Insectivorous Plants

On the Action of Distilled Water in Causing Inflection.

Although in all the more important experiments the difference between the leaves simultaneously immersed in water and in the several solutions will be described, nevertheless it may be well here to give a summary of the effects of water. The fact, moreover, of pure water acting on the glands deserves in itself some notice. Leaves to the number of 141 were immersed in water at the same time with those in the solutions, and their state recorded at short intervals of time. Thirty-two other leaves were separately observed in water, making altogether 173 experiments. Many scores of leaves were also immersed in water at other times, but no exact record of the effects produced was kept; yet these cursory observations support the conclusions arrived at in this chapter. A few of the long-headed tentacles, namely from one to about six, were commonly inflected within half an hour after immersion; as were occasionally a few, and rarely a considerable number of the exterior round-headed tentacles. After an immersion of from 5 to 8 hrs. the short tentacles surrounding the outer parts of the disc generally become inflected, so that their glands form a small dark ring on the disc; the exterior tentacles not partaking of this movement. Hence, excepting in a few cases hereafter to be specified, we can judge whether a solution produces any effect only by observing the exterior tentacles within the first 3 or 4 hrs. after immersion.

Now for a summary of the state of the 173 leaves after an immersion of 3 or 4 hrs. in pure water. One leaf had almost all its tentacles inflected; three leaves had most of them sub-inflected; and thirteen had on an average 36.5 tentacles inflected. Thus seventeen leaves out of the 173 were acted on in a marked manner. Eighteen leaves had from seven to nineteen tentacles inflected, the average being 9.3 tentacles for each leaf. Forty-four leaves had from one to six tentacles inflected, generally the long-headed ones. So that altogether of the 173 leaves carefully observed, seventy-nine were affected by the water in some degree, though commonly to a very slight degree; and ninety-four were not affected in the least degree. This amount of inflection is utterly insignificant, as we shall hereafter see, compared with that caused by very weak solutions of several salts of ammonia.

Plants which have lived for some time in a rather high temperature are far more sensitive to the action of water than those grown out of doors, or recently brought into a warm greenhouse. Thus in the above seventeen cases, in which the immersed leaves had a considerable number of tentacles inflected, the plants had been kept during the winter in a very warm greenhouse; and they bore in the early spring remarkably fine leaves, of a light red colour. Had I then known that the sensitiveness of plants was thus increased, perhaps I should not have used the leaves for my experiments with the very weak solutions of phosphate of ammonia; but my experiments are not thus vitiated, as I invariably used leaves from the same plants for simultaneous immersion in water. It often happened that some leaves on the same plant, and some tentacles on the same leaf, were more sensitive than others; but why this should be so, I do not know.

Besides the differences just indicated between the leaves immersed in water and in weak solutions of ammonia, the tentacles of the latter are in most cases much more closely inflected. The appearance of a leaf after immersion in a few drops of a solution of 1 grain of phosphate of ammonia to 200 oz. of water (i.e. one part to 87,500) is here reproduced: such energetic inflection is never caused by water alone. With leaves in the weak solutions, the blade or lamina often becomes inflected; and this is so rare a circumstance with leaves in water that I have seen only two instances; and in both of these the inflection was very feeble. Again, with leaves in the weak solutions, the inflection of the tentacles and blade often goes on steadily, though slowly, increasing during many hours; and this again is so rare a circumstance with leaves in water that I have seen only three instances of any such increase after the first 8 to 12 hrs.; and in these three instances the two outer rows of tentacles were not at all affected. Hence there is sometimes a much greater difference between the leaves in water and in the weak solutions, after from 8 hrs. to 24 hrs., than there was within the first 3 hrs.; though as a general rule it is best to trust to the difference observed within the shorter time.

With respect to the period of the re-expansion of the leaves, when left immersed either in water or in the weak solutions, nothing could be more variable. In both cases the exterior tentacles not rarely begin to re-expand, after an interval of only from 6 to 8 hrs.; that is just about the time when the short tentacles round the borders of the disc become inflected. On the other hand, the tentacles sometimes remain inflected for a whole day, or even two days; but as a general rule they remain inflected for a longer period in very weak solutions than in water. In solutions which are not extremely weak, they never re-expand within nearly so short a period as six or eight hours. From these statements it might be thought difficult to distinguish between the effects of water and the weaker solutions; but in truth there is not the slightest difficulty until excessively weak solutions are tried; and then the distinction, as might be expected, becomes very doubtful, and at last disappears. But as in all, except the simplest, cases the state of the leaves simultaneously immersed for an equal length of time in water and in the solutions will be described, the reader can judge for himself.]

CARBONATE OF AMMONIA

This salt, when absorbed by the roots, does not cause the tentacles to be inflected. A plant was so placed in a solution of one part of the carbonate to 146 of water that the young uninjured roots could be observed. The terminal cells, which were of a pink colour, instantly became colourless, and their limpid contents cloudy, like a mezzo-tinto engraving, so that some degree of aggregation was almost instantly caused; but no further change ensued, and the absorbent hairs were not visibly affected. The tentacles did not bend. Two other plants were placed with their roots surrounded by damp moss, in half an ounce (14.198 ml.) of a solution of one part of the carbonate to 218 of water, and were observed for 24 hrs.; but not a single tentacle was inflected. In order to produce this effect, the carbonate must be absorbed by the glands.

The vapour produces a powerful effect on the glands, and induces inflection. Three plants with their roots in bottles, so that the surrounding air could not have become very humid, were placed under a bell-glass (holding 122 fluid ounces), together with 4 grains of carbonate of ammonia in a watch-glass. After an interval of 6 hrs. 15 m. the leaves appeared unaffected; but next morning, after 20 hrs., the blackened glands were secreting copiously, and most of the tentacles were strongly inflected. These plants soon died. Two other plants were placed under the same bell-glass, together with half a grain of the carbonate, the air being rendered as damp as possible; and in 2 hrs. most of the leaves were affected, many of the glands being blackened and the tentacles inflected. But it is a curious fact that some of the closely adjoining tentacles on the same leaf, both on the disc and round the margins, were much, and some, apparently, not in the least affected. The plants were kept under the bell-glass for 24 hrs., but no further change ensued. One healthy leaf was hardly at all affected, though other leaves on the same plant were much affected. On some leaves all the tentacles on one side, but not those on the opposite side, were inflected. I doubt whether this extremely unequal action can be explained by supposing that the more active glands absorb all the vapour as quickly as it is generated, so that none is left for the others, for we shall meet with analogous cases with air thoroughly permeated with the vapours of chloroform and ether.

Minute particles of the carbonate were added to the secretion surrounding several glands. These instantly became black and secreted copiously; but, except in two instances, when extremely minute particles were given, there was no inflection. This result is analogous to that which follows from the immersion of leaves in a strong solution of one part of the carbonate to 109, or 146, or even 218 of water, for the leaves are then paralysed and no inflection ensues, though the glands are blackened, and the protoplasm in the cells of the tentacles undergoes strong aggregation.

[We will now turn to the effects of solutions of the carbonate. Half-minims of a solution of one part to 437 of water were placed on the discs of twelve leaves; so that each received 1/960 of a grain or .0675 mg. Ten of these had their tentacles well inflected; the blades of some being also much curved inwards. In two cases several of the exterior tentacles were inflected in 35 m.; but the movement was generally slower. These ten leaves re-expanded in periods varying between 21 hrs. and 45 hrs., but in one case not until 67 hrs. had elapsed; so that they re-expanded much more quickly than leaves which have caught insects.

The same-sized drops of a solution of one part to 875 of water were placed on the discs of eleven leaves; six remained quite unaffected, whilst five had from three to six or eight of their exterior tentacles inflected; but this degree of movement can hardly be considered as trustworthy. Each of these leaves received 1/1920 of a grain (.0337 mg.), distributed between the glands of the disc, but this was too small an amount to produce any decided effect on the exterior tentacles, the glands of which had not themselves received any of the salt.

Minute drops on the head of a small pin, of a solution of one part of the carbonate to 218 of water, were next tried in the manner above described. A drop of this kind equals on an average 1/20 of a minim, and therefore contains 1/4800 of a grain (.0135 mg.) of the carbonate. I touched with it the viscid secretion round three glands, so that each gland received only 1/14400 of a grain (.00445 mg.). Nevertheless, in two trials all the glands were plainly blackened; in one case all three tentacles were well inflected after an interval of 2 hrs. 40 m.; and in another case two of the three tentacles were inflected. I then tried drops of a weaker solution of one part to 292 of water on twenty-four glands, always touching the viscid secretion round three glands with the same little drop. Each gland thus received only the 1/19200 of a grain (.00337 mg.), yet some of them were a little darkened; but in no one instance were any of the tentacles inflected, though they were watched for 12 hrs. When a still weaker solution (viz. one part to 437 of water) was tried on six glands, no effect whatever was perceptible. We thus learn that the 1/14400 of a grain (.00445 mg.) of carbonate of ammonia, if absorbed by a gland, suffices to induce inflection in the basal part of the same tentacle; but as already stated, I was able to hold with a steady hand the minute drops in contact with the secretion only for a few seconds; and if more time had been allowed for diffusion and absorption, a much weaker solution would certainly have acted.

Some experiments were made by immersing cut-off leaves in solutions of different strengths. Thus four leaves were left for about 3 hrs. each in a drachm (3.549 ml.) of a solution of one part of the carbonate to 5250 of water; two of these had almost every tentacle inflected, the third had about half the tentacles and the fourth about one-third inflected; and all the glands were blackened. Another leaf was placed in the same quantity of a solution of one part to 7000 of water, and in 1 hr. 16 m. every single tentacle was well inflected, and all the glands blackened. Six leaves were immersed, each in thirty minims (1.774 ml.) of a solution of one part to 4375 of water, and the glands were all blackened in 31 m. All six leaves exhibited some slight inflection, and one was strongly inflected. Four leaves were then immersed in thirty minims of a solution of one part to 8750 of water, so that each leaf received the 1/320 of a grain (.2025 mg.). Only one became strongly inflected; but all the glands on all the leaves were of so dark a red after one hour as almost to deserve to be called black, whereas this did not occur with the leaves which were at the same time immersed in water; nor did water produce this effect on any other occasion in nearly so short a time as an hour. These cases of the simultaneous darkening or blackening of the glands from the action of weak solutions are important, as they show that all the glands absorbed the carbonate within the same time, which fact indeed there was not the least reason to doubt. So again, whenever all the tentacles become inflected within the same time, we have evidence, as before remarked, of simultaneous absorption. I did not count the number of glands on these four leaves; but as they were fine ones, and as we know that the average number of glands on thirty-one leaves was 192, we may safely assume that each bore on an average at least 170; and if so, each blackened gland could have absorbed only 1/54400 of a grain (.00119 mg.) of the carbonate.

A large number of trials had been previously made with solutions of one part of the nitrate and phosphate of ammonia to 43750 of water (i.e. one grain to 100 ounces), and these were found highly efficient. Fourteen leaves were therefore placed, each in thirty minims of a solution of one part of the carbonate to the above quantity of water; so that each leaf received 1/1600 of a grain (.0405 mg.). The glands were not much darkened. Ten of the leaves were not affected, or only very slightly so. Four, however, were strongly affected; the first having all the tentacles, except forty, inflected in 47 m.; in 6 hrs. 30 m. all except eight; and after 4 hrs. the blade itself. The second leaf after 9 m. had all its tentacles except nine inflected; after 6 hrs. 30 m. these nine were sub-inflected; the blade having become much inflected in 4 hrs. The third leaf after 1 hr. 6 m. had all but forty tentacles inflected. The fourth, after 2 hrs. 5 m., had about half its tentacles and after 4 hrs. all but forty-five inflected. Leaves which were immersed in water at the same time were not at all affected, with the exception of one; and this not until 8 hrs. had elapsed. Hence there can be no doubt that a highly sensitive leaf, if immersed in a solution, so that all the glands are enabled to absorb, is acted on by 1/1600 of a grain of the carbonate. Assuming that the leaf, which was a large one, and which had all its tentacles excepting eight inflected, bore 170 glands, each gland could have absorbed only 1/268800 of a grain (.00024 mg.); yet this sufficed to act on each of the 162 tentacles which were inflected. But as only four out of the above fourteen leaves were plainly affected, this is nearly the minimum dose which is efficient.

Aggregation of the Protoplasm from the Action of Carbonate of Ammonia. – I have fully described in the third chapter the remarkable effects of moderately strong doses of this salt in causing the aggregation of the protoplasm within the cells of the glands and tentacles; and here my object is merely to show what small doses suffice. A leaf was immersed in twenty minims (1.183 ml.) of a solution of one part to 1750 of water, and another leaf in the same quantity of a solution of one part to 3062; in the former case aggregation occurred in 4 m., in the latter in 11 m. A leaf was then immersed in twenty minims of a solution of one part to 4375 of water, so that it received 1/240 of a grain (.27 mg.); in 5 m. there was a slight change of colour in the glands, and in 15 m. small spheres of protoplasm were formed in the cells beneath the glands of all the tentacles. In these cases there could not be a shadow of a doubt about the action of the solution.

A solution was then made of one part to 5250 of water, and I experimented on fourteen leaves, but will give only a few of the cases. Eight young leaves were selected and examined with care, and they showed no trace of aggregation. Four of these were placed in a drachm (3.549 ml.) of distilled water; and four in a similar vessel, with a drachm of the solution. After a time the leaves were examined under a high power, being taken alternately from the solution and the water. The first leaf was taken out of the solution after an immersion of 2 hrs. 40 m., and the last leaf out of the water after 3 hrs. 50 m.; the examination lasting for 1 hr. 40 m. In the four leaves out of the water there was no trace of aggregation except in one specimen, in which a very few, extremely minute spheres of protoplasm were present beneath some of the round glands. All the glands were translucent and red. The four leaves which had been immersed in the solution, besides being inflected, presented a widely different appearance; for the contents of the cells of every single tentacle on all four leaves were conspicuously aggregated; the spheres and elongated masses of protoplasm in many cases extending halfway down the tentacles. All the glands, both those of the central and exterior tentacles, were opaque and blackened; and this shows that all had absorbed some of the carbonate. These four leaves were of very nearly the same size, and the glands were counted on one and found to be 167. This being the case, and the four leaves having been immersed in a drachm of the solution, each gland could have received on an average only 1/64128 of a grain (.001009 mg.) of the salt; and this quantity sufficed to induce within a short time conspicuous aggregation in the cells beneath all the glands.

A vigorous but rather small red leaf was placed in six minims of the same solution (viz. one part to 5250 of water), so that it received 1/960 of a grain (.0675 mg.). In 40 m. the glands appeared rather darker; and in 1 hr. from four to six spheres of protoplasm were formed in the cells beneath the glands of all the tentacles. I did not count the tentacles, but we may safely assume that there were at least 140; and if so, each gland could have received only the 1/134400 of a grain, or .00048 mg.

A weaker solution was then made of one part to 7000 of water, and four leaves were immersed in it; but I will give only one case. A leaf was placed in ten minims of this solution; after 1 hr. 37 m. the glands became somewhat darker, and the cells beneath all of them now contained many spheres of aggregated protoplasm. This leaf received 1/768 of a grain, and bore 166 glands. Each gland could, therefore, have received only 1/127488 of a grain (.00507 mg.) of the carbonate.

Two other experiments are worth giving. A leaf was immersed for 4 hrs. 15 m. in distilled water, and there was no aggregation; it was then placed for 1 hr. 15 m. in a little solution of one part to 5250 of water; and this excited well-marked aggregation and inflection. Another leaf, after having been immersed for 21 hrs. 15 m. in distilled water, had its glands blackened, but there was no aggregation in the cells beneath them; it was then left in six minims of the same solution, and in 1 hr. there was much aggregation in many of the tentacles; in 2 hrs. all the tentacles (146 in number) were affected – the aggregation extending down for a length equal to half or the whole of the glands. It is extremely improbable that these two leaves would have undergone aggregation if they had been left for a little longer in the water, namely for 1 hr. and 1 hr. 15 m., during which time they were immersed in the solution; for the process of aggregation seems invariably to supervene slowly and very gradually in water.]

Summary of the Results with Carbonate of Ammonia. – The roots absorb the solution, as shown by their changed colour, and by the aggregation of the contents of their cells. The vapour is absorbed by the glands; these are blackened, and the tentacles are inflected. The glands of the disc, when excited by a half-minim drop (.0296 ml.), containing 1/960 of a grain (.0675 mg.), transmit a motor impulse to the exterior tentacles, causing them to bend inwards. A minute drop, containing 1/14400 of a grain (.00445 mg.), if held for a few seconds in contact with a gland, soon causes the tentacle bearing it to be inflected. If a leaf is left immersed for a few hours in a solution, and a gland absorbs the 1/134400 of a grain (.0048 mg.), its colour becomes darker, though not actually black; and the contents of the cells beneath the gland are plainly aggregated. Lastly, under the same circumstances, the absorption by a gland of the 1/268800 of a grain (.00024 mg.) suffices to excite the tentacle bearing this gland into movement.

[NITRATE OF AMMONIA

With the salt I attended only to the inflection of the leaves, for it is far less efficient than the carbonate in causing aggregation, although considerably more potent in causing inflection. I experimented with half-minims (.0296 ml.) on the discs of fifty-two leaves, but will give only a few cases. A solution of one part to 109 of water was too strong, causing little inflection, and after 24 hrs. killing, or nearly killing, four out of six leaves which were thus tried; each of which received the 1/240 of a grain (or .27 mg.). A solution of one part to 218 of water acted most energetically, causing not only the tentacles of all the leaves, but the blades of some, to be strongly inflected. Fourteen leaves were tried with drops of a solution of one part to 875 of water, so that the disc of each received the 1/1920 of a grain (.0337 mg.). Of these leaves, seven were very strongly acted on, the edges being generally inflected; two were moderately acted on; and five not at all. I subsequently tried three of these latter five leaves with urine, saliva, and mucus, but they were only slightly affected; and this proves that they were not in an active condition. I mention this fact to show how necessary it is to experiment on several leaves. Two of the leaves, which were well inflected, re-expanded after 51 hrs.

In the following experiment I happened to select very sensitive leaves. Half-minims of a solution of one part to 1094 of water (i.e. 1 gr. to 2 1/2 oz.) were placed on the discs of nine leaves, so that each received the 1/2400 of a grain (.027 mg.). Three of them had their tentacles strongly inflected and their blades curled inwards; five were slightly and somewhat doubtfully affected, having from three to eight of their exterior tentacles inflected: one leaf was not at all affected, yet was afterwards acted on by saliva. In six of these cases, a trace of action was perceptible in 7 hrs., but the full effect was not produced until from 24 hrs. to 30 hrs. had elapsed. Two of the leaves, which were only slightly inflected, re-expanded after an additional interval of 19 hrs.

Half-minims of a rather weaker solution, viz. of one part to 1312 of water (1 gr. to 3 oz.) were tried on fourteen leaves; so that each received 1/2880 of a grain (.0225 mg.), instead of, as in the last experiment, 1/2400 of a grain. The blade of one was plainly inflected, as were six of the exterior tentacles; the blade of a second was slightly, and two of the exterior tentacles well, inflected, all the other tentacles being curled in at right angles to the disc; three other leaves had from five to eight tentacles inflected; five others only two or three, and occasionally, though very rarely, drops of pure water cause this much action; the four remaining leaves were in no way affected, yet three of them, when subsequently tried with urine, became greatly inflected. In most of these cases a slight effect was perceptible in from 6 hrs. to 7 hrs., but the full effect was not produced until from 24 hrs. to 30 hrs. had elapsed. It is obvious that we have here reached very nearly the minimum amount, which, distributed between the glands of the disc, acts on the exterior tentacles; these having themselves not received any of the solution.

In the next place, the viscid secretion round three of the exterior glands was touched with the same little drop (1/20 of a minim) of a solution of one part to 437 of water; and after an interval of 2 hrs. 50 m. all three tentacles were well inflected. Each of these glands could have received only the 1/28800 of a grain, or .00225 mg. A little drop of the same size and strength was also applied to four other glands, and in 1 hr. two became inflected, whilst the other two never moved. We here see, as in the case of the half-minims placed on the discs, that the nitrate of ammonia is more potent in causing inflection than the carbonate; for minute drops of the latter salt of this strength produced no effect. I tried minute drops of a still weaker solution of the nitrate, viz. one part to 875 of water, on twenty-one glands, but no effect whatever was produced, except perhaps in one instance.

Sixty-three leaves were immersed in solutions of various strengths; other leaves being immersed at the same time in the same pure water used in making the solutions. The results are so remarkable, though less so than with phosphate of ammonia, that I must describe the experiments in detail, but I will give only a few. In speaking of the successive periods when inflection occurred, I always reckon from the time of first immersion.

Having made some preliminary trials as a guide, five leaves were placed in the same little vessel in thirty minims of a solution of one part of the nitrate to 7875 of water (1 gr. to 18 oz.); and this amount of fluid just sufficed to cover them. After 2 hrs. 10 m. three of the leaves were considerably inflected, and the other two moderately. The glands of all became of so dark a red as almost to deserve to be called black. After 8 hrs. four of the leaves had all their tentacles more or less inflected; whilst the fifth, which I then perceived to be an old leaf, had only thirty tentacles inflected. Next morning, after 23 hrs. 40 m., all the leaves were in the same state, excepting that the old leaf had a few more tentacles inflected. Five leaves which had been placed at the same time in water were observed at the same intervals of time; after 2 hrs. 10 m. two of them had four, one had seven, one had ten, of the long-headed marginal tentacles, and the fifth had four round-headed tentacles, inflected. After 8 hrs. there was no change in these leaves, and after 24 hrs. all the marginal tentacles had re-expanded; but in one leaf, a dozen, and in a second leaf, half a dozen, submarginal tentacles had become inflected. As the glands of the five leaves in the solution were simultaneously darkened, no doubt they had all absorbed a nearly equal amount of the salt: and as 1/288 of a grain was given to the five leaves together, each got 1/1440 of a grain (.045 mg.). I did not count the tentacles on these leaves, which were moderately fine ones, but as the average number on thirty-one leaves was 192, it would be safe to assume that each bore on an average at least 160. If so, each of the darkened glands could have received only 1/230400 of a grain of the nitrate; and this caused the inflection of a great majority of the tentacles.

This plan of immersing several leaves in the same vessel is a bad one, as it is impossible to feel sure that the more vigorous leaves do not rob the weaker ones of their share of the salt. The glands, moreover, must often touch one another or the sides of the vessel, and movement may have been thus excited; but the corresponding leaves in water, which were little inflected, though rather more so than commonly occurs, were exposed in an almost equal degree to these same sources of error. I will, therefore, give only one other experiment made in this manner, though many were tried and all confirmed the foregoing and following results. Four leaves were placed in forty minims of a solution of one part to 10,500 of water; and assuming that they absorbed equally, each leaf received 1/1152 of a grain (.0562 mg.). After 1 hr. 20 m. many of the tentacles on all four leaves were somewhat inflected. After 5 hrs. 30 m. two leaves had all their tentacles inflected; a third leaf all except the extreme marginals, which seemed old and torpid; and the fourth a large number. After 21 hrs. every single tentacle, on all four leaves, was closely inflected. Of the four leaves placed at the same time in water, one had, after 5 hrs. 45 m., five marginal tentacles inflected; a second, ten; a third, nine marginals and submarginals; and the fourth, twelve, chiefly submarginals, inflected. After 21 hrs. all these marginal tentacles re-expanded, but a few of the submarginals on two of the leaves remained slightly curved inwards. The contrast was wonderfully great between these four leaves in water and those in the solution, the latter having every one of their tentacles closely inflected. Making the moderate assumption that each of these leaves bore 160 tentacles, each gland could have absorbed only 1/184320 of a grain (.000351 mg.). This experiment was repeated on three leaves with the same relative amount of the solution; and after 6 hrs. 15 m. all the tentacles except nine, on all three leaves taken together, were closely inflected. In this case the tentacles on each leaf were counted, and gave an average of 162 per leaf.

The following experiments were tried during the summer of 1873, by placing the leaves, each in a separate watch-glass and pouring over it thirty minims (1.775 ml.) of the solution; other leaves being treated in exactly the same manner with the doubly distilled water used in making the solutions. The trials above given were made several years before, and when I read over my notes, I could not believe in the results; so I resolved to begin again with moderately strong solutions. Six leaves were first immersed, each in thirty minims of a solution of one part of the nitrate to 8750 of water (1 gr. to 20 oz.), so that each received 1/320 of a grain (.2025 mg.). Before 30 m. had elapsed, four of these leaves were immensely, and two of them moderately, inflected. The glands were rendered of a dark red. The four corresponding leaves in water were not at all affected until 6 hrs. had elapsed, and then only the short tentacles on the borders of the disc; and their inflection, as previously explained, is never of any significance.

Four leaves were immersed, each in thirty minims of a solution of one part to 17,500 of water (1 gr. to 40 oz.), so that each received 1/640 of a grain (.101 mg.); and in less than 45 m. three of them had all their tentacles, except from four to ten, inflected; the blade of one being inflected after 6 hrs., and the blade of a second after 21 hrs. The fourth leaf was not at all affected. The glands of none were darkened. Of the corresponding leaves in water, only one had any of its exterior tentacles, namely five, inflected; after 6 hrs. in one case, and after 21 hrs. in two other cases, the short tentacles on the borders of the disc formed a ring, in the usual manner.

Four leaves were immersed, each in thirty minims of a solution of one part to 43,750 of water (1 gr. to 100 oz.), so that each leaf got 1/1600 of a grain (.0405 mg.). Of these, one was much inflected in 8 m., and after 2 hrs. 7 m. had all the tentacles, except thirteen, inflected. The second leaf, after 10 m., had all except three inflected. The third and fourth were hardly at all affected, scarcely more than the corresponding leaves in water. Of the latter, only one was affected, this having two tentacles inflected, with those on the outer parts of the disc forming a ring in the usual manner. In the leaf which had all its tentacles except three inflected in 10 m., each gland (assuming that the leaf bore 160 tentacles) could have absorbed only 1/251200 of a grain, or .000258 mg.

Four leaves were separately immersed as before in a solution of one part to 131,250 of water (1 gr. to 300 oz.), so that each received 1/4800 of a grain, or .0135 mg. After 50 m. one leaf had all its tentacles except sixteen, and after 8 hrs. 20 m. all but fourteen, inflected. The second leaf, after 40 m., had all but twenty inflected; and after 8 hrs. 10 m. began to re-expand. The third, in 3 hrs. had about half its tentacles inflected, which began to re-expand after 8 hrs. 15 m. The fourth leaf, after 3 hrs. 7 m., had only twenty-nine tentacles more or less inflected. Thus three out of the four leaves were strongly acted on. It is clear that very sensitive leaves had been accidentally selected. The day moreover was hot. The four corresponding leaves in water were likewise acted on rather more than is usual; for after 3 hrs. one had nine tentacles, another four, and another two, and the fourth none, inflected. With respect to the leaf of which all the tentacles, except sixteen, were inflected after 50 m., each gland (assuming that the leaf bore 160 tentacles) could have absorbed only 1/691200 of a grain (.0000937 mg.), and this appears to be about the least quantity of the nitrate which suffices to induce the inflection of a single tentacle.

As negative results are important in confirming the foregoing positive ones, eight leaves were immersed as before, each in thirty minims of a solution of one part to 175,000 of water (1 gr. to 400 oz.), so that each received only 1/6400 of a grain (.0101 mg.). This minute quantity produced a slight effect on only four of the eight leaves. One had fifty-six tentacles inflected after 2 hrs. 13 m.; a second, twenty-six inflected, or sub-inflected, after 38 m.; a third, eighteen inflected, after 1 hr.; and a fourth, ten inflected, after 35 m. The four other leaves were not in the least affected. Of the eight corresponding leaves in water, one had, after 2 hrs. 10 m., nine tentacles, and four others from one to four long-headed tentacles, inflected; the remaining three being unaffected. Hence, the 1/6400 of a grain given to a sensitive leaf during warm weather perhaps produces a slight effect; but we must bear in mind that occasionally water causes as great an amount of inflection as occurred in this last experiment.]

Summary of the Results with Nitrate of Ammonia. – The glands of the disc, when excited by a half-minim drop (.0296 ml.), containing 1/2400 of a grain of the nitrate (.027 mg.), transmit a motor impulse to the exterior tentacles, causing them to bend inwards. A minute drop, containing 1/28800 of a grain (.00225 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 only a few minutes, in a solution of such strength that each gland can absorb only the (1/691200 of a grain (.0000937 mg.), this small amount is enough to excite each tentacle into movement, and it becomes closely inflected.

PHOSPHATE OF AMMONIA

This salt is more powerful than the nitrate, even in a greater degree than the nitrate is more powerful than the carbonate. This is shown by weaker solutions of the phosphate acting when dropped on the discs, or applied to the glands of the exterior tentacles, or when leaves are immersed. The difference in the power of these three salts, as tried in three different ways, supports the results presently to be given, which are so surprising that their credibility requires every kind of support. In 1872 I experimented on twelve immersed leaves, giving each only ten minims of a solution; but this was a bad method, for so small a quantity hardly covered them. None of these experiments will, therefore, be given, though they indicate that excessively minute doses are efficient. When I read over my notes, in 1873, I entirely disbelieved them, and determined to make another set of experiments with scrupulous care, on the same plan as those made with the nitrate; namely by placing leaves in watch-glasses, and pouring over each thirty minims of the solution under trial, treating at the same time and in the same manner other leaves with the distilled water used in making the solutions. During 1873, seventy-one leaves were thus tried in solutions of various strengths, and the same number in water. Notwithstanding the care taken and the number of the trials made, when in the following year I looked merely at the results, without reading over my observations, I again thought that there must have been some error, and thirty-five fresh trials were made with the weakest solution; but the results were as plainly marked as before. Altogether, 106 carefully selected leaves were tried, both in water and in solutions of the phosphate. Hence, after the most anxious consideration, I can entertain no doubt of the substantial accuracy of my results.

[Before giving my experiments, it may be well to premise that crystallised phosphate of ammonia, such as I used, contains 35.33 per cent. of water of crystallisation; so that in all the following trials the efficient elements formed only 64.67 per cent. of the salt used.

Extremely minute particles of the dry phosphate were placed with the point of a needle on the secretion surrounding several glands. These poured forth much secretion, were blackened, and ultimately died; but the tentacles moved only slightly. The dose, small as it was, evidently was too great, and the result was the same as with particles of the carbonate of ammonia.

Half-minims of a solution of one part to 437 of water were placed on the discs of three leaves and acted most energetically, causing the tentacles of one to be inflected in 15 m., and the blades of all three to be much curved inwards in 2 hrs. 15 m. Similar drops of a solution of one part to 1312 of water, (1 gr. to 3 oz.) were then placed on the discs of five leaves, so that each received the 1/2880 of a grain (.0225 mg.). After 8 hrs. the tentacles of four of them were considerably inflected, and after 24 hrs. the blades of three. After 48 hrs. all five were almost fully re-expanded. I may mention with respect to one of these leaves, that a drop of water had been left during the previous 24 hrs. on its disc, but produced no effect; and that this was hardly dry when the solution was added.

Similar drops of a solution of one part to 1750 of water (1 gr. to 4 oz.) were next placed on the discs of six leaves; so that each received 1/3840 of a grain (.0169 mg.); after 8 hrs. three of them had many tentacles and their blades inflected; two others had only a few tentacles slightly inflected, and the sixth was not at all affected. After 24 hrs. most of the leaves had a few more tentacles inflected, but one had begun to re-expand. We thus see that with the more sensitive leaves the 1/3840 of a grain, absorbed by the central glands, is enough to make many of the exterior tentacles and the blades bend, whereas the 1/1920 of a grain of the carbonate similarly given produced no effect; and 1/2880 of a grain of the nitrate was only just sufficient to produce a well-marked effect.

A minute drop, about equal to 1/20 of a minim, of a solution of one part of the phosphate to 875 of water, was applied to the secretion on three glands, each of which thus received only 1/57600 of a grain (.00112 mg.), and all three tentacles became inflected. Similar drops of a solution of one part to 1312 of water (1 gr. to 3 oz.) were now tried on three leaves; a drop being applied to four glands on the same leaf. On the first leaf, three of the tentacles became slightly inflected in 6 m., and re-expanded after 8 hrs. 45 m. On the second, two tentacles became sub-inflected in 12 m. And on the third all four tentacles were decidedly inflected in 12 m.; they remained so for 8 hrs. 30 m., but by the next morning were fully re-expanded. In this latter case each gland could have received only the 1/115200 (or .000563 mg.) of a grain. Lastly, similar drops of a solution of one part to 1750 of water (1 gr. to 4 oz.) were tried on five leaves; a drop being applied to four glands on the same leaf. The tentacles on three of these leaves were not in the least affected; on the fourth leaf, two became inflected; whilst on the fifth, which happened to be a very sensitive one, all four tentacles were plainly inflected in 6 hrs. 15m.; but only one remained inflected after 24 hrs. I should, however, state that in this case an unusually large drop adhered to the head of the pin. Each of these glands could have received very little more than 1/153600 of a grain (or .000423); but this small quantity sufficed to cause inflection. We must bear in mind that these drops were applied to the viscid secretion for only from 10 to 15 seconds, and we have good reason to believe that all the phosphate in the solution would not be diffused and absorbed in this time. We have seen under the same circumstances that the absorption by a gland of 1/19200 of a grain of the carbonate, and of 1/57600 of a grain of the nitrate, did not cause the tentacle bearing the gland in question to be inflected; so that here again the phosphate is much more powerful than the other two salts.

We will now turn to the 106 experiments with immersed leaves. Having ascertained by repeated trials that moderately strong solutions were highly efficient, I commenced with sixteen leaves, each placed in thirty minims of a solution of one part to 43,750 of water (1 gr. to 100 oz.); so that each received 1/1600 of a grain, or .04058 mg. Of these leaves, eleven had nearly all or a great number of their tentacles inflected in 1 hr., and the twelfth leaf in 3 hrs. One of the eleven had every single tentacle closely inflected in 50 m. Two leaves out of the sixteen were only moderately affected, yet more so than any of those simultaneously immersed in water; and the remaining two, which were pale leaves, were hardly at all affected. Of the sixteen corresponding leaves in water, one had nine tentacles, another six, and two others two tentacles inflected, in the course of 5 hrs. So that the contrast in appearance between the two lots was extremely great.

Eighteen leaves were immersed, each in thirty minims of a solution of one part to 87,500 of water (1 gr. to 200 oz.), so that each received 1/3200 of a grain (.0202 mg.). Fourteen of these were strongly inflected within 2 hrs., and some of them within 15 m.; three out of the eighteen were only slightly affected, having twenty-one, nineteen, and twelve tentacles in- flected; and one was not at all acted on. By an accident only fifteen, instead of eighteen, leaves were immersed at the same time in water; these were observed for 24 hrs.; one had six, another four, and a third two, of their outer tentacles inflected; the remainder being quite unaffected.

The next experiment was tried under very favourable circumstances, for the day (July 8) was very warm, and I happened to have unusually fine leaves. Five were immersed as before in a solution of one part to 131,250 of water (1 gr. to 300 oz.), so that each received 1/4800 of a grain, or .0135 mg. After an immersion of 25 m. all five leaves were much inflected. After 1 hr. 25 m. one leaf had all but eight tentacles inflected; the second, all but three; the third, all but five; the fourth; all but twenty-three; the fifth, on the other hand, never had more than twenty-four inflected. Of the corresponding five leaves in water, one had seven, a second two, a third ten, a fourth one, and a fifth none inflected. Let it be observed what a contrast is presented between these latter leaves and those in the solution. I counted the glands on the second leaf in the solution, and the number was 217; assuming that the three tentacles which did not become inflected absorbed nothing, we find that each of the 214 remaining glands could have absorbed only 1/l027200 of a grain, or .0000631 mg. The third leaf bore 236 glands, and subtracting the five which did not become inflected, each of the remaining 231 glands could have absorbed only 1/1108800 of a grain (or .0000584 mg.), and this amount sufficed to cause the tentacles to bend.

Twelve leaves were tried as before in a solution of one part to 175,000 of water (1 gr. to 400 oz.), so that each leaf received 1/6400 of a grain (.0101 mg.). My plants were not at the time in a good state, and many of the leaves were young and pale. Nevertheless, two of them had all their tentacles, except three or four, closely inflected in under 1 hr. Seven were considerably affected, some within 1 hr., and others not until 3 hrs., 4 hrs. 30 m., and 8 hrs. had elapsed; and this slow action may be attributed to the leaves being young and pale. Of these nine leaves, four had their blades well inflected, and a fifth slightly so. The three remaining leaves were not affected. With respect to the twelve corresponding leaves in water, not one had its blade inflected; after from 1 to 2 hrs. one had thirteen of its outer tentacles inflected; a second six, and four others either one or two inflected. After 8 hrs. the outer tentacles did not become more inflected; whereas this occurred with the leaves in the solution. I record in my notes that after the 8 hrs. it was impossible to compare the two lots, and doubt for an instant the power of the solution.

Two of the above leaves in the solution had all their tentacles, except three and four, inflected within an hour. I counted their glands, and, on the same principle as before, each gland on one leaf could have absorbed only 1/1164800, and on the other leaf only 1/1472000, of a grain of the phosphate.

Twenty leaves were immersed in the usual manner, each in thirty minims of a solution of one part to 218,750 of water (1 gr. to 500 oz.). So many leaves were tried because I was then under the false impression that it was incredible that any weaker solution could produce an effect. Each leaf received 1/8000 of a grain, or .0081 mg. The first eight leaves which I tried both in the solution and in water were either young and pale or too old; and the weather was not hot. They were hardly at all affected; nevertheless, it would be unfair to exclude them. I then waited until I got eight pairs of fine leaves, and the weather was favourable; the temperature of the room where the leaves were immersed varying from 75o to 81o (23o.8 to 27o.2 Cent.) In another trial with four pairs (included in the above twenty pairs), the temperature in my room was rather low, about 60o (15o.5 Cent.); but the plants had been kept for several days in a very warm greenhouse and thus rendered extremely sensitive. Special precautions were taken for this set of experiments; a chemist weighed for me a grain in an excellent balance; and fresh water, given me by Prof. Frankland, was carefully measured. The leaves were selected from a large number of plants in the following manner: the four finest were immersed in water, and the next four finest in the solution, and so on till the twenty pairs were complete. The water specimens were thus a little favoured, but they did not undergo more inflection than in the previous cases, comparatively with those in the solution.

Of the twenty leaves in the solution, eleven became inflected within 40 m.; eight of them plainly and three rather doubtfully; but the latter had at least twenty of their outer tentacles inflected. Owing to the weakness of the solution, inflection occurred, except in No. 1, much more slowly than in the previous trials. The condition of the eleven leaves which were considerably inflected will now be given at stated intervals, always reckoning from the time of immersion: —

(1) After only 8 m. a large number of tentacles inflected, and after 17 m. all but fifteen; after 2 hrs. all but eight in- flected, or plainly sub-inflected. After 4 hrs. the tentacles began to re-expand, and such prompt re-expansion is unusual; after 7 hrs. 30 m. they were almost fully re-expanded.