would have struck the overarching labellum and been thrown back, as actually occurred with the single abnormal flower having a nearly straight column. If all the organs had not been twisted in opposite directions on the two sides of the crowded spike, so as to face always to the outside, there would not have been a clear space for the ejection of the pollinia and their adhesion to insects.

When the flower is mature the three sepals hang down, but the two upper petals remain nearly upright. The bases of the sepals, and especially of the two upper petals, are thick and swollen and have a yellowish tint; they are so gorged, when quite mature, with fluid, that, when punctured by a fine glass tube, the fluid rises to some height in it by capillary attraction. These swollen bases, as well as the footstalk of the labellum, have a decidedly sweet and pleasant taste; and I can hardly doubt that they are attractive to insects, for no free nectar is secreted.

I will now endeavour to show how all the parts of the flower are co-ordinated and act together. The pollinium, as in Catasetum, lies bowed round the rostellum; in that genus when freed it merely straightens itself with force, in this Mormodes something more takes place. If the reader will look forward to Fig. XXX., he will see a section of the flower-bud of another species of Mormodes, which differs only in the shape of the anther and in the viscid disc having a much deeper dependent curtain. Now let him suppose the pedicel of the pollinium to be so elastic that when freed it not only straightens itself, but suddenly bends backwards with a reversed curvature, so as to form an irregular hoop; he will then see that the exterior surface of the curtain, which is not viscid, will lie on the anther-case, and the viscid surface of the disc will be on the outside of the hoop.

This is exactly what takes place with our present Mormodes. But the pollinium assumes with such force its reversed curvature (aided, apparently, by a transverse curling outwards of the two margins of the pedicel),