Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/47

 and its lower part is produced into a nectary, which penetrates the ovarium.

Now for the action of these parts. If any body of size proportional to that of the tubular flower be forced into it—a dead humble-bee acted best—the tongue-shaped rostellum is depressed, and the object often gets slightly smeared with viscid matter; but in withdrawing the object, the tongue-formed rostellum is upturned, and a surprising quantity of viscid matter is forced over its edges and sides, and at the same time into the lip of the anther, which is slightly raised by the upturning of the rostellum. Thus the protruding tips of the caudicles are instantly glued to the retreating body, and the pollinia are withdrawn. This hardly ever failed to occur in my repeated trials. A living bee or other large insect alighting on the fringed edge of the labellum,and scrambling into the flower, would depress the labellum and would be less likely to disturb the rostellum, until it had sucked the nectar and began to retreat. When a dead bee, with the four waxy balls of pollen dangling by their caudicles from its back, is forced into another flower, some or all of these balls are surely caught by the broad, shallow, and very viscid stigmatic surface, which likewise tears off the grains of pollen from the threads of the caudicles.

That living humble-bees can thus remove the pollinia is certain. Sir W. C. Trevelyan sent to Mr. Smith of the British Museum (who forwarded to me) a Bombus hortorum—caught in his hot-house, where a Cattleya was in flower—with its whole back, between the wings, smeared with dried viscid matter, and with the four pollinia attached to it by their caudicles, and ready to be caught by the stigma of another flower if the bee had entered one.

The caudicles of the pollinia being free, and the viscid matter from the rostellum not coming into contact with them without mechanical aid, as well as the general manner of fertilisation, are the same in those species which I have seen in Lælia, Leptotes, Sophronitis, Barkeria, Phaius, Evelyna, Bletia and Coelogyne. In Coelogyne cristata the upper lip of the rostellum is much elongated. In Evelyna caravata eight balls of waxy pollen are all united to one caudicle. In the Barkeria the labellum, instead of enfolding the column, is pressed against it, and this would even more effectually compel insects to brush against the rostellum. In Epidendrum we have a slight difference; for the upper surface of the rostellum, instead of permanently remaining membranous, as in the above-named genera, is so tender that by a touch it breaks up, together with the whole lower surface, into a mass of viscid matter. In this case the whole of the rostellum, with the adherent pollinia, is removed by inects when they retreat from the flower. I observed in E. glaucum that viscid matter exuded from the upper surface of the rostellum when touched, as I have seen in Epipactis; in fact it is difficult to say, in these cases, whether the upper surface of the rostellum should be called membrane or viscid matter.

In Epidendrum floribundum there is a rather greater difference: the anterior horns of the clinandrum (i.e. the cup at the summit of the column in which the pollinia lie) approach each other so closely as to adhere to the two sides of the rostellum, which consequently lies in a nick, with the pollinia over it: and as, in this species, the upper surface of the rostellum resolves itself into viscid matter, the pollinia become glued to it without any mechanical aid. The pollinia, though thus attached, cannot, of course, be removed out of their anther-cells without the aid of insects. In this species it seems possible (though, from the position of parts, not probable) than an insect might drag the pollinia on to its own stigma. In all the other species of Epidendrum which I have examined, and in all the above-named genera, as the pollinia lie unattached above the rostellum, it is evident that the viscid matter has to be forced upwards into the lip of the anther by a retreating insect, which would thus necessarily carry the pollinia from one flower to the stigma of another flower.

MALAXEÆ

Turning now to the Malaxeæ: in Pleurothallis prolifera and ligulata (?) the pollinia have a minute caudicle, and mechanical aid is requisite to force the viscid matter from the under side of the rostellum into the anther, thus to catch the caudicles and remove the pollinia. On the other hand, in our British Malaxis and in the Microstylis Rhedii from India, the upper surface of the minute tongue-shaped rostellum becomes viscid and adheres to the pollinia without mechanical aid. In these two genera we have the curious case of the lower and flat surface of the rostellum being coated with a thin layer of viscid matter, apparently for the purpose of securing the pollinia in a proper position when brought by insects, so as to enter or remain in the slit-like stigma. In Stelis racemiflora the pollinia had also apparently (for the flowers were not in a good condition) become spontaneously attached to the rostellum; and I mention this latter flower chiefly because some insect in the hot-house at Kew had removed most of the pollinia, and had left some of them adhering to the lateral stigmas. These curious little flowers are widely expanded and much exposed; but afer a time the three petals close with perfect exactness and shut up the flower, so that it is scarcely possible to distinguish an old flower from a bud; yet, to my surprice, I found that the closed flowers opened under water.