Page:Darwin - The various contrivances by which orchids are fertilized by insects (1877).djvu/287

. IX. the tender inner coat with their proboscides, and suck the fluid contained in the inter-cellular spaces. This conclusion has been confirmed by Hermann Müller, and I have further shown that even Lepidoptera are able to penetrate other and tougher tissues. It is an interesting case of co-adaptation that in all the British species, in which the nectary does not contain free nectar, the viscid matter of the disc of the pollinium requires a minute or two in order to set hard; and it would be an advantage to the plant if insects were delayed thus long in obtaining the nectar by having to puncture the nectary at several points. On the other hand, in all the Ophreæ which have nectar ready stored within the nectary, the discs are sufficiently viscid for the attachment of the pollinia to insects, without the matter quickly setting hard; and there would therefore be no advantage to these plants in insects being delayed for a few minutes whilst sucking the flowers.

In the case of cultivated exotic Orchids which have a nectary, without any free nectar, it is of course impossible to feel absolutely sure that it would not contain any under more natural conditions. Nor have I made many comparative observations on the rate of the setting hard of the viscid matter of the disc in exotic forms. Nevertheless it seems that some Vandeæ are in the same predicament as our British species of Orchis; thus Calanthe masuca has a very long nectary, which in all the specimens examined by me was quite dry internally, and was inhabited by powdery Cocci; but in the intercellular spaces between the two coats there was much fluid; and in this species the viscid matter of the disc, after its surface had been disturbed, entirely lost its adhesiveness in two minutes. In an Oncidium the disc, similarly disturbed, became dry in