Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/26

The remarkable length of the nectary, containing much nectar, the white colour of the conspicuous flower, and the strong sweet odour emitted at night, all show that this plant depends for it fertilisation on the larger nocturnal Lepidoptera. I have often found spikes with almost all the pollinia removed. From the lateral position and distance of the two viscid discs from each other, the same moth would generally remove only one pollinium at a time; and in a spike which had not as yet been much visited, three flowers had both pollinia, and eigth flowers had only one pollinium, removed. From the position of the discs it might have been anticipated that they would adhere to the side of the head or face of moths; and Mr. F. Bond sent me a specimen of Hadena dentina with one eye covered and blinded by a disc, and a specimen of Plusia v. aureum with a disc attached to the edge of the eye. Although the discs are so extremely adhesive that almost all the pollinia of a bunch of flowers, when carried in the hand, will be removed by the shaken petals and sepals touching the discs, yet it is certain that moths, perhaps the smaller species, often visit these flowers without removing the pollinia; for on carefully examining the discs of a larger number of pollinia still in their cells I found minute Lepidopterous scales glued to them.

Habenaria bifolia, or Lesser Butterfly Orchis.

I am aware that this form and the last are considered by Mr. Bentham and some other botanists as mere varieties of each other; for it is said that intermediate gradations occur in the position of the viscid discs. But we shall immediately see that the two forms differ in a great number of characters, not to mention the differences in general aspect and in the stations inhabited; with which we are not here concerned. Should these two forms be hereafter proved to graduate at the present day into each other, it would be a remarkable case of variation; and I, for one, should be as much pleased as surprised at the fact, for these two forms certainly differ from each other more than do most species of the genus Orchis.

The viscid discs of the Lesser Butterfly Orchis are oval; they face each other, and stand far closer together than in the last species; so much so, that in the bud, when their surfaces are cellular, they almost touch. They are not placed so low down relatively to the mouth of the nectary. The viscid matter is of a somewhat different chemical nature, as shown by its much greater viscidity, when moistened, after having been long dried, or after being kept in weak spirits. The drum-like pedicel can hardly be said to be present, and is represented by a longitudinal ridge, truncated at the end where the caudicle is attached, and there is hardly a vestige of the rudimentary tail of the caudicle. In Fig. XII. the discs of both species, of the proper proportional sizes, are represented as seen from vertically above. The pollinia, after removal from their cells, undergo the same movements; but the inward movement seemed to be greater than in the last species, in conformity with the position of the stigma. In both forms the movement is well seen by removing, with a pair of pincers, a pollinium by the thick end, and holding it motionless under the microscope, when the plane of the viscid disc will be seen to move through an angle of at least 45°. The caudicles of the Lesser Butterfly Orchis are relatively very much shorter than in the other species; the little packets of pollen are shorter, whiter, and, in a mature flower, separate much more readily from each other. Lastly, the stigmatic surface is differently shaped, being more plainly tripartite, with two lateral prominences, situated beneath the viscid discs. These prominences contract the mouth of the nectary, making it subquadrangular. Hence I cannot doubt that the Larger and Lesser Butterfly Orchids are distinct species, masked by close external similarity.

As soon as I had examined the Lesser Butterfly Orchis, I felt convinced, from the position of the viscid discs, that it would be fertilised in a different manner from the Larger Butterfly Orchis; and now, owing to the kindness of Mr. F. Bond, I have examined two moths, namely, Agrotis segetum and Anaitis plagiata, one with three pollinia, and the other with five pollinia, attached not to the side of the face as in the last species, but to the base of the proboscis. I may remark that the pollinia of these two species of Habenaria, when attached to moths, could be distinguished at a glance.

We have now finished with the Ophrea; but before passing on to the next Division; I will recapitulate the chief facts on the movements of the pollinia, all due to the nicely regulated contraction of that small portion of membrane (together with the pedicel in the case of Habenaria) lying between the layer or ball of adhesive matter and the extremity of the caudicle. In most of the species of Orchis the stigma lies directly beneath the anther-cells, and the pollinia simply move vertically downwards. In Orchis pyramidalis and in Gymnadenia there are two lateral and inferior stigmas, and the pollinia move downwards and outwards, diverging at the proper angle (by a different mechanism in the two cases), so as to strike the two lateral stigmas; in Habenaria the stigmatic surface lies beneath and between the two widely separated anther-cells, and the pollinia here