Insectivorous Plants · Charles Darwin
CHAPTER IV. THE EFFECTS OF HEAT ON THE LEAVES.
It will be convenient first briefly to describe the effects of immersion for thirty seconds in boiling water. The leaves are rendered flaccid, with their tentacles bowed backwards, which, as we shall see in a future chapter, is probably due to their outer surfaces retaining their elasticity for a longer period than their inner surfaces retain the power of contraction. The purple fluid within the cells of the pedicels is rendered finely granular, but there is no true aggregation; nor does this follow [page 68] when the leaves are subsequently placed in a solution of carbonate of ammonia. But the most remarkable change is that the glands become opaque and uniformly white; and this may be attributed to the coagulation of their albuminous contents. My first and preliminary experiment consisted in putting seven leaves in the same vessel of water, and warming it slowly up to the temperature of 110° Fahr. (43°.3 Cent.); a leaf being taken out as soon as the temperature rose to 80° (26°.6 Cent.), another at 85°, another at 90°, and so on. Each leaf, when taken out, was placed in water at the temperature of my room, and the tentacles of all soon became slightly, though irregularly, inflected. They were now removed from the cold water and kept in damp air, with bits of meat placed on their discs. The leaf which had been exposed to the temperature of 110o became in 15 m. greatly inflected; and in 2 hrs. every single tentacle closely embraced the meat. So it was, but after rather longer intervals, with the six other leaves. It appears, therefore, that the warm bath had increased their sensitiveness when excited by meat.
I next observed the degree of inflection which leaves underwent within stated periods, whilst still immersed in warm water, kept as nearly as possible at the same temperature; but I will here and elsewhere give only a few of the many trials made. A leaf was left for 10 m. in water at 100° (37°.7 Cent.), but no inflection occurred. A second leaf, however, treated in the same manner, had a few of its exterior tentacles very slightly inflected in 6 m., and several irregularly but not closely inflected in 10 m. A third leaf, kept in water at 105° to 106° (40°.5 to 41°.1 Cent.), was very moderately inflected in 6 m. A fourth leaf, in water at 110° (43°.3 Cent.), was somewhat inflected in 4 m., and considerably so in from 6 to 7 m.
Three leaves were placed in water which was heated rather quickly, and by the time the temperature rose to 115°-116° (46°.1 to 46°.06 Cent.), all three were inflected. I then removed the lamp, and in a few minutes every single tentacle was closely inflected. The protoplasm within the cells was not killed, for it was seen to be in distinct movement; and the leaves, having been left in cold water for 20 hrs., re-expanded. Another leaf was immersed in water at 100o (37.7° Cent.), which was raised to 120° (48°.8 Cent.); and all the tentacles, except the extreme marginal ones, soon became closely inflected. The leaf was now placed in cold water, and in 7 hrs. 30 m. it had partly, and in 10 hrs. fully, re-expanded. On the following morning it was immersed in a weak solution of carbonate of [page 69] ammonia, and the glands quickly became black, with strongly marked aggregation in the tentacles, showing that the protoplasm was alive, and that the glands had not lost their power of absorption. Another leaf was placed in water at 110o (43°.3 Cent.) which was raised to 120° (48°.8 Cent.); and every tentacle, excepting one, was quickly and closely inflected.
This leaf was now immersed in a few drops of a strong solution of carbonate of ammonia (one part to 109 of water); in 10 m. all the glands became intensely black, and in 2 hrs. the protoplasm in the cells of the pedicels was well aggregated. Another leaf was suddenly plunged, and as usual waved about, in water at 120°, and the tentacles became inflected in from 2 m. to 3 m., but only so as to stand at right angles to the disc. The leaf was now placed in the same solution (viz. one part of carbonate of ammonia to 109 of water, or 4 grs. to 1 oz., which I will for the future designate as the strong solution), and when I looked at it again after the interval of an hour, the glands were blackened, and there was well-marked aggregation. After an additional interval of 4 hrs. the tentacles had become much more inflected. It deserves notice that a solution as strong as this never causes inflection in ordinary cases. Lastly a leaf was suddenly placed in water at 125° (51°.6 Cent.), and was left in it until the water cooled; the tentacles were rendered of a bright red and soon became inflected. The contents of the cells underwent some degree of aggregation, which in the course of three hours increased; but the masses of protoplasm did not become spherical, as almost always occurs with leaves immersed in a solution of carbonate of ammonia.]
We learn from these cases that a temperature of from 120° to 125° (48°.8 to 51°.6 Cent.) excites the tentacles into quick movement, but does not kill the leaves, as shown either by their subsequent re-expansion or by the aggregation of the protoplasm. We shall now see that a temperature of 130° (54°.4 Cent.) is too high to cause immediate inflection, yet does not kill the leaves. [Experiment 1.—A leaf was plunged, and as in all cases waved about for a few minutes, in water at 130° (54°.4 Cent.), but there was no trace of inflection; it was then placed in cold water, and after an interval of 15 m. very slow movement was [page 70] distinctly seen in a small mass of protoplasm in one of the cells of a tentacle.* After a few hours all the tentacles and the blade became inflected. Experiment 2.—Another leaf was plunged into water at 130o to 131o, and as before there was no inflection. After being kept in cold water for an hour, it was placed in the strong solution of ammonia, and in the course of 55 m. the tentacles were considerably inflected. The glands, which before had been rendered of a brighter red, were now blackened. The protoplasm in the cells of the tentacles was distinctly aggregated; but the spheres were much smaller than those generated in unheated leaves when subjected to carbonate of ammonia. After an additional 2 hrs. all the tentacles, excepting six or seven, were closely inflected.
Experiment 11.—A leaf was immersed in water at 145° (62°.7 Cent.), which was raised to 156° (68°.8 Cent.). The tentacles became bright red and somewhat reflexed, with almost all the glands like porcelain; those on the disc being still pinkish, those near the margin quite white. The leaf being placed as usual first in cold water and then in the strong solution, the cells in the tentacles became of a muddy greenish brown, with the protoplasm not aggregated. Nevertheless, four of the glands escaped being rendered like porcelain, and the pedicels of these glands were spirally curled, like a French horn, towards their upper ends; but this can by no means be considered as a case of true inflection. The protoplasm within the cells of the twisted portions was aggregated into distinct though excessively minute purple spheres. This case shows clearly that the protoplasm, after having been exposed to a high temperature for a few minutes, is capable of aggregation when afterwards subjected to the action of carbonate of ammonia, unless the heat has been sufficient to cause coagulation.]
Concluding Remarks.—As the hair-like tentacles are extremely thin and have delicate walls, and as the leaves were waved about for some minutes close to the bulb of the thermometer, it seems scarcely possible that they should not have been raised very nearly to the temperature which the instrument indicated. From the eleven last observations we see that a temperature of 130° (54°.4 Cent.) never causes the immediate inflection of the tentacles, though a temperature from 120° to 125° (48°.8 to 51°.6 Cent.) quickly produces this effect. But the leaves are paralysed only for a time by a temperature of 130°, as afterwards, whether left in simple water or in a solution of carbonate of ammonia, they become inflected and their protoplasm undergoes aggregation. This great difference in the effects of a higher and lower temperature may be compared with that from immersion in strong and weak solutions of the salts of ammonia; for the former do not excite movement, whereas the latter act energetically. A temporary suspension of the [page 73] power of movement due to heat is called by Sachs* heat-rigidity; and this in the case of the sensitive-plant (Mimosa) is induced by its exposure for a few minutes to humid air, raised to 120°-122° Fahr., or 49° to 50° Cent. It deserves notice that the leaves of Drosera, after being immersed in water at 130° Fahr., are excited into movement by a solution of the carbonate so strong that it would paralyse ordinary leaves and cause no inflection.
In all the following cases a drop was allowed to fall from the same pointed instrument on the centre of the leaf; and by repeated trials one of these drops was ascertained to be on an average very nearly half a minim, or 1/960 of a fluid ounce, or .0295 ml. But these measurements obviously do not pretend to any strict accuracy; moreover, the drops of the viscid fluids were plainly larger than those of water. Only one leaf on the same plant was tried, and the plants were collected from two distant localities. The experiments were made during August and September. In judging of the effects, one caution is necessary: if a drop of any adhesive fluid is placed on an old or feeble leaf, the glands of which have ceased to secrete copiously, the drop sometimes dries up, especially if the plant [page 77] is kept in a room, and some of the central and submarginal tentacles are thus drawn together, giving to them the false appearance of having become inflected. This sometimes occurs with water, as it is rendered adhesive by mingling with the viscid secretion. Hence the only safe criterion, and to this alone I have trusted, is the bending inwards of the exterior tentacles, which have not been touched by the fluid, or at most only at their bases. In this case the movement is wholly due to the central glands having been stimulated by the fluid, and transmitting a motor impulse to the exterior tentacles. The blade of the leaf likewise often curves inwards, in the same manner as when an insect or bit of meat is placed on the disc. This latter movement is never caused, as far as I have seen, by the mere drying up of an adhesive fluid and the consequent drawing together of the tentacles.
First for the non-nitrogenous fluids. As a preliminary trial, drops of distilled water were placed on between thirty and forty leaves, and no effect whatever was produced; nevertheless, in some other and rare cases, a few tentacles became for a short time inflected; but this may have been caused by the glands having been accidentally touched in getting the leaves into a proper position. That water should produce no effect might have been anticipated, as otherwise the leaves would have been excited into movement by every shower of rain. [Gum arabic.—Solutions of four degrees of strength were made; one of six grains to the ounce of water (one part to 73); a second rather stronger, yet very thin; a third moderately thick, and a fourth so thick that it would only just drop from a pointed instrument. These were tried on fourteen leaves; the drops being left on the discs from 24 hrs. to 44 hrs.; generally about [page 78] 30 hrs. Inflection was never thus caused. It is necessary to try pure gum arabic, for a friend tried a solution bought ready prepared, and this caused the tentacles to bend; but he afterwards ascertained that it contained much animal matter, probably glue. Sugar.—Drops of a solution of white sugar of three strengths (the weakest containing one part of sugar to 73 of water) were left on fourteen leaves from 32 hrs. to 48 hrs.; but no effect was produced.
Starch.—A mixture about as thick as cream was dropped on six leaves and left on them for 30 hrs., no effect being produced. I am surprised at this fact, as I believe that the starch of commerce generally contains a trace of gluten, and this nitrogenous substance causes inflection, as we shall see in the next chapter. Alcohol, Diluted.—One part of alcohol was added to seven of water, and the usual drops were placed on the discs of three leaves. No inflection ensued in the course of 48 hrs. To ascertain whether these leaves had been at all injured, bits of meat were placed on them, and after 24 hrs. they were closely inflected. I also put drops of sherry-wine on three other leaves; no inflection was caused, though two of them seemed somewhat injured. We shall hereafter see that cut off leaves immersed in diluted alcohol of the above strength do not become inflected. Olive Oil.—drops were placed on the discs of eleven leaves, and no effect was produced in from 24 hrs. to 48 hrs. Four of these leaves were then tested by bits of meat on their discs, and three of them were found after 24 hrs. with all their tentacles and blades closely inflected, whilst the fourth had only a few tentacles inflected. It will, however, be shown in a future place, that cut off leaves immersed in olive oil are powerfully affected.
Infusion and Decoction of Tea.—Drops of a strong infusion and decoction, as well as of a rather weak decoction, of tea were placed on ten leaves, none of which became inflected. I afterwards tested three of them by adding bits of meat to the drops which still remained on their discs, and when I examined them after 24 hrs. they were closely inflected. The chemical principle of tea, namely theine, was subsequently tried and produced no effect. The albuminous matter which the leaves must originally have contained, no doubt, had been rendered insoluble by their having been completely dried.] We thus see that, excluding the experiments with water, sixty-one leaves were tried with drops of the [page 79] above-named non-nitrogenous fluids; and the tentacles were not in a single case inflected. [With respect to nitrogenous fluids, the first which came to hand were tried. The experiments were made at the same time and in exactly the same manner as the foregoing. As it was immediately evident that these fluids produced a great effect, I neglected in most cases to record how soon the tentacles became inflected. But this always occurred in less than 24 hrs.; whilst the drops of non-nitrogenous fluids which produced no effect were observed in every case during a considerably longer period. Milk.—Drops were placed on sixteen leaves, and the tentacles of all, as well as the blades of several, soon became greatly inflected. The periods were recorded in only three cases, namely, with leaves on which unusually small drops had been placed. Their tentacles were somewhat inflected in 45 m.; and after 7 hrs. 45 m. the blades of two were so much curved inwards that they formed little cups enclosing the drops. These leaves re-expanded on the third day. On another occasion the blade of a leaf was much inflected in 5 hrs. after a drop of milk had been placed on it.
Human Urine.—Drops were placed on twelve leaves, and the tentacles of all, with a single exception, became greatly inflected. Owing, I presume, to differences in the chemical nature of the urine on different occasions, the time required for the movements of the tentacles varied much, but was always effected in under 24 hrs. In two instances I recorded that all the exterior tentacles were completely inflected in 17 hrs., but not the blade of the leaf. In another case the edges of a leaf, after 25 hrs. 30 m., became so strongly inflected that it was converted into a cup. The power of urine does not lie in the urea, which, as we shall hereafter see, is inoperative. Albumen (fresh from a hen’s egg), placed on seven leaves, caused the tentacles of six of them to be well inflected. In one case the edge of the leaf itself became much curled in after 20 hrs. The one leaf which was unaffected remained so for 26 hrs., and was then treated with a drop of milk, and this caused the tentacles to bend inwards in 12 hrs. Cold Filtered Infusion of Raw Meat.—This was tried only on a single leaf, which had most of its outer tentacles and the blade inflected in 19 hrs. During subsequent years, I repeatedly used this infusion to test leaves which had been experimented [page 80] on with other substances, and it was found to act most energetically, but as no exact account of these trials was kept, they are not here introduced.
Mucus.—Thick and thin mucus from the bronchial tubes, placed on three leaves, caused inflection. A leaf with thin mucus had its marginal tentacles and blade somewhat curved inward in 5 hrs. 30 m., and greatly so in 20 hrs. The action of this fluid no doubt is due either to the saliva or to some albuminous matter* mingled with it, and not, as we shall see in the next chapter, to mucin or the chemical principle of mucus. Saliva.—Human saliva, when evaporated, yields** from 1.14 to 1.19 per cent. of residue; and this yields 0.25 per cent. of ashes, so that the proportion of nitrogenous matter which saliva contains must be small. Nevertheless, drops placed on the discs of eight leaves acted on them all. In one case all the exterior tentacles, excepting nine, were inflected in 19 hrs. 30 m.; in another case a few became so in 2 hrs., and after 7 hrs. 30 m. all those situated near where the drop lay, as well as the blade, were acted on. Since making these trials, I have many scores of times just touched glands with the handle of my scalpel wetted with saliva, to ascertain whether a leaf was in an active condition; for this was shown in the course of a few minutes by the bending inwards of the tentacles. The edible nest of the Chinese swallow is formed of matter secreted by the salivary glands; two grains were added to one ounce of distilled water (one part to 218), which was boiled for several minutes, but did not dissolve the whole. The usual-sized drops were placed on three leaves, and these in 1 hr. 30 m. were well, and in 2 hrs. 15 m. closely, inflected.
Altogether I experimented on sixty-four leaves with the above nitrogenous fluids, the five leaves tried only with the extremely weak solution of isinglass not being included, nor the numerous trials subsequently made, of which no exact account was kept. Of these sixty-four leaves, sixty-three had their tentacles and often their blades well inflected. The one which failed was probably too old and torpid. But to obtain so large a proportion of successful cases, care must be taken to select young and active leaves. Leaves in this condition were chosen with equal care for the sixty-one trials with non-nitrogenous fluids (water not included); and we have seen that not one of these was in the least affected. We may therefore safely conclude that in the sixty-four experiments with nitrogenous fluids the inflection of the exterior tentacles was due to the absorption of [page 82] nitrogenous matter by the glands of the tentacles on the disc. Some of the leaves which were not affected by the non-nitrogenous fluids were, as above stated, immediately afterwards tested with bits of meat, and were thus proved to be in an active condition. But in addition to these trials, twenty-three of the leaves, with drops of gum, syrup, or starch, still lying on their discs, which had produced no effect in the course of between 24 hrs. and 48 hrs., were then tested with drops of milk, urine, or albumen. Of the twenty-three leaves thus treated, seventeen had their tentacles, and in some cases their blades, well inflected; but their powers were somewhat impaired, for the rate of movement was decidedly slower than when fresh leaves were treated with these same nitrogenous fluids. This impairment, as well as the insensibility of six of the leaves, may be attributed to injury from exosmose, caused by the density of the fluids placed on their discs.
On three occasions chopped cabbage-leaves* were boiled in distilled water for 1 hr. or for 1 1/4 hr.; and by decanting the decoction after it had been allowed to rest, a pale dirty green fluid was obtained. The usual-sized drops were placed on thirteen leaves. Their tentacles and blades were inflected after 4 hrs. to a quite extraordinary degree. Next day the protoplasm within the cells of the tentacles was found aggregated in the most strongly marked manner. I also touched the viscid secretion round the glands of several tentacles with minute drops of the decoction on the head of a small pin, and they became well inflected in a few minutes. The fluid proving so powerful, one part was diluted with three of water, and drops were placed on the discs of five leaves; and these next morning were so much acted on that their blades were completely doubled over. We thus see that a decoction of cabbage-leaves is nearly or quite as potent as an infusion of raw meat.
Text from Project Gutenberg, public domain in the United States.