Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/23

I am endebted to the Rev. B. S. Malden of Canterbury for two spikes of the Frog Orchis. Several of the flowers had one pollinium removed, and one flower had both removed.

We now come to two genera, namely, Gymnadenia and Habenaria, including four British species, which really have uncovered viscid discs. The viscid matter, as before remarked, is of a somewhat different nature from that in Orchis, and does not rapidly set hard. Their nectaries are stored with nectar. With respect to the uncovered condition of the discs, the last species, or Peristylus viridis, is in an almost intermediate condition. The four following forms compose a much broken series. In Gymnadenia conopsea the viscid discs are narrow and much elongated, and lie close together; in G. albida they are less elongated, but still approximate; in Habenaria bifolia they are oval and far apart; and, lastly, in Hab. chlorantha they are circular and much farther apart.

Gymnadenia conopsea

In general appearance this plant resembles pretty closely some species of true Orchis. The pollinia differ in having naked, narrow, strap-shaped discs, which are nearly as long as the caudicles.

When the pollinia are exposed to the air the caudicle is depressed in from 30 to 60 seconds; and as its anterior surface is slightly hollowed out, it closely clasps the upper membranous surface of the disc. The mechanism of this movement will be described in the last chapter. The elastic threads by which the packets of pollen are bound together are unusually weak, as is likewise the case with the two following species of Habenaria: this was well shown by the state of the specimens which had been kept in spirits of wine. This weakness apparently stands in relation to the viscid matter of the discs not setting hard and dry as in Orchis; so that a moth with a pollinium attached to its proboscis might be enabled to visit several flowers and not have the whole pollinium dragged off by the first stigma which was struck. The two strap-shaped discs lie close together, and form the roof of the mouth of the nectary. They are not enclosed, as in Orchis, by a lower lip or pouch, so that the structure of the rostellum is simpler. When we come to treat of the homologies of the rostellum we shall see that this difference is due to a small change, namely, to the lower and exterior cells of the rostellum resolving themselves into viscid matter; whereas in Orchis the exterior surface retains its early cellular or membranous condition.

As the two viscid discs form the roof of the mouth of the nectary, and are thus brought down near the labellum, the two stigmas, instead of being confluent and standing beneath the rostellum, are necessarily lateral and separate. They form two protuberant, almost horn-shaped, processes on each side of the mouth of the nectary. That their surfaces are really stigmatic I ascertained by finding them deeply penetrated by a multitude of pollen-tubes. As in the case of O. pyramidalis, it is a pretty little experiment to push a fine bristle into the narrow mouth of the nectary, and to observe how certainly the narrow elongated viscid discs, forming the roof, stick to the bristle. When the bristle is withdrawn, the pollinia are withdrawn, adhering to its upper side, and slightly divergent owing apparently to their original position in the anther-cells. They then quickly depress themselves till they lie in the same plane with the bristle; and if the bristle, held in the same relative position, be now inserted into the nectary of another flower, the two ends of the pollinia accurately strike the two stigmatic surfaces lying close on each side of the mouth of the nectary. I am, however, not quite sure that I understand the cause of the divergence of the pollinia, for I find that moths often remove one pollinium alone; and this fact leads me to suspect that they insert their probosces obliquely into the nectary.

The flowers smell sweet, and the abundant nectar always contained in their nectaries seems highly attractive to Lepidoptera, for the pollinia are soon and effectually removed. For instance, in a spike with forty-five open flowers, forty-one had their pollinia removed, or had pollen left on their stigmas: in another spike with fifty-four flowers, thirty-seven had both pollinia, and fifteen had one pollinium, removed; so that only two flowers in the whole spike had neither pollinium removed.

Gymnadenia albida