Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/51

 We now come to Lindley's immense Tribe of the Vandeæ, which includes many of the most extraordinary productions of our hot-houses, but has no British representative. I have examined twenty-four genera. The pollen consists of waxy masses, as in the two last Tribes, and each ball of pollen is furnished with a caudicle, which becomes, at an early period of growth, united to the rostellum. The caudicle is seldom attached directly to the viscid disc, as in the Ophree, but to the upper and posterior surface of the rostellum; and this part, together with the disc, is removed by insects. The imaginary diagram (Fig. XXII), with the parts separated, will best explain the type-structure of the Vandeæ. The middle organ (2) is the dorsal or posterior pistil of the three pistils always present in Orchids; its upper part is modified into and forms the rostellum, and is curved over the stigma. The stigma consists of two stigmas belonging to the two other confluent (3) pistils. On the left hand we have the filament (1) bearing the anther. The anther opens at an early period, and the tips of the two caudicles protrude through a small slit (only one caudicle and one pollen-mass is represented in the diagram) in a not fully-hardened condition, and adhere to the back of the rostellum. The surface of the rostellum is generally hollowed out for the reception of the pollen-masses; it is represented as smooth in the diagram, but is really often furnished with crests or knobs for the attachment of the two caudicles. The anther afterwards opens more widely on its under surface, and leaves the two pollen-masses unattached, excepting by their caudicles to the rostellum.

During this early period of growth, a remarkable change has been going on in the rostellum: either its extremity or its lower surface becomes extremely viscid, and a line of separation, at first appearing as a mere hyaline zone of tissue, gradually is formed, which separates the viscid extremity or disc, as well as the whole upper surface of the rostellum, as far back as the point of attachment of the caudicles. If any object now touches the viscid disc, it, the whole back of the rostellum, the caudicles and pollen-masses, can all be readily removed together. In botanical works the whole structure between the disc (generally called the gland) and the waxy balls of pollen is designated as the caudicle; but as these parts play an essential part in the fertilisation of the flower, and as they are fundamentally different in their origin and in their minute structure, I shall call the two elastic ropes, which are developed strictly within the anther-cells, the caudicles; and the portion of the rostellum to which the caudicles are attached (see diagram), and which is not viscid, the pedicel. The viscid portion of the rostellum I shall call, as heretofore, the viscid disc. The whole may be conveniently spoken of as the pollinium.

In the Ophreæ we always have (except in O. pyramidalis) two separate viscid discs. In the Vandee, with the exception of Angræcum, we have only one disc. The disc is naked, or is not enclosed in a pouch. In Habenaria the discs, as we have seen, are separated from the two caudicles by short drum-like pedicels, answering to the single and generally much more largely developed pedicel in the Vandeæ. In Ophreæ the caudicles of the pollinia, though elastic, are rigid, and serve to place the packets of pollen at the right distance from the insect's head or proboscis, so as to reach the stigma. In the Vandeæ this end is gained by the pedicel of the rostellum. The two caudicles in the Vandeæ are attached and embedded within a deep cleft in the pollen-masses, and until stretched are rarely visible, for the pollen-masses lie close to the pedicel of the rostellum. These caudicles answer both in position and function to the elastic threads, by which the packets of pollen are tied together in the Ophrea, at the point where they become confluent and where they form the upper part of the caudicle; for the function of the true caudicle in the Vandee is to break when the masses of pollen, transported by insects, adhere to the stigmatic surface.

In many Vandeæ the caudicles are easily ruptured, and the fertilisation of the flower, as far as this point is concerned, is a simple affair; but in other cases the strength of the caudicles and the length to which they can be stretched before they break is surprising. I was at first perplexed to understand what good purpose the great strength of the caudicles and their capacity of extension could serve. It is obvious that, when projecting far out from an insect' head, whilst flying about (and the insect, in the case of the larger Orchids, must be of considerable size), the strength of the caudicles would protect the pollen-masses from being brushed off and lost. So again, when an insect transporting a pollinium visits a flower either too young, with its stigma not yet sufficiently viscid, or one already impregnated, with its stigma beginning to dry, the strength of the caudicle would prevent the pollen-masses from being uselessly removed. It should be remembered that the pollen-masses are precious objects, for each flower, in most of the genera, produces only two; and in many cases, judging from the size of stigma, both pollen-masses would be left on one stigma, though in other cases the size of the orifice of the stigma allows the introduction of one pollen-mass alone; so that, in the latter case, the pollen from one flower probably suffices to fertilise two flowers.

lthough at the proper period the stigmatic surface is astonishingly viscid in many cases, as in Phalænopsis and Saccolabium, yet when, having removed the pollinia adhering to a rough scalpel by their viscid discs, I inserted the balls of pollen into the stigmatic chamber, they did not adhere to the surface with sufficient force to prevent their withdrawal. I even left them for some little time in contact with the viscid surface, as an insect would do whilst feeding; but when I pulled the pollinia straight out of the stigmatic chamber, the