I beg the reader to observe that the sensitiveness or irritability of the tentacles was ascertained by three different methods—­indirectly by drops placed on the disc, directly by drops applied to the glands of the outer tentacles, and by the immersion of whole leaves; and it was found by these three methods that the nitrate was more powerful than the carbonate, and the phosphate much more powerful than the nitrate; this result being intelligible from the difference in the amount of nitrogen in the first two salts, and from the presence of phosphorus in the third.  It may aid the [page 172] reader’s faith to turn to the experiments with a solution of one grain of the phosphate to 1000 oz. of water, and he will there find decisive evidence that the one-four-millionth of a grain is sufficient to cause the inflection of a single tentacle.  There is, therefore, nothing very improbable in the fifth of this weight, or the one-twenty-millionth of a grain, acting on the tentacle of a highly sensitive leaf.  Again, two of the leaves in the solution of one grain to 3000 oz., and three of the leaves in the solution of one grain to 5000 oz., were affected, not only far more than the leaves tried at the same time in water, but incomparably more than any five leaves which can be picked out of the 173 observed by me at different times in water.

There is nothing remarkable in the mere fact of the one-twenty-millionth of a grain of the phosphate, dissolved in above two-million times its weight of water, being absorbed by a gland.  All physiologists admit that the roots of plants absorb the salts of ammonia brought to them by the rain; and fourteen gallons of rain-water contain* a grain of ammonia, therefore only a little more than twice as much as in the weakest solution employed by me.  The fact which appears truly wonderful is, that the one-twenty-millionth of a grain of the phosphate of ammonia (including less than the one-thirty-millionth of efficient matter), when absorbed by a gland, should induce some change in it, which leads to a motor impulse being transmitted down the whole length of the tentacle, causing the basal part to bend, often through an angle of above 180 degrees.

Astonishing as is this result, there is no sound reason

* Miller’s ‘Elements of Chemistry,’ part ii. p. 107, 3rd edit. 1864. [page 173]

why we should reject it as incredible.  Prof.  Donders, of Utrecht, informs me that from experiments formerly made by him and Dr. De Ruyter, he inferred that less than the one-millionth of a grain of sulphate of atropine, in an extremely diluted state, if applied directly to the iris of a dog, paralyses the muscles of this organ.  But, in fact, every time that we perceive an odour, we have evidence that infinitely smaller particles act on our nerves.  When a dog stands a quarter of a mile to leeward of a deer or other animal, and perceives its presence, the odorous particles produce some change in the olfactory nerves;