Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/22

 answers to the minute oval bit of membrane to which the caudicle is attached in Orchis, and which is of larger size and convex in the the Fly Ophrys. When the lower part of the helmet is moved by any pointed object, the point so readily slips into the hollow base, and is there so firmly held by the viscid matter, that it appears adapted to stick to some prominent part of an insect's head. The caudicle is short and very elastic; it is attached not to the apex of the helmet, but to the hinder end; if it had been attached to the apex, the point of attachment would have been freely exposed to the air, and would not have contracted and caused the movement of depression in the pollinia when removed from their anther-cells. This movement is well marked, and is necessary to bring the end of the pollen-mass into a proper position to strike the stigma. The two viscid discs stand wide apart. There are two transverse stigmatic surfaces, meeting by their points in the middle; but the broad part of each lies directly under the disc.

The labellum is upturned, making the flower almost tubular. As far as I could ascertain, an insect, in crawling out of, or into, the flower, would be apt to strike against the upper and extraordinarily protuberant ends of the helmet-like discs, and so displace the inferior viscid surfaces, which would adhere to its head or body. There is so deep a hollow at the base of the labellum as almost to deserve to be called a nectary; but I could not perceive any nectar. The flowers are very small and inconspicuous, but emit a strong musky smell, especially at night. They seem highly attractive to insects; in a spike with only seven flowers recently open, four had both pollinia, and one had a single pollinium removed.

Peristylus (or Habenaria) viridis

The Frog Orchis has also been described as having its viscid discs naked, which is incorrect. The two small pouches stand wide apart. The ball of viscid matter is oval, and does not soon set hard; its surface is protected by a minute pouch. The upper membranous surface of the disc is large, and as in the Fly Ophrys (O. muscifera) the point of junction with the caudicle is freely exposed to the air, and does not cause the pollinium to undergo the often described movement of depression. But the caudicles are not doubly bent as in the Fly Ophrys.

The stigmatic surface is small and medial; and though the anther-cases are somewhat inclined backwards and converge a little at their upper ends, thus affecting the position of the pollinia when attached to any object, yet it is at first difficult to understand how the pollinia, when removed by insects, can strike the stigma.

The explanation is rather curious. The elongated labellum forms a rather deep hollow in front of the stigma, and in this hollow, but some way in advance of the stigma, a minute slit-like orifice (n) leads into the short bilobed nectary. Hence an insect, in order to suck the nectar with which the nectary is filled, would have to bend down its head in front of the stigma. The labellum has a medial ridge, which would probably induce the insect to alight on either side; but, apparently to make sure of this, besides the true nectary, there are two spots (n') on each side at the base of the labellum, bordered by its prominent edges, directly under the two pouches, which secrete drops of nectar. Now let us suppose an insect to alight, probably on one side of the labellum, and first lick up the drop of nectar on either side; from the position of the pouches exactly over these drops, it would almost certainly get the pollinium of that side attached to its head; if it were then to go to the mouth of the true nectary, it would assuredly strike the pollinium against the stigma. So that we see this unique case of nectar being secreted from the basal edges of the labellum as well as within the short medial nectary, replaces the power of movement in the pollinia, so general with other Orchids, and replaces the doubly bent caudicles of the Fly Ophrys.

As I have described the case, the flower would receive its own pollen; but if the insect first exhausted the richer source of nectar within the nectary, and afterwards licked up the lateral drops, it would not till then get the pollinia attached to its head, and, flying to another flower, a union would be effected between two distinct flowers or two distinct plants. If, indeed, the insect were to suck the lateral drops first, from what Sprengel has observed in the case of Listera, the insect would perhaps be disturbed by the attachment of the pollinium, and would not go on sucking immediately, but would fly to another flower, and thus a union between distinct individuals would ensue.