Volume II Part 60 (2/2)
He has described in the 'Autobiography' (volume i.), the general nature of these early experiments. He noticed insects sticking to the leaves, and finding that flies, etc., placed on the adhesive glands were held fast and embraced, he suspected that the leaves were adapted to supply nitrogenous food to the plant. He therefore tried the effect on the leaves of various nitrogenous fluids--with results which, as far as they went, verified his surmise. In September, 1860, he wrote to Dr. Gray:--
”I have been infinitely amused by working at Drosera: the movements are really curious; and the manner in which the leaves detect certain nitrogenous compounds is marvellous. You will laugh; but it is, at present, my full belief (after endless experiments) that they detect (and move in consequence of) the 1/2880 part of a single grain of nitrate of ammonia; but the muriate and sulphate of ammonia bother their chemical skill, and they cannot make anything of the nitrogen in these salts! I began this work on Drosera in relation to GRADATION as throwing light on Dionaea.”
Later in the autumn he was again obliged to leave home for Eastbourne, where he continued his work on Drosera. The work was so new to him that he found himself in difficulties in the preparation of solutions, and became puzzled over fluid and solid ounces, etc. etc. To a friend, the late Mr. E. Cresy, who came to his help in the matter of weights and measures, he wrote giving an account of the experiments. The extract (November 2, 1860) which follows ill.u.s.trates the almost superst.i.tious precautions he often applied to his researches:--
”Generally I have scrutinised every gland and hair on the leaf before experimenting; but it occurred to me that I might in some way affect the leaf; though this is almost impossible, as I scrutinised with equal care those that I put into distilled water (the same water being used for dissolving the carbonate of ammonia). I then cut off four leaves (not touching them with my fingers), and put them in plain water, and four other leaves into the weak solution, and after leaving them for an hour and a half, I examined every hair on all eight leaves; no change on the four in water; every gland and hair affected in those in ammonia.
”I had measured the quant.i.ty of weak solution, and I counted the glands which had absorbed the ammonia, and were plainly affected; the result convinced me that each gland could not have absorbed more than 1/64000 or 1/65000 of a grain. I have tried numbers of other experiments all pointing to the same result. Some experiments lead me to believe that very sensitive leaves are acted on by much smaller doses. Reflect how little ammonia a plant can get growing on poor soil--yet it is nourished. The really surprising part seems to me that the effect should be visible, and not under very high power; for after trying a high power, I thought it would be safer not to consider any effect which was not plainly visible under a two-thirds object gla.s.s and middle eye-piece. The effect which the carbonate of ammonia produces is the segregation of the h.o.m.ogeneous fluid in the cells into a cloud of granules and colourless fluid; and subsequently the granules coalesce into larger ma.s.ses, and for hours have the oddest movements--coalescing, dividing, coalescing ad infinitum. I do not know whether you will care for these ill-written details; but, as you asked, I am sure I am bound to comply, after all the very kind and great trouble which you have taken.”
On his return home he wrote to Sir J.D. Hooker (November 21, 1860):--
”I have been working like a madman at Drosera. Here is a fact for you which is certain as you stand where you are, though you won't believe it, that a bit of hair 1/78000 of one grain in weight placed on gland, will cause ONE of the gland-bearing hairs of Drosera to curve inwards, and will alter the condition of the contents of every cell in the foot-stalk of the gland.”
And a few days later to Lyell:--
”I will and must finish my Drosera MS., which will take me a week, for, at the present moment, I care more about Drosera than the origin of all the species in the world. But I will not publish on Drosera till next year, for I am frightened and astounded at my results. I declare it is a certain fact, that one organ is so sensitive to touch, that a weight seventy-eight times less than that, viz., 1/1000 of a grain, which will move the best chemical balance, suffices to cause a conspicuous movement. Is it not curious that a plant should be far more sensitive to the touch than any nerve in the human body? Yet I am perfectly sure that this is true. When I am on my hobby-horse, I never can resist telling my friends how well my hobby goes, so you must forgive the rider.”
The work was continued, as a holiday task, at Bournemouth, where he stayed during the autumn of 1862. The discussion in the following letter on ”nervous matter” in Drosera is of interest in relation to recent researches on the continuity of protoplasm from cell to cell:]
CHARLES DARWIN TO J.D. HOOKER. Cliff Cottage, Bournemouth. September 26 [1862].
My dear Hooker,
Do not read this till you have leisure. If that blessed moment ever comes, I should be very glad to have your opinion on the subject of this letter. I am led to the opinion that Drosera must have diffused matter in organic connection, closely a.n.a.logous to the nervous matter of animals. When the glands of one of the papillae or tentacles, in its natural position is supplied with nitrogenised fluid and certain other stimulants, or when loaded with an extremely slight weight, or when struck several times with a needle, the pedicel bends near its base in under one minute. These varied stimulants are conveyed down the pedicel by some means; it cannot be vibration, for drops of fluid put on quite quietly cause the movement; it cannot be absorption of the fluid from cell to cell, for I can see the rate of absorption, which though quick, is far slower, and in Dionaea the transmission is instantaneous; a.n.a.logy from animals would point to transmission through nervous matter.
Reflecting on the rapid power of absorption in the glands, the extreme sensibility of the whole organ, and the conspicuous movement caused by varied stimulants, I have tried a number of substances which are not caustic or corrosive,... but most of which are known to have a remarkable action on the nervous matter of animals. You will see the results in the enclosed paper. As the nervous matter of different animals are differently acted on by the same poisons, one would not expect the same action on plants and animals; only if plants have diffused nervous matter, some degree of a.n.a.logous action. And this is partially the case.
Considering these experiments, together with the previously made remarks on the functions of the parts, I cannot avoid the conclusion, that Drosera possesses matter at least in some degree a.n.a.logous in const.i.tution and function to nervous matter. Now do tell me what you think, as far as you can judge from my abstract; of course many more experiments would have to be tried; but in former years I tried on the whole leaf, instead of on separate glands, a number of innocuous (This line of investigation made him wish for information on the action of poisons on plants; as in many other cases he applied to Professor Oliver, and in reference to the result wrote to Hooker: ”Pray thank Oliver heartily for his heap of references on poisons.”) substances, such as sugar, gum, starch, etc., and they produced no effect. Your opinion will aid me in deciding some future year in going on with this subject. I should not have thought it worth attempting, but I had nothing on earth to do.
My dear Hooker, Yours very sincerely, CH. DARWIN.
P.S.--We return home on Monday 28th. Thank Heaven!
[A long break now ensued in his work on insectivorous plants, and it was not till 1872 that the subject seriously occupied him again. A pa.s.sage in a letter to Dr. Asa Gray, written in 1863 or 1864, shows, however, that the question was not altogether absent from his mind in the interim:--
”Depend on it you are unjust on the merits of my beloved Drosera; it is a wonderful plant, or rather a most sagacious animal. I will stick up for Drosera to the day of my death. Heaven knows whether I shall ever publish my pile of experiments on it.”
He notes in his diary that the last proof of the 'Expression of the Emotions' was finished on August 22, 1872, and that he began to work on Drosera on the following day.]
CHARLES DARWIN TO ASA GRAY. [Sevenoaks], October 22 [1872].
... I have worked pretty hard for four or five weeks on Drosera, and then broke down; so that we took a house near Sevenoaks for three weeks (where I now am) to get complete rest. I have very little power of working now, and must put off the rest of the work on Drosera till next spring, as my plants are dying. It is an endless subject, and I must cut it short, and for this reason shall not do much on Dionaea. The point which has interested me most is tracing the NERVES! which follow the vascular bundles. By a p.r.i.c.k with a sharp lancet at a certain point, I can paralyse one-half the leaf, so that a stimulus to the other half causes no movement. It is just like dividing the spinal marrow of a frog:--no stimulus can be sent from the brain or anterior part of the spine to the hind legs; but if these latter are stimulated, they move by reflex action. I find my old results about the astonis.h.i.+ng sensitiveness of the nervous system (!?)of Drosera to various stimulants fully confirmed and extended...
[His work on digestion in Drosera and other points in the physiology of the plant soon led him into regions where his knowledge was defective, and here the advice and a.s.sistance which he received from Dr. Burdon Sanderson was of much value:]
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