Applied for a few seconds directly to the gland of an outer tentacle :  1/14400 of a grain, or .00445 mg. :  1/28800 of a grain, or .0025 mg. grain, 1/153600 of a grain, or .000423 mg.

Leaf immersed, with time allowed for each gland to absorb all that it can :  1/268800 of a grain, or .00024 mg. :  1/691200 of a grain, or .0000937 mg. :  1/19760000 of a grain, or .00000328 mg.

Amount absorbed by a gland which suffices to cause the aggregation of the protoplasm in the adjoining cells of the tentacles. 1/134400 of a grain, or .00048 mg.

From the experiments tried in these three different ways, we see that the carbonate, which contains 23.7 per cent. of nitrogen, is less efficient than the nitrate, which contains 35 per cent.  The phosphate contains less nitrogen than either of these salts, namely, only 21.2 per cent., and yet is far more [page 169] efficient; its power no doubt depending quite as much on the phosphorus as on the nitrogen which it contains.  We may infer that this is the case, from the energetic manner in which bits of bone and phosphate of lime affect the leaves.  The inflection excited by the other salts of ammonia is probably due solely to their nitrogen,—­on the same principle that nitrogenous organic fluids act powerfully, whilst non-nitrogenous organic fluids are powerless.  As such minute doses of the salts of ammonia affect the leaves, we may feel almost sure that Drosera absorbs and profits by the amount, though small, which is present in rain-water, in the same manner as other plants absorb these same salts by their roots.

The smallness of the doses of the nitrate, and more especially of the phosphate of ammonia, which cause the tentacles of immersed leaves to be inflected, is perhaps the most remarkable fact recorded in this volume.  When we see that much less than the millionth* of a grain of the phosphate, absorbed by a gland of one of the exterior tentacles, causes it to bend, it may be thought that the effects of the solution on the glands of the disc have been overlooked; namely, the transmission of a motor impulse from them to the exterior tentacles.  No doubt the movements of the latter are thus aided; but the aid thus rendered must be insignificant; for we know that a drop containing as much as the 1/3840 of a grain placed on the disc is only just able to cause the outer tentacles of a highly sensitive leaf to bend.  It is cer-

* It is scarcely possible to realise what a million means.  The best illustration which I have met with is that given by Mr. Croll, who says, “Take a narrow strip of paper 83 ft. 4 in. in length, and stretch it along the wall of a large hall; then mark off at one end the tenth of an inch.  This tenth will represent a hundred, and the entire strip a million. [page 170]