Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/76

 the scheme of nature, as worked out by natural selection, that matter excreted to free the system from superfluous or injuring substances should be utilised for purposes of the highest importance. To give an example in strong contrast with flowers and honey, the larvae of certain beetles (Cassidæ, etc.) use their own excrement to make an umbrella-like projection for their tender bodies.

In Cypripedium the slipper-like labellum seems well adapted to hold and collect nectar; but I have found no such collection in the four previously named species; nor does C. calceolus ever secrete nectar according to Kurr. The labellum, however, in these four species is studded with hairs; and I have almost always noticed on their tips little drops of slightly viscid fluid, which if sweet would certainly suffice to attract insects; this viscid fluid when dried forms a brittle crust, but I could not perceive in it any traces of crystallisation.

It may be remembered that in the first chapter evidence was given proving that nectar is never found within the spur-like nectary of several species of Orchis, but that between its two membranes there is an abundant supply of fluid. In all the species thus characterised the viscid matter of the disc of the pollinium sets hard in a minute or two, and it would be an advantage to the plant if an insect were delayed in getting the nectar by having to puncture the nectary at several points, so that time should be allowed for the setting of the viscid matter. On the other hand, in all the Ophrea which have nectar ready stored within the nectary the viscid matter does not rapidly set hard, and there would be no advantage in insects being delayed.

In the case of cultivated exotic Orchids which have a nectary, without nectar, it is of course impossible to feel sure that it would be empty under more natural conditions. Nor have I made many comparative observations on the rate of setting of the viscid matter of the disc in exotic forms. Nevertheless it seems that certain 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 was internally quite dry, and was inhabited by powdery Cocci; but in the intercellular spaces between its two coats there was much fluid; 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 one minute and a half; in an Odontoglossum in two minutes; neither of these Orchids have free nectar. On the other hand, in Angrecum sesquipedale, which has free nectar stored within the lower end of its nectary, the disc of the pollinium, removed from the plant, and with its surface disturbed, was strongly adhesive after forty-eight hours.

Sarcanthus teretifolius offers a more curious case. The disc quite lost its viscidity, after the removal of the pollinium from the rostellum, in less than three minutes. Hence it might have been expected that fluid would have occurred in the intercellular spaces of the nectary, but none within the nectary; nevertheless there was fluid in both places, so that we here see the two conditions of the nectar-secreting organs combined in the same flower. It might perhaps have been thought that insects would have rapidly sucked the free nectar and neglected that between the two coats. But I strongly suspect that insects are delayed in sucking the free nectar, so as to allow the viscid matter to set hard, by totally different means. In this Orchid the labellum with its nectary is an extraordinary organ. I wished to have had a drawing of its structure; but found that it was as hopeless as to give one of the wards of a complicated lock: even the skilful Bauer, with numerous figures and sections on a large scale, hardly makes the structure intelligible. So complicated is the passage that I failed in repeated attempts to pass a bristle from the outside of the flower into the nectar-receptacle; or in a reversed direction from the cut-off end of the nectar-receptacle to the outside. No doubt an insect with a voluntarily flexible proboscis could wind it through the passages, and thus get the nectar; but in effecting this, some delay would be caused; and time would be allowed for the curious square disc to become securely semented to the insect's head or body.

As in Epipactis the cup at the base of the labellum serves as a nectar-receptacle, I expected to find that the analogous cup in Stanhopea, Acropera, etc., would serve for the same purpose; but I could never find a drop of nectar in it. According, also, to M. Ménirè, this is never the case in these genera, nor in Gongora, Cirrhea, and others. In Catasetum tridentatum, and in its female the Monachanthus, we see that the upturned cup cannot possibly serve as a nectar-receptacle. What then attracts insects to these flowers? That they must be attracted is certain; more especially in the case of Catasetum, in which the sexes stand on separate plants. In many genera of Vandeæ there is no trace of any nectar-secreting organ or receptacle; but in all these cases,