From the experiments tried in these three different ways, we see that the carbonate, which contains 23.7 per cent. of nitrogen, is less efficient than the nitrate, which contains 35 per cent. The phosphate contains less nitrogen than either of these salts, namely, only 21.2 per cent., and yet is far more [page 169] 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 nitrogen,--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 smallness 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 solution 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 movements 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 1/3840 of a grain placed on the disc is only just able to cause the outer tentacles of a highly sensitive leaf to bend. It is cer-

* It is scarcely possible to realise what a million means. The best ill.u.s.tration which I have met with is that given by Mr. Croll, who says, "Take a narrow strip of paper 83 ft. 4 in. in length, and stretch it along the wall of a large hall; then mark off at one end the tenth of an inch. This tenth will represent a hundred, and the entire strip a million. [page 170]

tainly a most surprising fact that the 1/19760000 of a grain, or in round numbers the one-twenty-millionth of a grain (.0000033 mg.), 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 1/30555126 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 tentacle, and often of the blade of the leaf.

I am well aware that this statement will at first appear incredible to almost everyone. 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 quant.i.ty of the phosphate of ammonia than the most skilful chemist can of any substance.* My results were for a long time incredible

* When my first observations were made on the nitrate of ammonia, fourteen years ago, the powers of the spectroscope had not been discovered; and I felt all the greater interest in the then unrivalled powers of Drosera. Now the spectroscope has altogether beaten Drosera; for according to Bunsen and Kirchhoff probably less than one 1/200000000 of a grain of sodium can be thus detected (see Balfour Stewart, 'Treatise on Heat,' 2nd edit. 1871, p. 228). With respect to ordinary chemical tests, I gather from Dr. Alfred Taylor's work on 'Poisons' that about 1/4000 of a grain of a.r.s.enic, 1/4400 of a grain of prussic acid, 1/1400 of iodine, and 1/2000 of tartarised antimony, can be detected; but the power of detection depends much on the solutions under trial not being extremely weak. [page 171]

even to myself, and I anxiously sought for every source of error. The salt was in some cases weighed for 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 appearance. I hope that some one may hereafter be induced 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-gla.s.ses, and a measured quant.i.ty of the solution and of water poured over each. The water used must be as absolutely pure as it can be made. It is to be especially observed that the experiments with the weaker solutions 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 [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; 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 influence 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 a.s.suming 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 1.678), weighing the one-twenty-millionth of a grain, and finds it to be 1/1644 of an inch.

Now, Dr. Klein informs me that the smallest Micrococci, which are distinctly discernible under a power of 800 diameters, are estimated to be from .0002 to .0005 of a millimetre--that is, from 1/50800 to 1/127000 of an inch--in diameter. Therefore, an object between 1/31 and 1/77 of the size of 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 ill.u.s.tration, could be seen under any power of the microscope.) [page 174]

CHAPTER VIII.

THE EFFECTS OF VARIOUS OTHER SALTS AND ACIDS ON THE LEAVES.

Salts of sodium, pota.s.sium, 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.

COLUMN 1 : SALTS CAUSING INFLECTION. COLUMN 2 : SALTS NOT CAUSING INFLECTION.

(Arranged in Groups according to the Chemical Cla.s.sification in Watts'

'Dictionary of Chemistry.')

Sodium carbonate, rapid inflection. : Pota.s.sium carbonate: slowly poisonous. Sodium nitrate, rapid inflection. : Pota.s.sium nitrate: somewhat poisonous. Sodium sulphate, moderately rapid inflection. : Pota.s.sium sulphate. Sodium phosphate, very rapid inflection. : Pota.s.sium phosphate. Sodium citrate, rapid inflection. : Pota.s.sium citrate. Sodium oxalate; rapid inflection. Sodium chloride, moderately rapid inflection. : Pota.s.sium chloride. [page 175]

COLUMN 1 : SALTS CAUSING INFLECTION. COLUMN 2 : SALTS NOT CAUSING INFLECTION.

(Arranged in Groups according to the Chemical Cla.s.sification in Watts'

'Dictionary of Chemistry.')

Sodium iodide, rather slow inflection. : Pota.s.sium iodide, a slight and doubtful amount of inflection. Sodium bromide, moderately rapid inflection. : Pota.s.sium bromide. Pota.s.sium oxalate, slow and doubtful inflection. : Lithium nitrate, moderately rapid inflection. : Lithium acetate. Caesium chloride, rather slow inflection. : Rubidium chloride. Silver nitrate, rapid inflection: quick poison. : Cadmium chloride, slow inflection. : Calcium acetate. Mercury perchloride, rapid inflection: quick poison. : Calcium nitrate.

: Magnesium acetate. : Magnesium nitrate. : Magnesium chloride. : Magnesium sulphate. : Barium acetate. : Barium nitrate. : Strontium acetate. : Strontium nitrate. : Zinc chloride.

Aluminium chloride, slow and doubtful inflection. : Aluminium nitrate, a trace of inflection. Gold chloride, rapid inflection: quick poison.

: Aluminium and pota.s.sium sulphate.

Tin chloride, slow inflection: poisonous. : Lead chloride.

Antimony tartrate, slow inflection: probably poisonous. a.r.s.enious acid, quick inflection: poisonous. Iron chloride, slow inflection: probably poisonous. : Manganese chloride. Chromic acid, quick inflection: highly poisonous. Copper chloride, rather slow in flection: poisonous. : Cobalt chloride. Nickel chloride, rapid inflection: probably poisonous. Platinum chloride, rapid inflection: poisonous. [page 176]

Sodium, Carbonate of (pure, given me by Prof. Hoffmann).--Half-minims (.0296 ml.) of a solution of one part to 218 of water (2 grs. to 1 oz.) were placed on the discs of twelve leaves. Seven of these became well inflected; three had only two or three of their outer tentacles inflected, and the remaining two were quite unaffected. But the dose, though only the 1/480 of a grain (.135 mg.), was evidently too strong, for three of the seven well-inflected leaves were killed. On the other hand, one of the seven, which had only a few tentacles inflected, re-expanded and seemed quite healthy after 48 hrs. By employing a weaker solution (viz. one part to 437 of water, or 1 gr. to 1 oz.), doses of 1/960 of a grain (.0675 mg.) were given to six leaves. Some of these were affected in 37 m.; and in 8 hrs. the outer tentacles of all, as well as the blades of two, were considerably inflected. After 23 hrs. 15 m. the tentacles had almost re-expanded, but the blades of the two were still just perceptibly curved inwards. After 48 hrs. all six leaves were fully re-expanded, and appeared perfectly healthy.

Three leaves were immersed, each in thirty minims of a solution of one part to 875 of water (1 gr. to 2 oz.), so that each received 1/32 of a grain (2.02 mg.); after 40 m. the three were much affected, and after 6 hrs. 45 m. the tentacles of all and the blade of one closely inflected.

Sodium, Nitrate of (pure).--Half-minims of a solution of one part to 437 of water, containing 1/960 of a grain (.0675 mg.), were placed on the discs of five leaves. After 1 hr. 25 m. the tentacles of nearly all, and the blade of one, were somewhat inflected. The inflection continued to increase, and in 21 hrs. 15 m. the tentacles and the blades of four of them were greatly affected, and the blade of the fifth to a slight extent. After an additional 24 hrs. the four leaves still remained closely inflected, whilst the fifth was beginning to expand. Four days after the solution had been applied, two of the leaves had quite, and one had partially, re-expanded; whilst the remaining two remained closely inflected and appeared injured.

Three leaves were immersed, each in thirty minims of a solution of one part to 875 of water; in 1 hr. there was great inflection, and after 8 hrs. 15 m. every tentacle and the blades of all three were most strongly inflected.

Sodium, Sulphate of.--Half-minims of a solution of one part to 437 of water were placed on the discs of six leaves. After 5 hrs. 30 m. the tentacles of three of them, (with the blade of one) were considerably; and those of the other three slightly, inflected. After 21 hrs. the inflection had a little decreased, [page 177] and in 45 hrs. the leaves were fully expanded, appearing quite healthy.

Three leaves were immersed, each in thirty minims of a solution of one part of the sulphate to 875 of water; after 1 hr. 30 m. there was some inflection, which increased so much that in 8 hrs. 10 m. all the tentacles and the blades of all three leaves were closely inflected.

Sodium, Phosphate of.--Half-minims of a solution of one part to 437 of water were placed on the discs of six leaves. The solution acted with extraordinary rapidity, for in 8 m. the outer tentacles on several of the leaves were much incurved. After 6 hrs. the tentacles of all six leaves, and the blades of two, were closely inflected. This state of things continued for 24 hrs., excepting that the blade of a third leaf became incurved. After 48 hrs. all the leaves re-expanded. It is clear that 1/960 of a grain of phosphate of soda has great power in causing inflection.

Sodium, Citrate of.--Half-minims of a solution of one part to 437 of water were placed on the discs of six leaves, but these were not observed until 22 hrs. had elapsed. The sub-marginal tentacles of five of them, and the blades of four, were then found inflected; but the outer rows of tentacles were not affected. One leaf, which appeared older than the others, was very little affected in any way. After 46 hrs. four of the leaves were almost re-expanded, including their blades. Three leaves were also immersed, each in thirty minims of a solution of one part of the citrate to 875 of water; they were much acted on in 25 m.; and after 6 hrs. 35 m. almost all the tentacles, including those of the outer rows, were inflected, but not the blades.

Sodium, Oxalate of.--Half-minims of a solution of one part to 437 of water were placed on the discs of seven leaves; after 5 hrs. 30 m. the tentacles of all, and the blades of most of them, were much affected.

In 22 hrs., besides the inflection of the tentacles, the blades of all seven leaves were so much doubled over that their tips and bases almost touched. On no other occasion have I seen the blades so strongly affected. Three leaves were also immersed, each in thirty minims of a solution of one part to 875 of water; after 30 m. there was much inflection, and after 6 hrs. 35 m. the blades of two and the tentacles of all were closely inflected.

Sodium, Chloride of (best culinary salt).--Half-minims of a solution of one part to 218 of water were placed on the discs [page 178] of four leaves. Two, apparently, were not at all affected in 48 hrs.; the third had its tentacles slightly inflected; whilst the fourth had almost all its tentacles inflected in 24 hrs., and these did not begin to re-expand until the fourth day, and were not perfectly expanded on the seventh day. I presume that this leaf was injured by the salt.

Half-minims of a weaker solution, of one part to 437 of water, were then dropped on the discs of six leaves, so that each received 1/960 of a grain. In 1 hr. 33 m. there was slight inflection; and after 5 hrs.

30 m. the tentacles of all six leaves were considerably, but not closely, inflected. After 23 hrs. 15 m. all had completely re-expanded, and did not appear in the least injured.

Three leaves were immersed, each in thirty minims of a solution of one part to 875 of water, so that each received 1/32 of a grain, or 2.02 mg. After 1 hr. there was much inflection; after 8 hrs. 30 m. all the tentacles and the blades of all three were closely inflected. Four other leaves were also immersed in the solution, each receiving the same amount of salt as before, viz. 1/32 of a grain. They all soon became inflected; after 48 hrs. they began to re-expand, and appeared quite uninjured, though the solution was sufficiently strong to taste saline.

Sodium, Iodide of.--Half-minims of a solution of one part to 437 of water were placed on the discs of six leaves. After 24 hrs. four of them had their blades and many tentacles inflected. The other two had only their submarginal tentacles inflected; the outer ones in most of the leaves being but little affected. After 46 hrs. the leaves had nearly re-expanded. Three leaves were also immersed, each in thirty minims of a solution of one part to 875 of water. After 6 hrs. 30 m.

almost all the tentacles, and the blade of one leaf, were closely inflected.

Sodium, Bromide of.--Half-minims of a solution of one part to 437 of water were placed on six leaves. After 7 hrs. there was some inflection; after 22 hrs. three of the leaves had their blades and most of their tentacles inflected; the fourth leaf was very slightly, and the fifth and sixth hardly at all, affected. Three leaves were also immersed, each in thirty minims of a solution of one part to 875 of water; after 40 m. there was some inflection; after 4 hrs. the tentacles of all three leaves and the blades of two were inflected.

These leaves were then placed in water, and after 17 hrs. 30 m. two of them were almost completely, and the third partially, re-expanded; so that apparently they were not injured. [page 179]

Pota.s.sium, Carbonate of (pure).--Half-minims of a solution of one part to 437 of water were placed on six leaves. No effect was produced in 24 hrs.; but after 48 hrs. some of the leaves had their tentacles, and one the blade, considerably inflected. This, however, seemed the result of their being injured; for on the third day after the solution was given, three of the leaves were dead, and one was very unhealthy; the other two were recovering, but with several of their tentacles apparently injured, and these remained permanently inflected. It is evident that the 1/960 of a grain of this salt acts as a poison. Three leaves were also immersed, each in thirty minims of a solution of one part to 875 of water, though only for 9 hrs.; and, very differently from what occurs with the salts of soda, no inflection ensued.

Pota.s.sium, Nitrate of.--Half-minims of a strong solution, of one part to 109 of water (4 grs. to 1 oz.), were placed on the discs of four leaves; two were much injured, but no inflection ensued. Eight leaves were treated in the same manner, with drops of a weaker solution, of one part to 218 of water. After 50 hrs. there was no inflection, but two of the leaves seemed injured. Five of these leaves were subsequently tested with drops of milk and a solution of gelatine on their discs, and only one became inflected; so that the solution of the nitrate of the above strength, acting for 50 hrs., apparently had injured or paralysed the leaves. Six leaves were then treated in the same manner with a still weaker solution, of one part to 437 of water, and these, after 48 hrs., were in no way affected, with the exception of perhaps a single leaf. Three leaves were next immersed for 25 hrs., each in thirty minims of a solution of one part to 875 of water, and this produced no apparent effect. They were then put into a solution of one part of carbonate of ammonia to 218 of water; the glands were immediately blackened, and after 1 hr. there was some inflection, and the protoplasmic contents of the cells became plainly aggregated. This shows that the leaves had not been much injured by their immersion for 25 hrs. in the nitrate.

Pota.s.sium, Sulphate of.--Half-minims of a solution of one part to 437 of water were placed on the discs of six leaves. After 20 hrs. 30 m. no effect was produced; after an additional 24 hrs. three remained quite unaffected; two seemed injured, and the sixth seemed almost dead with its tentacles inflected. Nevertheless, after two additional days, all six leaves recovered. The immersion of three leaves for 24 hrs., each in thirty minims of [page 180] a solution of one part to 875 of water, produced no apparent effect. They were then treated with the same solution of carbonate of ammonia, with the same result as in the case of the nitrate of potash.

Pota.s.sium, Phosphate of.--Half-minims of a solution of one part to 437 of water were placed on the discs of six leaves, which were observed during three days; but no effect was produced. The partial drying up of the fluid on the disc slightly drew together the tentacles on it, as often occurs in experiments of this kind. The leaves on the third day appeared quite healthy.