Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/292

272 is seen from above highly magnified in the upper of the two adjoining figures; 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 longitudinally elongated cells. The end of the caudicle is attached to the steep sides of this depression and ridge. 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, which then lies parallel to the elongated tapering part of the disc. If placed in water the ridge rises, re-elevating the caudicle, and when re-exposed to the air it sinks again, but each time with somewhat enfeebled power. During each sinking and rising of the caudicle, the whole pollinium is of course depressed and elevated.

That the power of movement lies exclusively in the surface of the disc is well shown in the case of the saddle-shaped disc of Orchis pyramidalis; for whilst it was held under water I removed the attached caudicles and the layer of viscid matter from the inferior surface, and immediately that the disc was exposed to the air the proper contraction ensued. The disc is formed of several layers of minute cells, which are best seen in specimens that have been 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. 3,