Manner of performing the experiments—­Action of distilled water in comparison with the solutions—­Carbonate of ammonia, absorbed by the roots—­The vapour absorbed by the glands- -Drops on the disc—­Minute drops applied to separate glands—­Leaves immersed in weak solutions—­Minuteness of the doses which induce aggregation of the protoplasm—­Nitrate of ammonia, analogous experiments with—­Phosphate of ammonia, analogous experiments with- -Other salts of ammonia—­Summary and concluding remarks on the action of salts of ammonia.

The chief object in this chapter is to show how powerfully the salts of ammonia act on the leaves of Drosera, and more especially to show what an extraordinarily small quantity suffices to excite inflection.  I shall, therefore, be compelled to enter into full details.  Doubly distilled water was always used; and for the more delicate experiments, water which had been prepared with the utmost possible care was given me by Professor Frankland.  The graduated measures were tested, and found as accurate as such measures can be.  The salts were carefully weighed, and in all the more delicate experiments, by Borda’s double method.  But extreme accuracy would have been superfluous, as the leaves differ greatly in irritability, according to age, condition, and constitution.  Even the tentacles on the same leaf differ in irritability to a marked degree.  My experiments were tried in the following several ways.

[Firstly.—­Drops which were ascertained by repeated trials to be on an average about half a minim, or the 1/960 of a fluid ounce (.0296 ml.), were placed by the same pointed instrument on the [page 137] discs of the leaves, and the inflection of the exterior rows of tentacles observed at successive intervals of time.  It was first ascertained, from between thirty and forty trials, that distilled water dropped in this manner produces no effect, except that sometimes, though rarely, two or three tentacles become inflected.  In fact all the many trials with solutions which were so weak as to produce no effect lead to the same result that water is inefficient.

Secondly.—­The head of a small pin, fixed into a handle, was dipped into the solution under trial.  The small drop which adhered to it, and which was much too small to fall off, was cautiously placed, by the aid of a lens, in contact with the secretion surrounding the glands of one, two, three, or four of the exterior tentacles of the same leaf.  Great care was taken that the glands themselves should not be touched.  I had supposed that the drops were of nearly the same size; but on trial this proved a great mistake.  I first measured some water, and removed 300 drops, touching the pin’s head each time on blotting-paper; and on again measuring the water, a drop was found to equal on an average about the 1/60 of a minim.  Some water in a small vessel was weighed (and this is a more accurate method), and 300 drops removed as before; and on again weighing