Insectivorous Plants

(7) After 4 hrs. 30 m. only thirty-five tentacles inflected or sub-inflected, and this small amount of inflection never increased. (1) After 4 hrs. thirty-eight tentacles inflected ; but after 7 hrs. these, with the exception of six, re-expanded. (2) After 4 hrs. 20 m. twenty inflected ; these after 9 hrs. partially re-expanded. (3) After 4 hrs. five inflected, which began to re-expand after 7 hrs. (5), (6) and (7) Not at all affected, though observed for 24 hrs., excepting the short tentacles on the borders of the disc, which as usual formed a ring.

A comparison of the leaves in the solution, especially of the first five or even six on the list, with those in the water, after 1 hr. or after 4 hrs., and in a still more marked degree after 7 hrs. or 8 hrs., could not leave the least doubt that the solution had produced a great eifect. This was shown not only by the vastly greater number of inflected tentacles, but by the degree or closeness of their inflection, and by that of their blades. Yet each gland on leaf No. 1 (which bore 255 glands, all of which, excepting five, were inflected in 30 m.) could not have received more than one-four-millionth of a grain ('0000162 mg.) of the salt. Again, each gland on leaf No. 3 (which bore 233 glands, all of which, except nine, were inflected in 2 hrs. 30 m.) could have received at most only the 3B8^000 of a grain, or '0000181 mg.

Four leaves were immersed as before in a solution of one part to 656,250 of water (1 gr. to 1500 oz.) ; but on this occasion I happened to select leaves which were very little sensitive, as on other occasions I chanced to select unusually sensitive leaves. The leaves were not more affected after 12 hrs. than the four corresponding ones in water; but after 24 hrs. they were slightly more inflected. Such evidence, however, is not at all trustworthy.

Twelve leaves were immersed, each in thirty minims of a solu- tion of one part to 1,312,500 of water (1 gr. to 3000 oz.); so that each leaf received 48ooo of a grain ('00135 mg.). The leaves were not in very good condition ; four of them were too old and of a dark red colour ; four were too pale, yet one of these latter acted well ; the four others, as far as could be told by the eye, seemed in excellent condition. The result was as follows : — (1) This was a pale leaf ; after 40 m. about thirty -eight ten- tacles inflected ; after 3 hrs. 30 m. the blade and many of the outer tentacles inflected; after 10 hrs. 15 m. all the tentacles but seventeen inflected, and the blade quite doubled up; after 24 hrs. all the tentacles but ten more or less inflected. Most of them were closely inflected, but twenty-five were only sub- inflected.

(2) After 1 hr. 40 m. twenty-five tentacles inflected; after G hrs. all but twenty-one inflected ; after 10 hrs. all but sixteen more or less inflected ; after 24 hrs. re-expanded. (3) After 1 hr. 40 m. thirty-five inflected; after 6 hrs. "a large number" (to quote my own memorandum) inflected, but from want of time they were not counted ; after 24 hrs. re- expanded. (4) After 1 hr. 40 m. about thirty inflected ; after 6 hrs. " a large number all round the leaf " inflected, but they were not counted ; after 10 hrs. began to re-expand.

(5) to (12) These were not more inflected than leaves often are in water, having respectively 16, 8, 10, 8, 4, 9, 14, and 0 ten- tacles inflected. Two of these leaves, however, were remarkable from having their blades slightly inflected after 6 hrs. • With respect to the twelve corresponding leaves in water, (1) had, after 1 hr. 35 m., fifty tentacles inflected, but after 11 hrs. only twenty-two remained so, and these formed a group, with the blade at this point slightly inflected. It appeared as if this leaf had been in some manner accidentally excited, for instance by a particle of animal matter which was dissolved by the water. (2) After 1 hr. 45 m. thirty-two tentacles inflected, but after 5 hrs. 30 m. only twenty-five inflected, and these after 10 hrs, all re-expanded; (3) after 1 hr. twenty-five inflected, which after 10 hrs. 20 m. were all re-expanded ; (4) and (5) after 1 hr. 35 m. six and seven tentacles inflected, which re-expanded after 11 hrs.; (6), (7) and (8) from one to three inflected, which

Comparing the states of the twelve leaves in water with those in the solution, there could be no doubt that in the latter a larger number of tentacles were inflected, and these to a greater degree ; but the evidence was by no means so clear as in the former ex- periments with stronger solutions. It deserves attention that the inflection of four of the leaves in the solution went on increasing during the first 6 hrs., and with some of them for a longer time ; whereas in the water the inflection of the three leaves which •were the most affected, as well as of all the others, began to de- crease during this same interval. It is also remarkable that the blades of three of the leaves in the solution were slightly in- flected, and this is a most rare event with leaves in water, though it occurred to a slight extent in one (No. 1), which seemed to have been in some manner accidentally excited. All this shows that the solution produced some effect, though less and at a much slower rate than in the previous cases. The small effect produced may, however, be accounted for in large part by the majority of the leaves having been in a poor con- dition.

Of the leaves in the solution, No. 1 bore 200 glands and received 4So00 of a grain of the salt. Subtracting the seventeen tentacles which were not inflected, each gland could have absorbed only *ne sTslooo °f a grain ('00000738 mg.). This amount caused the tentacle bearing each gland to be greatly inflected. The blade was also inflected. Lastly, eight leaves were immersed, each in thirty minims of a solution of one part of the phosphate to 21,875,000 of water (1 gr. to 5000 oz.). Each leaf thus received ^ tro^o °f a grain of the salt, or '00081 mg. I took especial pains in selecting the finest leaves from the hot-house for immersion, both in the solution and the water, and almost all proved extremely sensitive. Beginning as before with those in the solution :—

(1) After 2 hrs. 30 m. all the tentacles but twenty-two in- flected, but some only sub-inflected ; the blade much inflected; after 6 hrs. 30 m. all but thirteen inflected, with the blade immensely inflected ; and remained so for 48 hrs. (2) No change for the first 12 hrs., but after 24 hrs. all the tentacles inflected, excepting those of the outermost row, of which only eleven were inflected. The inflection continued to increase, and after 48 hrs. all the tentacles except three were inflected.

and most of them rather closely, four or five being only sub- inflected. (3) No change for the first 12 hrs.; but after 24 hrs. all the tentacles excepting those of the outermost row were sub-inflected, with the blade inflected. After 3u hrs. blade strongly inflected, with all the tentacles, except three, inflected or sub -inflected. After 48 hrs. in the same state. (4) to (8) These leaves, after 2 hrs. 30 m., had respectively 32, 17, 7, 4, and 0 tentacles inflected, most of which, after a few hours, re-expanded, with the exception of No. 4, which retained its thirty-two tentacles inflected for 48 hrs.

(1) After 2 hrs. 40 m. this had twenty of its outer tentacles inflected, five of which re-expanded after 6 hrs. 30 m. After 10 hrs. 15 m. a most unusual circumstance occurred, namely, the whole blade became slightly bowed towards the footstalk, and so remained for 48 hrs. The exterior tentacles, excepting those of the three or four outermost rows, were now also in- flected to an unusual degree. When the two lots of eight leaves in the solution and in the water were compared after the lapse of 24 hrs., they undoubt- edly differed much in appearance. The few tentacles on the leaves in water which were inflected had after this interval re- expanded, with the exception of one leaf; and this presented the very unusual case of the blade being somewhat inflected, though in a degree hardly approaching that of the two leaves in the solution. Of these latter leaves, No. 1 had almost all its tentacles, together with its blade, inflected after an immersion of 2 hrs. 30 in. Leaves No. 2 and 3 were affected at a much slower rate; but after from 24 hrs. to 48 hrs. almost all their tentacles were closely inflected, and the blade of one quite doubled up. We must therefore admit, incredible as the fact may at first appear, that this extremely weak solution acted on the more sensitive leaves ; each of which received only the 80000 of a grain ('00081 nig.) of the phosphate. Now, leaf No. 3 bore 178 tentacles, and subtracting the three which were not inflected, each gland could have absorbed only the 1 4060000 of a grain, or '00000463 mg. Leaf No. 1, which was strongly acted on within 2 hrs. 30 m., and had all its oiiter tentacles, except thirteen, inflected within 6 hrs. 30 m., bore 260 tentacles; and on the same principle as before, each gland could hstva

absorbed only m 7^0000 of a grain> or '00000328 mg. ; and this excessively minute amount sufficed to cause all the tentaclea 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 nil.), containing -g-gVo °f a grain (-0109 mg.) of this salt, transmit a motor impulse to the exterior tentacles, causing them to bend inwards. A minute drop, containing ,-3-^7^0 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 ., 97ti'OOUo 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 9^ 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, 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 yyVo 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 Ammou ia.— 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 oi some of the leaves, and this became more decided after 1 hr. with all the leaves; but the tentacles were never closely in- flected. 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 tho period of inflection in this and the following case is remark- able.

Chloride of Ammonium. — Half-minims of a solution of one part to 437 of water were placed on the discs of six leaves. A decided degree of inflection in the outer and submarginal tentacles was perceptible in 25 m. ; and this increased during the next three or four hours, but never became strongly marked. After only 8 hrs. 30 m. the tentacles began to re-expand, ami by the next morning, after 24 hrs., were fully re-expanded on four of the leaves, but still slightly inflected on two.

General Summary and Concluding Remarks on the Sails of Ammonia. — We have now seen that the nine salts of ammonia which were tried, all cause the in- flection of the tentacles, and often of the blade of the leaf. As far as can be ascertained from the superficial trials with the last six salts, the citrate is the least powerful, and the phosphate certainly by far the most. The tartrate and chloride are remarkable from the short duration of their action. The rela- tive efficiency of the carbonate, nitrate, and phos- phate, is shown in the following table by the smallest amount which suffices to cause the inflection of the tentacles.

From the experiments tried in these three dif- ferent ways, we see that the carbonate, which con- tains 23'7 per cent, of nitrogen, is less efficient than the nitrate, which contains 35 per cent. The phos- phate contains less nitrogen than either of these salts, namely, only 21 -2 per cent., and yet is far more efficient ; its power no doubt depending quite as much on the phosphorus as on the nitrogen which it contains. We may infer that this is the case, from the energetic manner in which bits of bone and phosphate of lime affect the leaves. The inflection excited by the other salts of ammonia is probably due solely to their nitro- gen,— on the same principle that nitrogenous organic fluids act powerfully, whilst non-nitrogenous organic fluids are powerless. As such minute doses of the salts of ammonia affect the leaves, we may feel almost sure that Drosera absorbs and profits by the amount, though small, which is present in rain-water, in the same manner as other plants absorb these same salts by their roots.

The srnallness of the doses of the nitrate, and more especially of the phosphate of ammonia, which cause the tentacles of immersed leaves to be inflected, is perhaps the most remarkable fact recorded in this volume. When we see that much less than the millionth* of a grain of the phosphate, absorbed by a gland of one of the exterior tentacles, causes it to bend, it may be thought that the effects of the solu- tion on the glands of the disc have been overlooked ; namely, the transmission of a motor impulse from them to the exterior tentacles. No doubt the move- ments of the latter are thus aided ; but the aid thus rendered must be insignificant ; for we know that a drop containing as much as the -3-^4-0- OI> a grain placed on the disc is only just able to cause the outer ten- tacles of a highly sensitive leaf to bend. It is cer-

* It is scarcely possible to real- stretch it along the wall of a large ise what a million means. The hall; then mark off at one end best illustration which I have met the tenth of an inch. This tenth with is that given by Mr. Croll, will represent a hundred, and tli<' who says, — Take a narrow strip of entire strip a million. paper 83 ft. 4 in. in length, and taiiily a most surprising fact that the -rirr^o^nr-n of a grain, or in round numbers the one-twenty-millionth of a grain ('0000033 nig.), of the phosphate should affect any plant, or indeed any animal ; and as this salt contains 35'33 per cent, of water of crystallisation, the efficient elements are reduced to -3-H5 0'5., 3ri of a grain, or in round numbers to one-thirty-millionth of a grain (-00000216 mg.). The solution, moreover, in these experiments was diluted in the proportion of one part of the salt to 2,187,500 of water, or one grain to 5000 oz. The reader will perhaps best realise this degree of dilution by remembering that 5000 oz. would more than fill a 31-gallon cask ; and that to this large body of water one grain of the salt was added ; only half a drachm, or thirty minims, of the solution being poured over a leaf. Yet this amount sufficed to cause the inflection of almost every ten- tacle, and often of the blade of the leaf.

I am well aware that this statement will at first appear incredible to almost every one. Drosera is far from rivalling the power of the spectroscope, but it can detect, as shown by the movements of its leaves, a very much smaller quantity of the phosphate of am- monia than the most skilful chemist can of any substance.* My results were for a long time incredible raonia, fourteen years ago, the ordinary chemical tests, I gather powers of the spectroscope had from Dr. Alfred Taylor's work

not been discovered ; and I felt on ' Poisons ' that about ^ of a all the greater interest in the grain of arsenic, ^ of a grain then unrivalled powers of Drosera. of prussic acid, -^ of iodine, together beaten Drosera ; for ac- can be detected ; but the power cording to Bunsen and Kirchhoff of detection depends much on the probably less than one Wiia'<iM of solutions under trial not being a grain of sodium can be thus extremely weak, detected (see Balfour Stewart,

even to myself, and I anxiously sought for every source of error. The salt was in some cases weighed fo>* me by a chemist in an excellent balance ; and fresh water was measured many times with care. The observations were repeated during several years. Two of my sons, who were as incredulous as myself, compared several lots of leaves simultaneously immersed in the weaker solutions and in water, and declared that there could be no doubt about the difference in their ap- pearance. I hope that some one may hereafter be in- duced to repeat my experiments ; in this case he should select young and vigorous leaves, with the glands surrounded by abundant secretion. The leaves should be carefully cut off and laid gently in watch-glasses, and a measured quantity of the solution and of water poured over each. The water used must be as ab- solutely pure as it can be made. It is to be especially observed that the experiments with the weaker solu- tions ought to be tried after several days of very warm weather. Those with the weakest solutions should be made on plants which have been kept for a considerable time in a warm greenhouse, or cool hothouse ; but this is by no means necessary for trials with solutions of moderate strength.

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

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-niillionth of a grain is sufficient to cause the inflection of a single tentacle. There is, there- fore, 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 effi- cient 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.

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 atro- pine, 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 in- finitely smaller * than those of the phosphate of am- monia weighing the one-twenty-millionth of a grain. These nerves then transmit some influence to the brain of the dog, which leads to action on its part. With Dro- sera, 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 assiimiug 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 from SQ^QO to TF^OTHT of an inch calculated for me the diameter of — in diameter. Therefore, an ob- a sphere of phosphate of ammonia ject between -fo and -fa of the (specific gravity 1 • 678), weigh- size of a sphere of the phos- ing the one-twenty-millionth of phate of ammonia of the above a grain, and finds it to be -^ of weight can be seen under a high me that the smallest Micrococci, that odorous particles, such as

which are distinctly discernible those emitted from the deer in under a power of 800 diameters, the above illustration, could be are estimated to be from -0002 to ?een under any power of the mi- Baits of sodium, potassium, and other alkaline, earthy, and metallic salts — Summary on the action of these salts — Various acids — Summary on their action. HAVING found that the salts of ammonia were so powerful, I was led to investigate the action of some other salts. It will be convenient, first, to give a list of the substances tried (including forty-nine salts and two metallic acids), divided into two columns, showing those which cause inflection, and those which do not do so, or only doubtfully. My experiments were made by placing half-minim drops on the discs of leaves, or, more commonly, by immersing them in the solutions ; and sometimes by both methods. A summary of the results, with some concluding remarks, will then be given. The action of various acids will afterwards be described.