General Summary and Concluding Remarks on the Salts of Ammonia.—We have now seen that the nine [page 168] salts of ammonia which were tried, all cause the inflection of the tentacles, and often of the blade of the leaf. As far as can be ascertained from the superficial trials with the last six salts, the citrate is the least powerful, and the phosphate certainly by far the most. The tartrate and chloride are remarkable from the short duration of their action. The relative efficiency of the carbonate, nitrate, and phosphate, is shown in the following table by the smallest amount which suffices to cause the inflection of the tentacles. Column 1 : Solutions, how applied. Column 2 : Carbonate of Ammonia. Column 3 : Nitrate of Ammonia. Column 4 : Phosphate of Ammonia. Placed on the glands of the disc, so as to act indirectly on the outer tentacles : 1/960 of a grain, or 0675 mg. : 1/2400 of a grain, or .027 mg. : 1/3840 of a grain, or .0169 mg. 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.