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Show 172 DROSERA ROTUNDIFOLIA. CHAP. VII. 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 wh:lch 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-millionth of its weight of water, being absorbed by' a gland. A~l 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-twentyInillionth 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. CHAI'. yii. SUMMARY, SALTS OF AMMONIA. 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; yet these particles must be infinitely smaller* than those of the phosphate of ammonia weighing the one-twenty-millionth of a grain. These nerves then transmit some in:fl uence to the brain of the dog, which leads to action on its part. With Drosera, the really marvellous fact is, that a plant without any specialised nervous system should be affected by such minute particles; but we have no grounds for assuming that other tissues could not be rendered as exquisitely susceptible to impressions from without if this were beneficial to the organism, as is the nervous system of the higher animals. * My son, George Darwin, has calculated for me the diameter of a sphere of phosphate of ammonia ~specific gravity I· 678), weighmg the one-twenty-millionth of a g.rain, and finds it to be T'h:r of au mch. Now, Dr. Klein informs me that the smallest Micrococci which are distinctly discernibl~ under a power of 800 diameters are estimated to be from · 0002 t~ · 0005 of a millimetre- that is, from ;.;~00 to 12-fnrm of an inch-in diameter. Therefore, an object from thirty-one to seventy-seven times as large as a sphere of the phosphate of ammonia of the above weight can be seen under a high power; and no one supposes that odorous particles, such as those emitted from the deer in the above illu tration, could. be seen under any power of the microscope. |