Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/89

 magnificent Vandeæ as the highest. When we look within this tribe at the elaborate mechanism of Catasetum for the ejection and transportal of the pollinia, with the sensitive rostellum so wonderfully modified, with the sexes borne on distinct plants, we may perhaps give to this genus the palm of victory.

A few miscellaneous points, which could not elsewhere have been conveniently introduced, deserve to be noticed. First, for the mechanism by which the pollinia in so many Orchids undergo a movement of depression, when removed from their places of attachment and exposed for a few seconds to the air. This is always due to the contraction of a portion, sometimes, as in Orchis, to an exceedingly minute portion, of the exterior surface of the rostellum, which has retained a membranous condition. This membrane, as we have seen, is likewise sensitive to a touch. In one Maxillaria the middle of the pedicel, and in Habenaria the whole drum-like pedicel, contracts. The point of contraction in all other cases seen by me is either close to the surface of attachment of the caudicle to the disc, or at the point where the pedicel is united to the disc; but both the disc and pedicel are parts of the exterior surface of the rostellum. In these remarks I do not refer to the movements simply due to the elasticity of the pollinia in the Vandeæ.

The long strap-formed disc of Gymnadenia conopsea is well adapted to show the mechanism of the movement of depression. The whole pollinium, both in its upright and depressed (but not closely depressed) position, has been shown by Fig. X. The disc, highly magnified, in its uncontracted condition, is seen from above in the upper figure here given, with the caudicle removed; and in the lower figure we have a longitudinal section of the uncontracted disc, together with the base of the attached and upright caudicle. At the broad end of the disc there is a deep crescent-shaped depression, bordered by a slight ridge formed of elongated cells. The end of the caudicle is attached to the steep sides of this depression and ridge. Now, when the disc is exposed to the air for about thirty seconds, the ridge contracts and sinks flat down; in sinking, it drags with it the caudicle. When placed again in water the ridge rises, and when re-exposed to the air it sinks, but each time with somewhat enfeebled power. With each sinking and rising of the caudicle, the whole pollinium is depressed and elevated.

That the power of movement lies exclusively in the surface of the rostellum is well shown by the saddle-shaped disc of Orchis pyramidalis; for I removed under water the attached caudicles, as well as the layer of viscid matter from its under surface, and immediately that it was exposed to the air the proper contraction ensued. The disc is here formed of several layers of minute cells (and I believe this to be the case with the disc of the Gymnadenia), which are best seen in specimens kept in spirits of wine, for their contents are thus rendered more opaque. The cells in the flaps of the saddle are a little elongated. As long as the saddle is kept damp its upper surface is nearly flat, but when exposed to the air (see Fig. III. E) the surface contracts immediately beneath the point of attachment of the truncated end of each caudicle, and becomes oblique; and two valleys are likewise formed in front of the two caudicles. By this contraction the caudicles are thrown down, almost in the same way as if trenches were dug in front of two upright poles, and the ground at the same time undermined beneath them. As far as I could perceive, an analogous contraction causes the depression of the pollinia in Orchis mascula.

Some pollinia which had been gummed on card for several months, when placed in water, underwent the movement of depression; and a fresh pollinium, when alternately damped and exposed to the air, can be made to rise and sink several times. Before I had ascertained these facts, which seem to show that the movement is hygrometric, I thought that it was a vital action, and tried vapour of chloroform and prussic acid, and immersion in laudanum; but these reagents did not check the movement. Nevertheless, there are considerable difficulties in understanding how the movement can be simply hygrometric. The flaps of the saddle in Orchis pyramidalis (see Fig. III. D) curl completely inwards in nine seconds, which is surprisingly quick for the action of mere evaporation; and it is the under surface which curls inwards and ought to dry so quickly; but this cannot happen, as it is covered with a thick layer of viscid matter: the edges, however, of the saddle might become slightly dry in the nine seconds. When the saddle-formed disc is placed in spirits of wine it contracts energetically, and when placed in water opens again. This does not look as if the action was simply hydrometric. Whether the contraction is hygrometric, or is due to endosmose, or to some other unknown cause, the movements of depression in the pollinia thus produced are admirably regulated in each