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Show 360 CONCLUDING REMARKS OuAP. XV. power was transferred to the basal parts of tho hairs, or that the surface of the leaf has been prolonged upwards at numerous points, so as ~o unite with the hairs, thus forming the bases of the Inner tentacles. The above named three genera, namely Drosophyllum, Roridula, .and By~l~s, whi~h appear to have retained a primordial condition, still bear glandular hairs on both surfaces of their leaves; but those on the lower surface have since disappeared in the more highly developed genera, with the partial exception of one species, Drosera binata. The sinall sessile glands have also disappeared in soine of the genera, being replaced in Roridula by hairs, ancl in most species of Drosera by absorbent papillm. Drosera binata, with its linear and bifurcating loaves, is in an intermediate condition. It still b ars so1no sessile glands on both surfaces of the leaves, and on the lower surface a few irregularly placed tentacles, which are incapable of movement. A further slight change would con vert the linear leaves of this latter species into the oblong leaves of Drosera anglica, and these Inight easily pass into orbicular ones with footstalks, like those of Drosera rotundifolia. The footstalks of this latter species bear multicellular hairs, whi0h we have good reason to believer present aborted tentacles. The parent form of Dionrea and Alclrovanda seems to have been closely alli d to Drosera, and to have had rounded leaves, supported on distinct footstalks, and furnished with tentacles all round the circumference, with other tentacles and sessile glands on the upper surface. I think so because the marginal spikes of Dionrea apparently represent the extreme marginal tentacles of Drosera, the six (sometimes eight) sensitive filaments on the upper surface, as well as the more numerous ones in Aldrovanda, r presenting the central CHAP. XV. ON THE DROSERACE~. 361 tentacles of Drosera, with their glands aborted, but their sensitiveness retained. Under this .point of view we should bear in mind that the summits of the tentacles of Drosera, close beneath the glands, are sensitive. The three most remarkable characters possessed by the several members of the Droseracere consist in the leaves of so1ne having the power of movement when excited, in their ·glands secreting a fluid which digests animal matter, and in their absorption of the digested matter. Can any light be thrown on the steps by which these remarkable powers were gradually acquired? As the walls of the cells are necessarily permeable to fluids, in ordoc to allow the glands to secrete, it is not surprising that they should readily allow fluids to pass inwards; and this inward passage would deserve to be called an act of absorption, if the fluids combined with the contents of the glands. Judging from the evidence above given, the secreting glands of many other plants can absorb salts of ammonia, of which they must receive small qua~tities from the rain. This is the case with two species of Saxifraga, and the glands of one of them apparently absorb matter from captured insects, and certainly from an infusion of raw meat. There is, therefore, nothing anomalous in the ?roseracere having acquired the power of absorption In a much more highly developed degree. It is a far more remarkable problem how the members of this family, and Pinguicula, and, as Dr. Hoo~er has recently shown, Nepenthes, could all have acquned the power of secreting a fluid which dissolves or digests animal matter. The six genera of the Droseraceoo have probably inherited this power from a common progenitor, but this cannot apply to |
