Page:On the Various Contrivances by Which British and Foreign Orchids are Fertilised by Insects, and on the Good Effects of Intercrossing.djvu/55

 If a thick needle be inserted into the mouth of the nectary (Fig. A), and then withdrawn, the viscid disc will be withdrawn, bearing with it the elegant fan of radiating pollen-masses. These undergo no change in position. But if the needle be now inserted into the nectary of another flower the ends of the pollen-masses necessarily hit the upper and laterally sloping sides of the rostellum, and, glancing off both ways, they strike down into the two lateral pit-like stigmas. The thin caudicles being easily ruptured, the pollen-masses are left adhering like little darts (see left-hand stigma in Fig. C) to the viscid surface of both stigmas, and the fertilisation of the flower is completed in a simple manner pleasing to behold.

I should have stated that a narrow transverse rim of stigmatic tissue, running beneath the rostellum, connects the two lateral stigmas; and it is probable that some of the middle pollen-masses may be inserted through the notch in the rostellum or beneath its surface. I am more inclined to this opinion from having found in the elegant Calanthe vestita that the rostellum extends so widely over the two lateral stigmas, that apparently all the pollen-masses would have been inserted beneath its surface.

I fear that the reader will be wearied, but I must say a few words on the Angræcum sesquipedale, of which the large six-rayed flowers, like stars formed of snow-white wax, have excited the admiration of travellers in Madagascar. A whip-like green nectary of astonishing length hangs down beneath the labellum. In several flowers sent me by Mr. Bateman I found the nectaries eleven and a half inches long, with only the lower inch and a half filled with very sweet nectar. What can be the use, it may be asked, of a nectary of such disproportional length? We shall, I think, see that the fertilisation of the plant depends on this length and on nectar being contained only within the lower and attenuated extremity. It is, however, surprising that any insect should be able to reach the nectar: our English sphinxes have probosces as long as their bodies: but in Madagascar there must be moths with probosces capable of extension to a length of between ten and eleven inches!

The rostellum is broad and foliaceous, and arches rectangularly over the stigma and over the orifice of the nectary: it is deeply cleft, with the cleft enlarged or widened at the end. Hence the rostellum pretty closely resembles (see Fig. XXIV., C) that of Calanthe after the disc has been removed. The under surfaces of both margins of the cleft near its end are bordered by narrow strips of viscid membrane, easily removed; so that there are two distinct viscid discs. To the middle of each disc a short membranous pedicel is attached; and each pedicel carries at its other end a pollen-mass. Beneath the rostellum a narrow, ledge-like, viscid stigma is seated.

I could not for some time understand how the pollinia of this Orchid were removed, or how it could be fertilised. I passed bristles and needles down the open entrance into the nectary and through the cleft in the rostellum with no result. It then occurred to me that, from the length of the nectary, the flower must be visited by large moths, with a proboscis thick at the base; and that to drain the last drop of nectar even the largest moth would have to force its proboscis as far down as possible. To effect this, whether or not the moth first inserted its proboscis by the open entrance into the nectary (as is most probable, from the shape of the flower, etc.) or through the cleft in the rostellum, it would ultimately force its proboscis into this cleft, for this is the straightest course, and by slight pressure the whole foliaceous rostellum can be depressed: the distance from the outside of the flower to the extremity of the nectary can be thus shortened by about a quarter of an inch. Hence I took a cylinder, one-tenth of an inch in diameter, and pushed it down through the cleft in the rostellum: the margins readily separated, and were pushed downwards together with the whole rostellum. When I slowly withdrew the cylinder the rostellum rose from its elasticity, and the margins of the cleft upturned and clasped the cylinder. Thus the viscid strips of membrane on the under sides of the cleft rostellum came into contact with the cylinder, and firmly adhered to it; and the pollen-masses were withdrawn. By this means alone I succeeded in each case in withdrawing the pollinia; and it cannot, I think, be doubted that a large moth must thus act; namely, by driving its proboscis up to the very base, through the cleft of the rostellum, so as to reach the extremity of the nectary; and then withdrawing its proboscis with the pollinia attached to it.

I did not succeed in imitating the fertilisation of the flower so well as I did in withdrawing the pollinia, but I effected it twice. As the margins of the cleft rostellum must be upturned before the discs adhere to the cylinder, they become, during its withdrawal, affixed some little way from its actual base. The two discs did not always adhere at exactly corresponding points. Now, when a moth inserts its proboscis, with the pollinia affixed to it near the base, into the mouth of the nectary, the pollen-masses will probably be first inserted beneath the rostellum; and during the final exertion, when the moth pushes its proboscis through the cleft of the rostellum, the pollen-masses will almost necessarily be placed on the narrow, ledge-like stigma projecting beneath the rostellum. By acting thus with the pollinia attached to the cylinder the pollen-masses were twice torn off and left glued to the stigmatic surface.