If the Angræcum in its native forests secretes more nectar than did the vigorous plants sent me by Mr. Bateman, so that the nectary becomes filled, small moths might obtain their share, but they would not benefit the plant. The pollinia would not be withdrawn until some huge moth, with a wonderfully long proboscis, tried to drain the last drop. If such great moths were to become extinct in Madagascar, assuredly the Angræcum would become extinct. On the other hand, as the nectar, at least in the lower part of the nectary, is stored safe from depredation by other insects, the extinction of the Angrecum would probably be a serious loss to these moths. We can thus partially understand how the astonishing length of the nectary may have been acquired by successive modifications. As certain moths of Madagascar became larger through natural selection in relation to their general conditions of life, either in the larval or mature state, or as the proboscis alone was lengthened to obtain honey from the Angrecum and other deep tubular flowers, those individual plants of the Angrecum which had the longest nectaries (and the nectary varies much in length in some Orchids), and which, consequently, compelled the moths to insert their probosces up to the very base, would be fertilised. These plants would yield most seed, and the seedlings would generally inherit longer nectaries; and so it would be in successive generations of the plant and moth. Thus it would appear that there has been a race in gaining length between the nectary of the Angrecum and the proboscis of certain moths; but the Angræcum has triumphed, for it flourishes and abounds in the forests of Madagascar, and still troubles each moth to insert its proboscis as far as possible in order to drain the last drop of nectar.
Finally, another genus, Acropera, must be noticed from an independent reason. Although Dr. Hooker sent me, over and over again, fresh flowers of two species (A. luteola[1] and Loddigesii), this genus for a long time remained the opprobrium of my work. All the parts seemed determinately contrived that the plant should never be fertilised. I believe I have at last partly solved the mystery; yet the use of some important parts remains quite unintelligible. But I do not suppose that I completely understand the contrivances in any one Orchid; for I find that the more I study our most commonest British species, continually new and admirable adaptations become apparent.
The rostellum of Acropera is thin and elongated, projecting at right angles to the column (see diagram, Fig. XXII.); the pedicel of the pollinium is of course equally long and very thin; the disc is extraordinarily small, and forms a little cap, viscid within, fitting the extremity of the rostellum. After repeated trials I find that the disc does not adhere to any object until it is drawn quite off the tip of the rostellum; and this can only be well effected by the whole rostellum being pushed upwards so as to slide over and against the touching object: when the small disc is thus removed it adheres well to the object. The upper sepal forms a hood enclosing and protecting the column. The labellum is an extraordinary organ, baffling description: it is articulated to the base of the column by a thin strap, so elastic and flexible that a breath of wind sets it vibrating. It hangs downwards; and this seems to be of importance, for the plant is pendulous, and to place the labellum in this position the footstalk (ovarium) of each flower is curved into a semicircle. The two upper petals serve as lateral guides leading into the hood-like upper sepal, But how all these parts concur in leading an insect to push some part of its body into the hood-like upper sepal and then to raise the rostellum, thus brushing off the little sticky disc, I do not in the least understand.
The pollinium, when adhering by its disc to any object, undergoes the common movement of depression; and this seems superfluous, for the stigmatic cavity lies (see diagram, Fig. XXII.) high up at the base of the rectangularly projecting rostellum. But this is a comparatively trifling difficulty; the real difficulty lies in the orifice of the stigmatic chamber being so narrow that the pollen-masses can hardly be forced in. I repeatedly tried, and succeeded only three or four times. Even after allowing the pollen-masses to dry for an hour, and thus to shrink a little, I rarely succeeded in forcing them in. I examined young flowers and almost withered flowers, for I imagined that the mouth of the chamber at different periods of growth might become larger or smaller, for we have seen that the mouth actually closes up in one species of Bolbophyllum; but the difficulty of insertion always remained the same. Now when we observe, that the viscid disc is extraordinarily small, and consequently its power of attachment not so firm as with Orchids having a large disc, and that the pedicel is long and thin, it would seem almost indispensable that the stigmatic chamber should be unusually large for the easy insertion of the pollinium. Far from this being the case, it is, as just stated, so much contracted that rarely by any force could even one pollen-mass be forced in. Moreover the stigmatic surface, as Dr. Hooker also observed, is singularly little viscid!
I had given up the whole case as inexplicable, when it occurred to me that, although no instance of the separation of the two sexes was known in Orchids, yet that Acropera might be a male plant. I first examined the utriculi of the stigmatic surface from specimens which had been kept in spirits of wine, and I found them empty like little glass cases, but generally with a faint areola or nucleus visible.[2] Now I have looked at the
- ↑ Dr. Lindley informs me that he knows of no species so named; nor is the origin of the name known at Kew. This species or variety differs in little or nothing from A. Loddigesii, excepting in its uniform yellow colour.
- ↑ * See R. Brown in 'Linn. Transact.,' vol. xvi. p. 710, on the nucleus of the stigmatic utriculi; and Bauer's beautiful drawing in Lindley's great work.
