Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/152

132 rostellum above, is united on each side behind to a green membranous expansion, convex exteriorly and concave interiorly, of which the summits on each side are pointed and stand a little above the crest of the rostellum. These two membranes sweep round (see back views, figs. C and D), and are united to the filament or base of the anther; they thus form a cup-like clinandrum behind the rostellum. The use of this cup is to protect laterally the pollen-masses. When I have to treat of the homologies of the different parts, it will be shown by the course of the spiral vessels that these two membranes consist of the two upper anthers of the inner whorl, in a rudimentary condition, but utilised for this special purpose.

In a flower before it expands, a little mass or drop of viscid fluid maybe seen on the crest of the rostellum, rather overhanging its front surface. After the flower has remained open for a little time, this drop shrinks and becomes more viscid. Its chemical nature is different from that of the viscid matter in most Orchids, for it remains fluid for many days, though fully exposed to the air. From these facts I concluded that the viscid fluid exuded from the crest of the rostellum; but fortunately I examined a closely-allied Indian form, namely, the Microstylis rhedii (sent me from Kew by Dr. Hooker), and in this, before the flower opened, there was a similar drop of viscid matter; but on opening a still younger bud, I found a minute, regular, tongue-shaped projection on the crest of the rostellum, formed of cells, which when slightly disturbed resolved themselves into a drop of viscid matter. At this age, also, the front surface of the whole rostellum, between its crest and the pocket-like stigma, was coated with cells filled with similar brown viscid matter; so that there can