Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/84

 converted into a highly elastic and tender membrane, which is in itself slightly viscid, and allows the underlying viscid matter readily to exude; yet it acts as a membrane, with its under surface lined with much more viscid matter: in Habenaria chlorantha the exterior surface is highly viscid, but still closely resembles, under the microscope, the exterior membrane of Epipactis. Lastly, in some species of Oncidium, etc., the viscid exterior surface differs, as far as appearance under the microscope goes, from the underlying viscid layer only in colour; but it must have some essential difference; for I find that, until this very thin exterior layer is disturbed, the underlying matter remains viscid; whilst, after it has been disturbed, it rapidly sets hard. The gradation in the state of the surface of the rostellum is not surprising, for in all cases in the bud the surface is cellular; so that an early condition has only to be more or less perfectly retained.

The nature of the viscid matter differs remarkably in different Orchids: in Listera it sets hard almost instantaneously, more quickly than plaster of Paris; in Malaxis and Angrecum it remains fluid and viscid for several days; but these two states pass into each other by many gradations: in an Oncidium I have observed the viscid matter to dry in a minute and a half; in some species of Orchis in two or three minutes; in Epipactis in ten minutes; in Gymnadenia in two hours; and in Habenaria in over twenty-four hours. After the viscid matter of Listera has set hard, neither water nor weak spirits of wine has any effect on it; whereas that of Habenaria bifolia, after being kept in spirits, and after having been dried for several months, when moistened was as sticky as ever; the viscid matter in some species of Orchis, when remoistened, presented an intermediate condition.

One of the most important differences in the state of the rostellum is, whether or not the pollinia are congenitally attached to it. I do not allude to those cases in which the upper surface of the rostellum becomes viscid, as in Malaxis and some Epidendrums, and adheres without mechanical aid to the pollen-masses; for these cases present no difficulty, and can be graduated together. But I refer to the so-called congenital attachment of the pollinia by their caudicles. It is not, however, strictly correct to speak of congenital attachment, for the pollinia at an early period are invariably free, and become attached either earlier or later in different Orchids. No actual gradation is at present known in the process of attachment; but it can be shown to depend on very simple conditions and modifications. In the Epidendreæ the pollinia consist of a ball of waxy pollen, with a long caudicle (formed of elastic threads with adherent pollen-grains), which never becomes spontaneously attached to the rostellum. Cymbidium giganteum, on the other hand, has a congenitally attached caudicle, but its structure is identically the same, with the sole difference, that the elastic threads near its base adhere to, instead of simply lying on, the upper lip of the rostellum.

In an allied form, the Oncidium unguiculatum, I examined the development of the caudicles. At an early period the pollen-masses are enclosed in membranous cases; which soon rupture at one point. At this early period, within the cleft of each pollen-mass, a layer of cellular matter may be detected, with the cells of rather large size, including remarkably opaque matter. The contained matter may be traced undergoing all the stages of development into the translucent substance forming the threads of the caudicles. As this change progresses, the cells themselves disappear. The threads at the one end finally adhere to the waxy pollen, and at the other end, whilst still in a semi-developed state, they protrude through the small opening of the membranous case. and adhere to the rostellum, against which the anther is pressed. So that the adhesion of the caudicle to the back of the rostellum seems to depend solely on the early rupturing of the anther-case, and on the slight protrusion of the caudicles, before they have become fully developed and hardened.

In all Orchids a portion of the rostellum is, in fact, removed by insects when the pollinia are removed; for the viscid matter, though conveniently spoken of as a secretion, is a part of the rostellum in a modified condition.

But in those Orchids, which have their caudicles at an early period attached to the rostellum, a membranous or solid portion of its exterior surface in an unmodified condition is likewise removed. In the Vandeæ this portion is sometimes of considerable size (forming the disc and pedicel of the pollinium), and gives to their pollinia their most remarkable character; but the differences in the shape and size of the removed portions of the rostellum can be finely graduated together, even within the Vandee; and still more closely by commencing with the minute oval atom of membrane to which the caudicle of Orchis adheres, passing thense to that of Habenaria bifolia, to that of H. chlorantha with its drum-like pedicel, and thense through many forms to the great disc and pedicel of Catasetum.

In all the cases in which a portion of the exterior surface of the rostellum is removed attached to the caudicles, definite and often complicated lines of separation are formed, or are at least prepared by being weak, in order to allow of the easy separation of the removeable portions. But the formation of the lines of weakness does not differ much from certain definite portions of the exterior surface of the rostellum assuming a condition intermediate between that of unaltered membrane and of viscid matter, which has been already alluded to. The actual separation of the lines depends in many, perhaps in all, cases on the excitement