minute a quantity as the one-twenty-millionth of a grain of phosphate of ammonia should induce some change in a gland of Drosera, sufficient to cause a motor impulse to be sent down the whole length of the tentacle; this impulse exciting movement often through an angle of above 180o.  I know not whether to be most astonished at this fact, or that the pressure of a minute bit of hair, supported by the dense secretion, should quickly cause conspicuous movement.  Moreover, this extreme sensitiveness, exceeding that of the most delicate part of the human body, as well as the power of transmitting various impulses from one part of the leaf to another, have been acquired without the intervention of any nervous system.

As few plants are at present known to possess glands specially adapted for absorption, it seemed worth while to try the effects on Drosera of various other salts, besides those of ammonia, and of various acids.  Their action, as described in the eighth chapter, does not correspond at all strictly with their chemical affinities, as inferred from the classification commonly followed.  The nature of the base is far more influential than that of the acid; and this is known to hold good with animals.  For instance, nine salts of sodium all caused well-marked inflection, and none of them were poisonous in small doses; whereas seven of the nine corre- [page 273] sponding salts of potassium produced no effect, two causing slight inflection.  Small doses, moreover, of some of the latter salts were poisonous.  The salts of sodium and potassium, when injected into the veins of animals, likewise differ widely in their action.  The so-called earthy salts produce hardly any effect on Drosera.  On the other hand, most of the metallic salts cause rapid and strong inflection, and are highly poisonous; but there are some odd exceptions to this rule; thus chloride of lead and zinc, as well as two salts of barium, did not cause inflection, and were not poisonous.

Most of the acids which were tried, though much diluted (one part to 437 of water), and given in small doses, acted powerfully on Drosera; nineteen, out of the twenty-four, causing the tentacles to be more or less inflected.  Most of them, even the organic acids, are poisonous, often highly so; and this is remarkable, as the juices of so many plants contain acids.  Benzoic acid, which is innocuous to animals, seems to be as poisonous to Drosera as hydrocyanic.  On the other hand, hydrochloric acid is not poisonous either to animals or to Drosera, and induces only a moderate amount of inflection.  Many acids excite the glands to secrete an extraordinary quantity of mucus; and the protoplasm within their cells seems to be often killed, as may be inferred from the surrounding fluid soon becoming pink.  It is strange that allied acids act very differently:  formic acid induces very slight inflection, and is not poisonous; whereas acetic acid of the same strength acts most powerfully and is poisonous.  Lactic acid is also poisonous, but causes inflection only after a considerable lapse of time.  Malic acid acts slightly, whereas citric and tartaric acids produce no effect. [page 274]