These dorsal tentacles differ in one important respect from those on the upper surface, namely, in not possessing any power of movement, in whatever manner they may be stimulated. Thus, portions of four leaves were placed at different times in solutions of carbonate of ammonia (one part to 437 or 218 of water), and all the tentacles on the upper surface soon became closely inflected; but the dorsal ones did not move, though the leaves were left in the solution for many hours, and though their glands from their blackened colour had obviously absorbed some of the salt. Rather young leaves should be selected for such trials, for the dorsal tentacles, as they grow old and begin to wither, often spontaneously incline towards the middle of the leaf. If these tentacles had possessed the power of movement, they would not have been thus rendered more serviceable to the plant; for they are not long enough to bend round the margin of the leaf so as to reach an insect caught on the upper surface, Nor would it have been of any use if these tentacles could have [page 284] moved towards the middle of the lower surface, for there are no viscid glands there by which insects can be caught. Although they have no power of movement, they are probably of some use by absorbing animal matter from any minute insect which may be caught by them, and by absorbing ammonia from the rain-water. But their varying presence and size, and their irregular position, indicate that they are not of much service, and that they are tending towards abortion. In a future chapter we shall see that Drosophyllum, with its elongated leaves, probably represents the condition of an early progenitor of the genus Drosera; and none of the tentacles of Drosophyllum, neither those on the upper nor lower surface of the leaves, are capable of movement when excited, though they capture numerous insects, which serve as nutriment. Therefore it seems that Drosera binata has retained remnants of certain ancestral characters--namely a few motionless tentacles on the backs of the leaves, and fairly well developed sessile glands--which have been lost by most or all of the other species of the genus.]
Concluding Remarks.--From what we have now seen, there can be little doubt that most or probably all the species of Drosera are adapted for catching insects by nearly the same means. Besides the two Australian species above described, it is said* that two other species from this country, namely Drosera pallida and Drosera sulphurea, "close their leaves upon insects with "great rapidity: and the same phenomenon is mani-"fested by an Indian species, D. lunata, and by several "of those of the Cape of Good Hope, especially by "D. trinervis." Another Australian species, Drosera heterophylla (made by Lindley into a distinct genus, Sondera) is remarkable from its peculiarly shaped leaves, but I know nothing of its power of catching insects, for I have seen only dried specimens. The leaves form minute flattened cups, with the footstalks attached not to one margin, but to the bottom. The
* 'Gardener's Chronicle,' 1874, p. 209. [page 285]
inner surface and the edges of the cups are studded with tentacles, which include fibro-vascular bundles, rather different from those seen by me in any other species; for some of the vessels are barred and punctured, instead of being spiral. The glands secrete copiously, judging from the quantity of dried secretion adhering to them. [page 286]
CHAPTER XIII.
DIONAEA MUSCIPULA.
Structure of the leaves--Sensitiveness of the filaments--Rapid movement of the lobes caused by irritation of the filaments--Glands, their power of secretion--Slow movement caused by the absorption of animal matter--Evidence of absorption from the aggregated condition of the glands--Digestive power of the secretion--Action of chloroform, ether, and hydrocyanic acid- -The manner in which insects are captured--Use of the marginal spikes--Kinds of insects captured--The transmission of the motor impulse and mechanism of the movements-- Re-expansion of the lobes.