Insectivorous Plants Part 10

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 [page 154]

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 quant.i.ty 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-gla.s.ses, 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 [page 155]

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. [page 156]

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 quant.i.ty 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 circ.u.mstances 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- [page 157] 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 circ.u.mstances, 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; a.s.suming 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 [page 158] 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- [page 159] 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.

(2) After 39 m. a large number of tentacles inflected; after 2 hrs. 18 m. all but twenty-five inflected; after 4 hrs. 17 m. all but sixteen inflected. The leaf remained in this state for many hours.

(3) After 12 m. a considerable amount of inflection; after 4 hrs. all the tentacles inflected except those of the two outer rows, and the leaf remained in this state for some time; after 23 hrs. began to re-expand.

(4) After 40 m. much inflection; after 4 hrs. 13 m. fully half the tentacles inflected; after 23 hrs. still slightly inflected.

(5) After 40 m. much inflection; after 4 hrs. 22 m. fully half the tentacles inflected; after 23 hrs. still slightly inflected.

(6) After 40 m. some inflection; after 2 hrs. 18 m. about twenty-eight outer tentacles inflected; after 5 hrs. 20 m. about a third of the tentacles inflected; after 8 hrs. much re-expanded.

(7) After 20 m. some inflection; after 2 hrs. a considerable number of tentacles inflected; after 7 hrs. 45 m. began to re-expand.

(8) After 38 m. twenty-eight tentacles inflected; after 3 hrs. 45 m.

thirty-three inflected, with most of the submarginal tentacles sub-inflected; continued so for two days, and then partially re-expanded.

(9) After 38 m. forty-two tentacles inflected; after 3 hrs. 12 m.

sixty-six inflected or sub-inflected; after 6 hrs. 40 m. all but twenty-four inflected or sub-inflected; after 9 hrs. 40 m. all but seventeen inflected; after 24 hrs. all but four inflected or sub-inflected, only a few being closely inflected; after 27 hrs. 40 m.

the blade inflected. The leaf remained in this state for two days, and then began to re-expand.

(10) After 38 m. twenty-one tentacles inflected; after 3 hrs. 12 m.

forty-six tentacles inflected or sub-inflected; after 6 hrs. 40 m. all but seventeen inflected, though none closely; after 24 hrs. every tentacle slightly curved inwards; after 27 hrs. 40 m. blade strongly inflected, and so continued for two days, and then the tentacles and blade very slowly re-expanded.

(11) This fine dark red and rather old leaf, though not very large, bore an extraordinary number of tentacles (viz. 252), and behaved in an anomalous manner. After 6 hrs. 40 m. only the short tentacles round the outer part of the disc were inflected, forming a ring, as so often occurs in from 8 to 24 hrs. With leaves both in water and the weaker solutions. But after 9 hrs. [page 160] 40 m. all the outer tentacles except twenty-five were inflected; as was the blade in a strongly marked manner. After 24 hrs. every tentacle except one was closely inflected, and the blade was completely doubled over. Thus the leaf remained for two days, when it began to re-expand. I may add that the three latter leaves (Nos. 9, 10, and 11) were still somewhat inflected after three days. The tentacles in but few of these eleven leaves became closelyinflected within so short a time as in the previous experiments with stronger solutions.

We will now turn to the twenty corresponding leaves in water. Nine had none of their outer tentacles inflected; nine others had from one to three inflected; and these re-expanded after 8 hrs. The remaining two leaves were moderately affected; one having six tentacles inflected in 34 m.; the other twenty-three inflected in 2 hrs. 12 m.; and both thus remained for 24 hrs. None of these leaves had their blades inflected.

So that the contrast between the twenty leaves in water and the twenty in the solution was very great, both within the first hour and after from 8 to 12 hrs. had elapsed.

Of the leaves in the solution, the glands on leaf No. 1, which in 2 hrs. had all its tentacles except eight inflected, were counted and found to be 202. Subtracting the eight, each gland could have received only the 1/1552000 grain (.0000411 mg.) of the phosphate. Leaf No. 9 had 213 tentacles, all of which, with the exception of four, were inflected after 24 hrs., but none of them closely; the blade was also inflected; each gland could have received only the 1/1672000 of a grain, or .0000387 mg. Lastly, leaf No. 11, which had after 24 hrs. all its tentacles, except one, closely inflected, as well as the blade, bore the unusually large number of 252 tentacles; and on the same principle as before, each gland could have absorbed only the 1/2008000 of a grain, or .0000322 mg.

With respect to the following experiments, I must premise that the leaves, both those placed in the solutions and in water, were taken from plants which had been kept in a very warm greenhouse during the winter. They were thus rendered extremely sensitive, as was shown by water exciting them much more than in the previous experiments. Before giving my observations, it may be well to remind the reader that, judging from thirty-one fine leaves, the average number of tentacles is 192, and that the outer or exterior ones, the movements of which are alone significant, are to the short ones on the disc in the proportion of about sixteen to nine. [page 161]

Four leaves were immersed as before, each in thirty minims of a solution of one part to 328,125 of water (1 gr. to 750 oz.). Each leaf thus received 1/12000 of a grain (.0054 mg.) of the salt; and all four were greatly inflected.

(1) After 1 hr. all the outer tentacles but one inflected, and the blade greatly so; after 7 hrs. began to re-expand.

(2) After 1 hr. all the outer tentacles but eight inflected; after 12 hrs. all re-expanded.

(3) After 1 hr. much inflection; after 2 hrs. 30 m. all the tentacles but thirty-six inflected; after 6 hrs. all but twenty-two inflected; after 12 hrs. partly re-expanded.

(4) After 1 hr. all the tentacles but thirty-two inflected; after 2 hrs. 30 m. all but twenty-one inflected; after 6 hrs. almost re-expanded.

Of the four corresponding leaves in water:--

(1) After 1 hr. forty-five tentacles inflected; but after 7 hrs. so many had re-expanded that only ten remained much inflected.

(2) After 1 hr. seven tentacles inflected; these were almost re-expanded in 6 hrs.

(3) and (4) Not affected, except that, as usual, after 11 hrs. the short tentacles on the borders of the disc formed a ring.

There can, therefore, be no doubt about the efficiency of the above solution; and it follows as before that each gland of No. 1 could have absorbed only 1/2412000 of a grain (.0000268 mg.) and of No. 2 only 1/2460000 of a grain (.0000263 mg.) of the phosphate.

Seven leaves were immersed, each in thirty minims of a solution of one part to 437,500 of water (1 gr. to 1000 oz.). Each leaf thus received 1/16000 of a grain (.00405 mg.). The day was warm, and the leaves were very fine, so that all circ.u.mstances were favourable.

(1) After 30 m. all the outer tentacles except five inflected, and most of them closely; after 1 hr. blade slightly inflected; after 9 hrs. 30 m. began to re-expand.

(2) After 33 m. all the outer tentacles but twenty-five inflected, and blade slightly so; after 1 hr. 30 m. blade strongly inflected and remained so for 24 hrs.; but some of the tentacles had then re-expanded.

(3) After 1 hr. all but twelve tentacles inflected; after 2 hrs. 30 m.