Summary of the Results with Nitrate of Ammonia.—­The glands of the disc, when excited by a half-minim drop (.0296 ml.), containing 1/2400 of a grain of the nitrate (.027 mg.), transmit a motor impulse to the exterior tentacles, causing them to bend inwards.  A minute drop, containing 1/28800 of a grain (.00225 mg.), if held for a few seconds in contact with a gland, causes the tentacle bearing this gland to be inflected.  If a leaf is left immersed for a few hours, and sometimes for only a few minutes, in a solution of such strength that each gland can absorb only the (1/691200 of a grain (.0000937 mg.), this small amount is enough to excite each tentacle into movement, and it becomes closely inflected.

Phosphateof ammonia.

This salt is more powerful than the nitrate, even in a greater degree than the nitrate is more powerful than the carbonate.  This is shown by weaker solutions of the phosphate acting when dropped on the discs, or applied to the glands of the exterior tentacles, or when leaves are immersed.  The difference in the power of these three salts, as tried in three different ways, supports the results presently to be [page 154] given, which are so surprising that their credibility requires every kind of support.  In 1872 I experimented on twelve immersed leaves, giving each only ten minims of a solution; but this was a bad method, for so small a quantity hardly covered them.  None of these experiments will, therefore, be given, though they indicate that excessively minute doses are efficient.  When I read over my notes, in 1873, I entirely disbelieved them, and determined to make another set of experiments with scrupulous care, on the same plan as those made with the nitrate; namely by placing leaves in watch-glasses, and pouring over each thirty minims of the solution under trial, treating at the same time and in the same manner other leaves with the distilled water used in making the solutions.  During 1873, seventy-one leaves were thus tried in solutions of various strengths, and the same number in water.  Notwithstanding the care taken and the number of the trials made, when in the following year I looked merely at the results, without reading over my observations, I again thought that there must have been some error, and thirty-five fresh trials were made with the weakest solution; but the results were as plainly marked as before.  Altogether, 106 carefully selected leaves were tried, both in water and in solutions of the phosphate.  Hence, after the most anxious consideration, I can entertain no doubt of the substantial accuracy of my results.

[Before giving my experiments, it may be well to premise that crystallised phosphate of ammonia, such as I used, contains 35.33 per cent. of water of crystallisation; so that in all the following trials the efficient elements formed only 64.67 per cent. of the salt used.

Extremely minute particles of the dry phosphate were placed [page 155] with the point of a needle on the secretion surrounding several glands.  These poured forth much secretion, were blackened, and ultimately died; but the tentacles moved only slightly.  The dose, small as it was, evidently was too great, and the result was the same as with particles of the carbonate of ammonia.