Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/53

 weight, they had become in the least degree depressed. This is all that takes place in Lycaste Skinnerii, Cymbidium giganteum, Zygopetalum Mackai, Angræcum eburneum, Miltonia Clowesii, in a Warrea, and, I believe in Galeandra Funkii. But if in our diagram we suppose the stigma to be seated lower down at the bottom of a deep cavity, or suppose the anther to be seated higher up so that the pedicel of the rostellum sloped upwards, and under other contingencies not worth detailing, all of which occur, in such cases, an insect with the pollinium attached to its head, on flying to another flower would not strike the stigma with the pollen-masses unless some great change had intervened in their position after attachment.

This change is effected in many Vandeæ in the same manner as is so general with the Ophreæ, namely, by a movement of depression in the pollinium in the course of about half a minute after its removal from the rostellum. I have seen this movement conspicuously displayed, generally causing the pollinium after detachment to rotate through about a quarter of a circle, or 90°, in several species of Oncidium, Odontoglossum, Vanda, Aerides, Sarcanthus, Saccolabium, Acropera, and Maxillaria. In Rodriguezia suaveolens the movement of depression is remarkable from its extreme slowness; in Eulophia viridis from its small extent. In some, indeed, of the cases specified of the pollinia undergoing no movement, I am not sure that there is not a very slight depression. In the Ophrea the anther-cells are sometimes seated exteriorly and sometimes interiorly with respect to the stigma; and we accordingly have outward and inward movements in the pollinia; but in the Vandee the anther-cells lie directly over the stigma, and the movement of the pollinium is always directly downwards. In Calanthe, however, the two stigmas are placed exteriorly to the anther-cells, but the pollinia, as we shall see, are made to strike them by a mechanical arrangement of the parts.

In the Ophreæ the seat of contraction, which causes the act of depression, is in the upper surface of the viscid disc, near the point of attachment of the caudicle: in the Vandee the seat is likewise in the upper surface of the viscid disc, but at the point where the pedicel is united to it, and therefore at a considerable distance from the point of attachment of the true caudicles. The contraction and consequent movement is hygrometric in its nature (but the subject, as we shall see in the seventh chapter, is rather obscure), and consequently does not take place until the pollinium is removed from the rostellum, and the point of union of the disc and pedicel has been exposed for a few seconds to the air. If, after the contraction and consequent movement of the pedicel, the whole body be placed into water, the pedicel slowly moves back and resumes the same position with respect to the viscid disc which is held when forming part of the rostellum. When taken out of the water, it again undegoes the movement of depression. It is of importance to notice these facts, as we thus get a test by which this movement can be distinguished from certain other movements.

In one species of Maxillaria, viz. in the M. ornithorhyncha, we have a unique case. The pedicel of the rostellum is much elongated, and is entirely covered by the produced front lip of the anther, and is thus kept damp. When removed there is no movement at the junction of the disc and the pedicel; but the pedicel itself, at a point rather more than half-way up, quickly bends backwards on itself, in a reversed direction compared with all other cases. When the pedicel is placed in water it resumes its original straight form. If we suppose the long upright neck of a bird to represent the pedicel, the head representing the balls of pollen, then in all ordinary cases the movement is that of a bird picking up food from the ground, but bending only the lower vertebrae close to its body; whereas, in this Maxillaria, the movement is as if the bird threw its head backwards so as nearly to touch its own back, the middle vertebrae of the neck being alone bent. I stated above that, when the pedicel is long and the stigmatic cavity shallow, as in this Maxillaria, we have a compensating action; and here we have one instance.

The labellum has a square projection in front of the stigma, the passage into the flower being thus contracted; and if the pedicel of the rostellum did not somehow become shortened, the flower could hardly be fertilised. After the reversed movement just described, and the consequent shortening of the pedicel, the pollinium, when attached to any small object, can be inserted into the flower, and the balls of pollen are so placed as readily to adhere to the stigmatic surface.

In some cases, besides these hygrometric movements, elasticity comes into play. In Aerides odorata and virens, and in Oncidium (roseum?), the pedicel of the rostellum is fastened down in a straight line, by the disc at one extremity and by the anther at the other; it has, however, a strong natural elastic tendency to spring up at right angles to the disc. Consequently, when the pollinium is removed by its viscid disc sticking to any object, the pedicel instantly springs up and stands at nearly right angles to its former position, with the pollen-masses carried aloft. This has been noticed by other observers; and I agree with then that the object gained is to free the pollen-masses from their anther-cells. After this upward elastic spring, the downward hygrometric movement immediately commences, which, oddly enough, carries the pedicel back again into almost the same position, relative to the disc, which it held whilst adhering to the rostellum. The end of the pedicel, however, which in Aerides carries the pollen-mass by short dangling caudicles, remains curved a