30 m., bore 260 tentacles; and on the same principle as before, each gland could have [page 166] absorbed only 1/19760000 of a grain, or .00000328 mg.; and this excessively minute amount sufficed to cause all the tentacles bearing these glands to be greatly inflected. The blade was also inflected.]

Summary of the Results with Phosphate of Ammonia.--The glands of the disc, when excited by a half-minim drop (.0296 ml.), containing 1/3840 of a grain (.0169 mg.) of this salt, transmit a motor impulse to the exterior tentacles, causing them to bend inwards. A minute drop, containing 1/153600 of a grain (.000423 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 a shorter time, in a solution so weak that each gland can absorb only the 1/9760000 of a grain (.00000328 mg.), this is enough to excite the tentacle into movement, so that it becomes closely inflected, as does sometimes the blade. In the general summary to this chapter a few remarks will be added, showing that the efficiency of such extremely minute doses is not so incredible as it must at first appear.

[Sulphate of Ammonia.--The few trials made with this and the following five salts of ammonia were undertaken merely to ascertain whether they induced inflection. Half-minims of a solution of one part of the sulphate of ammonia to 437 of water were placed on the discs of seven leaves, so that each received 1/960 of a grain, or .0675 mg. After 1 hr. the tentacles of five of them, as well as the blade of one, were strongly inflected. The leaves were not afterwards observed.

Citrate of Ammonia.--Half-minims of a solution of one part to 437 of water were placed on the discs of six leaves. In 1 hr. the short outer tentacles round the discs were a little inflected, with the glands on the discs blackened. After 3 hrs. 25 m. one leaf had its blade inflected, but none of the exterior tentacles. All six leaves remained in nearly the same state during the day, the submarginal tentacles, however, [page 167] becoming more inflected. After 23 hrs. three of the leaves had their blades somewhat inflected; and the submarginal tentacles of all considerably inflected, but in none were the two, three, or four outer rows affected. I have rarely seen cases like this, except from the action of a decoction of grass. The glands on the discs of the above leaves, instead of being almost black, as after the first hour, were now after 23 hrs. very pale. I next tried on four leaves half-minims of a weaker solution, of one part to 1312 of water (1 gr. to 3 oz.); so that each received 1/2880 of a grain (.0225 mg.). After 2 hrs. 18 m. the glands on the disc were very dark-coloured; after 24 hrs. two of the leaves were slightly affected; the other two not at all.

Acetate of Ammonia.--Half-minims of a solution of about one part to 109 of water were placed on the discs of two leaves, both of which were acted on in 5 hrs. 30 m., and after 23 hrs. had every single tentacle closely inflected.

Oxalate of Ammonia.--Half-minims of a solution of one part to 218 of water were placed on two leaves, which, after 7 hrs., became moderately, and after 23 hrs. strongly, inflected. Two other leaves were tried with a weaker solution of one part to 437 of water; one was strongly inflected in 7 hrs.; the other not until 30 hrs. had elapsed.

Tartrate of Ammonia.--Half-minims of a solution of one part to 437 of water were placed on the discs of five leaves. In 31 m. there was a trace of inflection in the exterior tentacles of some of the leaves, and this became more decided after 1 hr. with all the leaves; but the tentacles were never closely inflected. After 8 hrs. 30 m. they began to re-expand. Next morning, after 23 hrs., all were fully re-expanded, excepting one which was still slightly inflected. The shortness of the period of inflection in this and the following case is remarkable.

Chloride of Ammonium.--Half-minims of a solution of one part to 437 of water were placed on the discs of six leaves.

Charles Darwin

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