It has been demonstrated, by experiment, that the secretion of the glands of the sundew completely dissolves albumen, muscle, fibrine, cartilage, the fibrous portion of bone, gelatin, and the casein of milk. That is to say, little cubes of hard-boiled egg, fragments of roast meat, tough cartilage from a leg-bone of mutton, small pieces of the bone of a fowl, and of a mutton-chop bone, the latter being so softened that it might be penetrated by a blunt needle, or compressed, and these became dissolved as they would have been in the stomach of some of the higher animals.
It is only necessary to cite one experiment which was performed on a most unpromising substance under somewhat unfavourable conditions. "Three cubes of white translucent, extremely tough cartilage were cut from the end of a slightly-roasted leg-bone of a sheep. These were placed on three leaves, borne by poor small plants in my greenhouse during November; and it seemed in the highest degree improbable that so hard a substance would be digested under such unfavourable circumstances. Nevertheless, after forty-eight hours, the cubes were largely dissolved, and converted into minute spheres, surrounded by transparent, very acid fluid. Two of these spheres were completely softened to their centres, whilst the third still contained a very small irregularly-shaped core of solid cartilage. Their surfaces were seen under the microscope to be curiously marked by prominent ridges, showing that the cartilage had been unequally corroded by the secretion. I need hardly say that cubes of the same cartilage, kept in water for the same length of time, were not in the least affected."
The fact, therefore, is clearly established, that the secretion from the glands of the Sundew, under certain conditions of stimulation, is capable of dissolving animal substances, in precisely the same manner as they are acted upon during the process of digestion in the stomach of animals. It remains to be seen what evidence there is in support of the absorption, and assimilation, of the substances so digested. Here it would be essential to show, in the first instance, that the glands in question possess the power of absorption at all. Because if it can be demonstrated that they are capable of absorbing fluids, especially nitrogenous fluids, it would be easy to believe that no exception would be made to the exclusion of dissolved animal substances.
All the experiments made in this direction are exceedingly interesting and instructive, some of them truly marvellous in their results. It would be somewhat tedious to narrate them in detail, in a popular exposition of the reasons why certain plants have been called "carnivorous plants," but it will be necessary to allude to one or two. Solutions of certain chemical substances, called salts of ammonia, were applied to the leaves of living plants. Some of these quickly discoloured the glands, but all caused the characteristic inflection of the tentacles. Yet these salts were applied in a very diluted state, for less than one-millionth part of a grain, absorbed by a gland of one of the exterior tentacles, was sufficient to cause it to bend. In order that some idea might be formed of what a million means, the following illustration is given in a foot-note to Mr. Darwin's book. "Take a narrow strip of paper, eighty-three feet four inches in length, and stretch it along the wall of a large hall; then mark off at one end the tenth of an inch. This tenth will represent a hundred, and the entire strip a million." The experiments alluded to were performed in three ways. Small drops were placed on the disc of the leaves; very minute drops were gently placed on one or more of the exterior tentacles; and whole leaves were cut off and immersed in the solutions. In all ways the results harmonised. How exceedingly sensitive the leaves were to some of these solutions may be inferred from their great dilution. As an illustration, it was stated that five thousand fluid ounces would more than fill a thirty-one gallon cask, and that to this large body of water one grain of the salt was added; only half a drachm, or thirty minims, of the solution being poured over a leaf. "Yet this amount sufficed to cause the inflection of almost every tentacle, and often of the blade of the leaf."
The solution of many salts, acids, and alkaloids, were tried, and produced in some instances unexpected results, inasmuch as some substances which are wholly harmless to animals were poisonous to the plant, and others which are poisonous in their effects upon animals were almost inert upon the Sundew. Abundant evidence was supplied that the fluids must have been absorbed by the glands, and their influence transmitted to other tentacles which were not touched. In cases of poisoning, for instance, it must be conceded that the deleterious substance was absorbed, for the parts became blackened, and all the phenomena of poisoning were exhibited. It is but fair to conclude that, if deleterious substances actually become absorbed, the result of absorption being plainly traced, that also other substances may be absorbed, which would either be neutral or beneficial to the plant, but which cannot so easily be traced in their course.
There is another remarkable phenomenon in which the tentacles perform a conspicuous part, which must be briefly alluded to, as affording evidence of the great changes which take place in the internal organism of these plants under excitement. If a resting tentacle is examined, selecting one which has not been excited or inflected, the cells of the pedicel will be seen to present a completely uniform appearance, filled with a purple fluid, retaining throughout one uniform character, and a thin layer of uncoloured circulating fluid, passing along the walls of the cell. These cells, with a diffused colour, impart to the pedicel a continuous purple tint. But if a tentacle is examined after it has been excited, from whatever cause, the cells will present quite a changed appearance. Even to the naked eye they will not present the same uniform, even, purple tint, but seem to be speckled, mottled, or variegated. Examination of these cells, so changed, under the microscope will reveal the cause of the mottling, in the aggregation of the purple matter, which they contain, in variously-shaped masses, suspended in a colourless medium. Each cell, which before was suffused with a uniform tint, now holds a clear and colourless fluid, in which floats an elongated dark-coloured body, formed by the aggregation of the colouring matter. This aggregated body, sometimes of a single mass, sometimes of two, is constantly changing its form, slowly but gradually, like that curious little animal found in stagnant waters called an Amœba. Finally, the movement ceases, the masses again dissolve, and become diffused through the contents of the cell, which again assume a uniform tint and appearance. In this cycle we have a manifestation of a great molecular change which is wrought within the cells of the tentacles, in response to some external irritation. Whatever causes the tentacle to become inflected seems also sufficient to induce this phenomenon of the aggregation of masses within the cells. When the influence of that irritation has passed away, and the tentacle has assumed its original erect position, the contents of the cells assume also their homogeneity, or uniform density.
This aggregation commences in a tentacle at the upper end, in immediate proximity to the gland, and proceeds from above downwards. It accompanies the bending over of the tentacle, but that it neither causes the inflection, nor is caused by it, is evident from the fact that aggregation may take place when there is no inflection of the tentacle. Some acids will produce a rapid inflection but no aggregation. Hence, then, this aggregation is neither a cause nor a consequence of inflection. Whatever its cause may be, it appears to be invariably accompanied by an increased secretion of the glands, and the dispersion of the masses, in like manner, indicates a diminution in the amount of viscid matter secreted by the glands. The shorter central tentacles, which have a green pedicel, exhibit the same phenomenon, with the exception that the aggregated masses partake of the green colour of the cells. The colour of the aggregated masses being of course dependent upon the colour of the contents of the cells.
The experiments on the Sundews have been for the most part conducted with the little round-leaved Sundew, but the other two English species have likewise been examined, and found to correspond in all essentials with its fellow species. This has been done both in England and America. A species with very long slender leaves, which grows abundantly in New Jersey, has been tested in a similar manner. Mrs. Mary Treat writes: "I found it in full bloom, and growing as thick as it could well stand, on either side of an extensive cranberry plantation. This charming plant with its pretty pink blossoms, together with the dewlike substance exuding from the glands (the glands surmount the bristles or hairs which cover the long thread-like leaves) was one of the most beautiful sights I ever beheld. From former observations I had supposed this plant caught only small insects, but now found I was mistaken; great Asilus flies were held firm prisoners, innumerable moths and butterflies, many of them two inches across, were alike held captive until they died—the bright flowers and brilliant glistening dew luring them on to sure death. But what is the use of this wholesale destruction of insect life? can the plants use them? Upon examination I find that after the death of the larger insects, they fall around the roots of the plants, and so fertilise them, but the smaller flies remain sticking to the leaves."
And again, "At ten o'clock I pinned some living flies half an inch from the leaves, near the apex. In forty minutes the leaves had bent perceptibly toward the flies. At twelve o'clock the leaves had reached the flies, and their legs were entangled among the bristles and held fast. I then removed the flies three quarters of an inch further from the leaves. The leaves still remained bent away from the direction of the light toward the flies, but did not reach them at this distance."
Mr. Darwin also examined this species, which he says, "had thread-like leaves from six to twelve inches in length, with the upper surface convex and the lower flat and slightly channelled. The whole convex surface down to the roots—for there is no distinct footstalk—is covered with short gland-bearing tentacles, those on the margin being the longest and reflexed. Bits of meat placed on the glands of some tentacles caused them to be slightly inflected in twenty minutes, but the plant was not in a vigorous state." Two Australian species have also exhibited the same propensities, so that it is probable that all the Sundews, from whatever part of the world they may come, are equally fly-catchers, as well as our own species continue to be, when found flourishing in countries far remote.
In Mr. Darwin's book all the facts resulting from observation and experiment are brought to bear upon the theory which he advanced, that the power of catching and digesting insects is of advantage to the plants themselves. Some continental botanists have denied that the case is proved. Subsequent to the volume in question Mr. Francis Darwin instituted some experiments with the view of ascertaining what effect the indulgence in carnivorous propensities had upon the Sundew. The plants were isolated and protected. Half the plants, or 91 plants, were not fed, whilst 86 plants were supplied with roast meat, cut into thin slices across the grain, and the fibre torn into fragments exceedingly minute. The first difference observed was that in August the starved plants had only produced 116 flowering stems whilst the fed plants had produced 173. Another difference observed was that the fed plants contained a larger number of healthy leaves than the starved plants, and, finally, it is stated that from these experiments "it would seem that the great advantage accruing to carnivorous plants from a supply of nitrogenous food to the leaves is the power of producing a vastly superior yield of seeds; and," the author adds, "I venture to think that the above experiments prove beyond question that the supply of meat to Drosera is of signal advantage to the plants." Similar experiments in Germany in which the plants were fed with plant-lice instead of meat resulted in a similar conclusion, that numerous and striking advantages accrued to the fed plants.