Insectivorous Plants

A full account will be given in the next chapter of the effects of heat on the leaves, and I need here only state that leaves immersed for a short time in water at a temperature of 120oFahr. (48o.8 Cent.), which, as we have seen, does not immediately induce aggregation, were then placed in a few drops of a strong solution of one part of carbonate of ammonia to 109 of water, and became finely aggregated. On the other hand, leaves, after an immersion in water at 150o (65o.5 Cent.), on being placed in the same strong solution, did not undergo aggregation, the cells becoming filled with brownish, pulpy, or muddy matter. With leaves subjected to temperatures between these two extremes of 120o and 150o Fahr. (48o.8 and 65o.5 Cent.), there were gradations in the completeness of the process; the former temperature not preventing aggregation from the subsequent action of carbonate of ammonia, the latter quite stopping it. Thus, leaves immersed in water, heated to 130o (54o.4 Cent.), and then in the solution, formed perfectly defined spheres, but these were decidedly smaller than in ordinary cases. With other leaves heated to 140o (60 °Cent.), the spheres were extremely small, yet well defined, but many of the cells contained, in addition, some brownish pulpy matter. In two cases of leaves heated to 145o (62o.7 Cent.), a few tentacles could be found with some of their cells containing a few minute spheres; whilst the other cells and other whole tentacles included only the brownish, disintegrated or pulpy matter.

The fluid within the cells of the tentacles must be in an oxygenated condition, in order that the force or influence which induces aggregation should be transmitted at the proper rate from cell to cell. A plant, with its roots in water, was left for 45 m. in a vessel containing 122 oz. of carbonic acid. A leaf from this plant, and, for comparison, one from a fresh plant, were both immersed for 1 hr. in a rather strong solution of carbonate of ammonia. They were then compared, and certainly there was much less aggregation in the leaf which had been subjected to the carbonic acid than in the other. Another plant was exposed in the same vessel for 2 hrs. to carbonic acid, and one of its leaves was then placed in a solution of one part of the carbonate to 437 of water; the glands were instantly blackened, showing that they had absorbed, and that their contents were aggregated; but in the cells close beneath the glands there was no aggregation even after an interval of 3 hrs. After 4 hrs. 15 m. a few minute spheres of protoplasm were formed in these cells, but even after 5 hrs. 30 m. the aggregation did not extend down the pedicels for a length equal to that of the glands. After numberless trials with fresh leaves immersed in a solution of this strength, I have never seen the aggregating action transmitted at nearly so slow a rate. Another plant was left for 2 hrs. in carbonic acid, but was then exposed for 20 m. to the open air, during which time the leaves, being of a red colour, would have absorbed some oxygen. One of them, as well as a fresh leaf for comparison, were now immersed in the same solution as before. The former were looked at repeatedly, and after an interval of 65 m. a few spheres of protoplasm were first observed in the cells close beneath the glands, but only in two or three of the longer tentacles. After 3 hrs. the aggregation had travelled down the pedicels of a few of the tentacles for a length equal to that of the glands. On the other hand, in the fresh leaf similarly treated, aggregation was plain in many of the tentacles after 15 m.; after 65 m. it had extended down the pedicels for four, five, or more times the lengths of the glands; and after 3 hrs. the cells of all the tentacles were affected for one-third or one-half of their entire lengths. Hence there can be no doubt that the exposure of leaves to carbonic acid either stops for a time the process of aggregation, or checks the transmission of the proper influence when the glands are subsequently excited by carbonate of ammonia; and this substance acts more promptly and energetically than any other. It is known that the protoplasm of plants exhibits its spontaneous movements only as long as it is in an oxygenated condition; and so it is with the white corpuscles of the blood, only as long as they receive oxygen from the red corpuscles;[9 - With respect to plants, Sachs, 'Trait de Bot.' 3rd edit., 1874, p. 864. On blood corpuscles, see 'Quarterly Journal of Microscopical Science,' April 1874, p. 185.'] but the cases above given are somewhat different, as they relate to the delay in the generation or aggregation of the masses of protoplasm by the exclusion of oxygen.

Summary and Concluding Remarks. – The process of aggregation is independent of the inflection of the tentacles and of increased secretion from the glands. It commences within the glands, whether these have been directly excited, or indirectly by a stimulus received from other glands. In both cases the process is transmitted from cell to cell down the whole length of the tentacles, being arrested for a short time at each transverse partition. With pale-coloured leaves the first change which is perceptible, but only under a high power, is the appearance of the finest granules in the fluid within the cells, making it slightly cloudy. These granules soon aggregate into small globular masses. I have seen a cloud of this kind appear in 10 s. after a drop of a solution of carbonate of ammonia had been given to a gland. With dark red leaves the first visible change often is the conversion of the outer layer of the fluid within the cells into bag-like masses. The aggregated masses, however they may have been developed, incessantly change their forms and positions. They are not filled with fluid, but are solid to their centres. Ultimately the colourless granules in the protoplasm which flows round the walls coalesce with the central spheres or masses; but there is still a current of limpid fluid flowing within the cells. As soon as the tentacles fully re-expand, the aggregated masses are redissolved, and the cells become filled with homogeneous purple fluid, as they were at first. The process of redissolution commences at the bases of the tentacles, thence proceeding upwards to the glands; and, therefore, in a reversed direction to that of aggregation.

Aggregation is excited by the most diversified causes, – by the glands being several times touched, – by the pressure of particles of any kind, and as these are supported by the dense secretion, they can hardly press on the glands with the weight of a millionth of a grain,[10 - According to Hofmeister (as quoted by Sachs, 'Trait de Bot.' 1874, p. 958), very slight pressure on the cell-membrane arrests immediately the movements of the protoplasm, and even determines its separation from the walls. But the process of aggregation is a different phenomenon, as it relates to the contents of the cells, and only secondarily to the layer of protoplasm which flows along the walls; though no doubt the effects of pressure or of a touch on the outside must be transmitted through this layer.]– by the tentacles being cut off close beneath the glands, – by the glands absorbing various fluids or matter dissolved out of certain bodies, – by exosmose, – and by a certain degree of heat. On the other hand, a temperature of about 150o Fahr. (65o.5 Cent.) does not excite aggregation; nor does the sudden crushing of a gland. If a cell is ruptured, neither the exuded matter nor that which still remains within the cell undergoes aggregation when carbonate of ammonia is added. A very strong solution of this salt and rather large bits of raw meat prevent the aggregated masses being well developed. From these facts we may conclude that the protoplasmic fluid within a cell does not become aggregated unless it be in a living state, and only imperfectly if the cell has been injured. We have also seen that the fluid must be in an oxygenated state, in order that the process of aggregation should travel from cell to cell at the proper rate.

Various nitrogenous organic fluids and salts of ammonia induce aggregation, but in different degrees and at very different rates. Carbonate of ammonia is the most powerful of all known substances; the absorption of 1/134400 of a grain (.000482 mg.) by a gland suffices to cause all the cells of the same tentacle to become aggregated. The first effect of the carbonate and of certain other salts of ammonia, as well as of some other fluids, is the darkening or blackening of the glands. This follows even from long immersion in cold distilled water. It apparently depends in chief part on the strong aggregation of their cell-contents, which thus become opaque, and do not reflect light. Some other fluids render the glands of a brighter red; whilst certain acids, though much diluted, the poison of the cobra-snake, &c., make the glands perfectly white and opaque; and this seems to depend on the coagulation of their contents without any aggregation. Nevertheless, before being thus affected, they are able, at least in some cases, to excite aggregation in their own tentacles.

That the central glands, if irritated, send centrifugally some influence to the exterior glands, causing them to send back a centripetal influence inducing aggregation, is perhaps the most interesting fact given in this chapter. But the whole process of aggregation is in itself a striking phenomenon. Whenever the peripheral extremity of a nerve is touched or pressed, and a sensation is felt, it is believed that an invisible molecular change is sent from one end of the nerve to the other; but when a gland of Drosera is repeatedly touched or gently pressed, we can actually see a molecular change proceeding from the gland down the tentacle; though this change is probably of a very different nature from that in a nerve. Finally, as so many and such widely different causes excite aggregation, it would appear that the living matter within the gland-cells is in so unstable a condition that almost any disturbance suffices to change its molecular nature, as in the case of certain chemical compounds. And this change in the glands, whether excited directly, or indirectly by a stimulus received from other glands, is transmitted from cell to cell, causing granules of protoplasm either to be actually generated in the previously limpid fluid or to coalesce and thus to become visible.

Supplementary Observations on the Process of Aggregation in the Roots of Plants.

It will hereafter be seen that a weak solution of the carbonate of ammonia induces aggregation in the cells of the roots of Drosera; and this led me to make a few trials on the roots of other plants. I dug up in the latter part of October the first weed which I met with, viz. Euphorbia peplus, being care- ful not to injure the roots; these were washed and placed in a little solution of one part of carbonate of ammonia to 146 of water. In less than one minute I saw a cloud travelling from cell to cell up the roots, with wonderful rapidity. After from 8 m. to 9 m. the fine granules, which caused this cloudy appearance, became aggregated towards the extremities of the roots into quadrangular masses of brown matter; and some of these soon changed their forms and became spherical. Some of the cells, however, remained unaffected. I repeated the experiment with another plant of the same species, but before I could get the specimen into focus under the microscope, clouds of granules and quadrangular masses of reddish and brown matter were formed, and had run far up all the roots. A fresh root was now left for 18 hrs. in a drachm of a solution of one part of the carbonate to 437 of water, so that it received 1/8 of a grain, or 2.024 mg. When examined, the cells of all the roots throughout their whole length contained aggregated masses of reddish and brown matter. Before making these experiments, several roots were closely examined, and not a trace of the cloudy appearance or of the granular masses could be seen in any of them. Roots were also immersed for 35 m. in a solution of one part of carbonate of potash to 218 of water; but this salt produced no effect.

I may here add that thin slices of the stem of the Euphorbia were placed in the same solution, and the cells which were green instantly became cloudy, whilst others which were before colourless were clouded with brown, owing to the formation of numerous granules of this tint. I have also seen with various kinds of leaves, left for some time in a solution of carbonate of ammonia, that the grains of chlorophyll ran together and partially coalesced; and this seems to be a form of aggregation.

Plants of duck-weed (Lemna) were left for between 30 m. and 45 m. in a solution of one part of this same salt to 146 of water, and three of their roots were then examined. In two of them, all the cells which had previously contained only limpid fluid now included little green spheres. After from 1 1/2 hr. to 2 hrs. similar spheres appeared in the cells on the borders of the leaves; but whether the ammonia had travelled up the roots or had been directly absorbed by the leaves, I cannot say. As one species, Lemna arrhiza, produces no roots, the latter alternative is perhaps the most probable. After about 2 1/2 hrs. some of the little green spheres in the roots were broken up into small granules which exhibited Brownian movements. Some duck-weed was also left for 1 hr. 30 m. in a solution of one part of carbonate of potash to 218 of water, and no decided change could be perceived in the cells of the roots; but when these same roots were placed for 25 m. in a solution of carbonate of ammonia of the same strength, little green spheres were formed.

A green marine alga was left for some time in this same solution, but was very doubtfully affected. On the other hand, a red marine alga, with finely pinnated fronds, was strongly affected. The contents of the cells aggregated themselves into broken rings, still of a red colour, which very slowly and slightly changed their shapes, and the central spaces within these rings became cloudy with red granular matter. The facts here given (whether they are new, I know not) indicate that interesting results would perhaps be gained by observing the action of various saline solutions and other fluids on the roots of plants.

CHAPTER IV

THE EFFECTS OF HEAT ON THE LEAVES

Nature of the experiments – Effects of boiling water – Warm water causes rapid inflection – Water at a higher temperature does not cause immediate inflection, but does not kill the leaves, as shown by their subsequent re-expansion and by the aggregation of the protoplasm – A still higher temperature kills the leaves and coagulates the albuminous contents of the glands.

IN my observations on Drosera rotundifolia, the leaves seemed to be more quickly inflected over animal substances, and to remain inflected for a longer period during very warm than during cold weather. I wished, therefore, to ascertain whether heat alone would induce inflection, and what temperature was the most efficient. Another interesting point presented itself, namely, at what degree life was extinguished; for Drosera offers unusual facilities in this respect, not in the loss of the power of inflection, but in that of subsequent re-expansion, and more especially in the failure of the protoplasm to become aggregated, when the leaves after being heated are immersed in a solution of carbonate of ammonia.[11 - When my experiments on the effects of heat were made, I was not aware that the subject had been carefully investigated by several observers. For instance, Sachs is convinced ('Trait de Botanique,' 1874, pp. 772, 854) that the most different kinds of plants all perish if kept for 10 m. in water at 45o to 46 °Cent., or 113o to 115o Fahr.; and he concludes that the protoplasm within their cells always coagulates, if in a damp condition, at a temperature of between 50oand 60 °Cent., or 122o to 140o Fahr. Max Schultze and Khne (as quoted by Dr. Bastian in 'Contemp. Review,' 1874, p. 528) "found that the protoplasm of plant-cells, with which they experimented, was always killed and [] altered by a very brief exposure to a temperature of 118 1/2o Fahr. as a maximum." As my results are deduced from special phenomena, namely, the subsequent aggregation of the protoplasm and the re-expansion of the tentacles, they seem to me worth giving. We shall find that Drosera resists heat somewhat better than most other plants. That there should be considerable differences in this respect is not surprising, considering that some low vegetable organisms grow in hot springs – cases of which have been collected by Prof. Wyman ('American Journal of Science,' vol. xliv. 1867). Thus, Dr. Hooker found Confervae in water at 168o Fahr.; Humboldt, at 185o Fahr.; and Descloizeaux, at 208o Fahr.)]

[My experiments were tried in the following manner. Leaves were cut off, and this does not in the least interfere with their powers; for instance, three cut off leaves, with bits of meat placed on them, were kept in a damp atmosphere, and after 23 hrs. closely embraced the meat both with their tentacles and blades; and the protoplasm within their cells was well aggregated. Three ounces of doubly distilled water was heated in a porcelain vessel, with a delicate thermometer having a long bulb obliquely suspended in it. The water was gradually raised to the required temperature by a spirit-lamp moved about under the vessel; and in all cases the leaves were continually waved for some minutes close to the bulb. They were then placed in cold water, or in a solution of carbonate of ammonia. In other cases they were left in the water, which had been raised to a certain temperature, until it cooled. Again in other cases the leaves were suddenly plunged into water of a certain temperature, and kept there for a specified time. Considering that the tentacles are extremely delicate, and that their coats are very thin, it seems scarcely possible that the fluid contents of their cells should not have been heated to within a degree or two of the temperature of the surrounding water. Any further precautions would, I think, have been superfluous, as the leaves from age or constitutional causes differ slightly in their sensitiveness to heat.

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 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 110o Fahr. (43o.3 Cent.); a leaf being taken out as soon as the temperature rose to 80o (26o.6 Cent.), another at 85o, another at 90o, 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 100o (37o.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 105o to 106o (40o.5 to 41o.1 Cent.), was very moderately inflected in 6 m. A fourth leaf, in water at 110o (43o.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 115o-116o (46o.1 to 46o.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 120o (48o.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 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 (43o.3 Cent.) which was raised to 120o (48o.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 120o, 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 125o (51o.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 120o to 125o (48o.8 to 51o.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 130o (54o.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 130o (54o.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 distinctly seen in a small mass of protoplasm in one of the cells of a tentacle.[12 - Sachs states ('Trait de Botanique,' 1874, p. 855) that the movements of the protoplasm in the hairs of a Cucurbita ceased after they were exposed for 1 m. in water to a temperature of 47o to 48 °Cent., or 117o to 119o Fahr.] 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 3. – A similar experiment to the last, with exactly the same results.

Experiment 4. – A fine leaf was placed in water at 100o (37o.7 Cent.), which was then raised to 145o (62o.7 Cent.). Soon after immersion, there was, as might have been expected, strong inflection. The leaf was now removed and left in cold water; but from having been exposed to so high a temperature, it never re-expanded.

Experiment 5. – Leaf immersed at 130o (54o.4 Cent.), and the water raised to 145o (62o.7 Cent.), there was no immediate inflection; it was then placed in cold water, and after 1 hr. 20 m. some of the tentacles on one side became inflected. This leaf was now placed in the strong solution, and in 40 m. all the submarginal tentacles were well inflected, and the glands blackened. After an additional interval of 2 hrs. 45 m. all the tentacles, except eight or ten, were closely inflected, with their cells exhibiting a slight degree of aggregation; but the spheres of protoplasm were very small, and the cells of the exterior tentacles contained some pulpy or disintegrated brownish matter.

Experiments 6 and 7. – Two leaves were plunged in water at 135o (57o.2 Cent.) which was raised to 145o (62o.7 Cent.); neither became inflected. One of these, however, after having been left for 31 m. in cold water, exhibited some slight inflection, which increased after an additional interval of 1 hr. 45 m., until all the tentacles, except sixteen or seventeen, were more or less inflected; but the leaf was so much injured that it never re-expanded. The other leaf, after having been left for half an hour in cold water, was put into the strong solution, but no inflection ensued; the glands, however, were blackened, and in some cells there was a little aggregation, the spheres of protoplasm being extremely small; in other cells, especially in the exterior tentacles, there was much greenish-brown pulpy matter.

Experiment 8. – A leaf was plunged and waved about for a few minutes in water at 140o (60 °Cent.), and was then left for half an hour in cold water, but there was no inflection. It was now placed in the strong solution, and after 2 hrs. 30 m. the inner submarginal tentacles were well inflected, with their glands blackened, and some imperfect aggregation in the cells of the pedicels. Three or four of the glands were spotted with the white porcelain-like structure, like that produced by boiling water. I have seen this result in no other instance after an immersion of only a few minutes in water at so low a temperature as 140o, and in only one leaf out of four, after a similar immersion at a temperature of 145o Fahr. On the other hand, with two leaves, one placed in water at 145o (62o.7 Cent.), and the other in water at 140o (60 °Cent.), both being left therein until the water cooled, the glands of both became white and porcelain-like. So that the duration of the immersion is an important element in the result.

Experiment 9. – A leaf was placed in water at 140o (60 °Cent.), which was raised to 150o(65o.5 Cent.); there was no inflection; on the contrary, the outer tentacles were somewhat bowed backwards. The glands became like porcelain, but some of them were a little mottled with purple. The bases of the glands were often more affected than their summits. This leaf having been left in the strong solution did not undergo any inflection or aggregation.

Experiment 10. – A leaf was plunged in water at 150o to 150 1/2o (65o.5 Cent.); it became somewhat flaccid, with the outer tentacles slightly reflexed, and the inner ones a little bent inwards, but only towards their tips; and this latter fact shows that the movement was not one of true inflection, as the basal part alone normally bends. The tentacles were as usual rendered of a very bright red, with the glands almost white like porcelain, yet tinged with pink. The leaf having been placed in the strong solution, the cell-contents of the tentacles became of a muddy-brown, with no trace of aggregation.

Experiment 11. – A leaf was immersed in water at 145o (62o.7 Cent.), which was raised to 156o (68o.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 130o (54o.4 Cent.) never causes the immediate inflection of the tentacles, though a temperature from 120o to 125o (48o.8 to 51o.6 Cent.) quickly produces this effect. But the leaves are paralysed only for a time by a temperature of 130o, 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 power of movement due to heat is called by Sachs[13 - 'Trait de Bot.' 1874, p. 1034.] 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 120o-122o Fahr., or 49o to 50 °Cent. It deserves notice that the leaves of Drosera, after being immersed in water at 130o Fahr., are excited into movement by a solution of the carbonate so strong that it would paralyse ordinary leaves and cause no inflection.

The exposure of the leaves for a few minutes even to a temperature of 145o Fahr. (62o.7 Cent.) does not always kill them; as when afterwards left in cold water, or in a strong solution of carbonate of ammonia, they generally, though not always, become inflected; and the protoplasm within their cells undergoes aggregation, though the spheres thus formed are extremely small, with many of the cells partly filled with brownish muddy matter. In two instances, when leaves were immersed in water, at a lower temperature than 130o (54o.4 Cent.), which was then raised to 145o (62o.7 Cent.), they became during the earlier period of immersion inflected, but on being afterwards left in cold water were incapable of re-expansion. Exposure for a few minutes to a temperature of 145o sometimes causes some few of the more sensitive glands to be speckled with the porcelain-like appearance; and on one occasion this occurred at a temperature of 140o (60 °Cent.). On another occasion, when a leaf was placed in water at this temperature of only 140o, and left therein till the water cooled, every gland became like porcelain. Exposure for a few minutes to a temperature of 150o (65o.5 Cent.) generally produces this effect, yet many glands retain a pinkish colour, and many present a speckled appearance. This high temperature never causes true inflection; on the contrary, the tentacles commonly become reflexed, though to a less degree than when immersed in boiling water; and this apparently is due to their passive power of elasticity. After exposure to a temperature of 150o Fahr., the protoplasm, if subsequently subjected to carbonate of ammonia, instead of undergoing aggregation, is converted into disintegrated or pulpy discoloured matter. In short, the leaves are generally killed by this degree of heat; but owing to differences of age or constitution, they vary somewhat in this respect. In one anomalous case, four out of the many glands on a leaf, which had been immersed in water raised to 156o (68o.8 Cent.), escaped being rendered porcellanous;[14 - As the opacity and porcelain-like appearance of the glands is probably due to the coagulation of the albumen, I may add, on the authority of Dr. Burdon Sanderson, that albumen coagulates at about 155o, but, in presence of acids, the temperature of coagulation is lower. The leaves of Drosera contain an acid, and perhaps a difference in the amount contained may account for the slight differences in the results above recorded.It appears that cold-blooded animals are, as might have been expected, far more sensitive to an increase of temperature than is Drosera. Thus, as I hear from Dr. Burdon Sanderson, a frog begins to be distressed in water at a temperature of only 85o Fahr. At 95o the muscles become rigid, and the animal dies in a stiffened condition.] and the protoplasm in the cells close beneath these glands underwent some slight, though imperfect, degree of aggregation.

Finally, it is a remarkable fact that the leaves of Drosera rotundifolia, which flourishes on bleak upland moors throughout Great Britain, and exists (Hooker) within the Arctic Circle, should be able to withstand for even a short time immersion in water heated to a temperature of 145o.

It may be worth adding that immersion in cold water does not cause any inflection: I suddenly placed four leaves, taken from plants which had been kept for several days at a high temperature, generally about 75o Fahr. (23o.8 Cent.), in water at 45o (7o.2 Cent.), but they were hardly at all affected; not so much as some other leaves from the same plants, which were at the same time immersed in water at 75o; for these became in a slight degree inflected.

CHAPTER V

THE EFFECTS OF NON-NITROGENOUS AND NITROGENOUS ORGANIC FLUIDS ON THE LEAVES

Non-nitrogenous fluids – Solutions of gum arabic – Sugar – Starch – Diluted alcohol – Olive oil – Infusion and decoction of tea – Nitrogenous fluids – Milk – Urine – Liquid albumen – Infusion of raw meat – Impure mucus – Saliva – Solution of isinglass – Difference in the action of these two sets of fluids – Decoction of green peas – Decoction and infusion of cabbage – Decoction of grass leaves.

WHEN, in 1860, I first observed Drosera, and was led to believe that the leaves absorbed nutritious matter from the insects which they captured, it seemed to me a good plan to make some preliminary trials with a few common fluids, containing and not containing nitrogenous matter; and the results are worth giving.

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 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 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 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.