A leaf studded with sparkling glandular hairs is not in itself very extraordinary, but when we discover that these hairs, or tentacles, can be moved in a particular direction in response to some exciting cause, we have to deal with a phenomenon by no means common in plant life, and we naturally become curious to discover the cause. When any object, living or dead, comes in contact with one of these tentacles it commences to bend over towards the centre of the leaf (fig. 3). The power of responding to irritation, moreover, is not confined to the single tentacle which has been touched, for it possesses the capacity of communicating with the surrounding tentacles, and they also bend over, as if in sympathy with and to assist their companion. The minute fragment of a human hair 1/800th of an inch in length, laid upon a gland, has been shown to be sufficient to excite a tentacle to bend over. Minute particles of glass, chalk, and other inorganic substances, placed on the glands of the outer tentacles, will cause them to bend. So also will small fragments of meat, and minute drops of stimulating fluids. When a tentacle is touched three or four times it will also bend, but not when only touched once or twice, although the sustained pressure of a gnat's foot is sufficient to produce the movement. After remaining bent down for some time the excited tentacles again slowly return to their original erect position. This return is much more speedy when an inorganic body has been the cause of the inflection, than when a small insect, or a fragment of meat, has been the exciting cause. These facts have been proved by numerous experiments, which place them beyond question. First, that the tentacles are sensitive (if we may use that expression) to the sustained pressure of one millionth part of a grain. That they will respond to such pressure, and bend towards the centre of the leaf. That this irritation will also be communicated to neighbouring tentacles, which will bend in the same direction. And that after this operation is performed the inflected tentacles will return to their former position. If we suppose, then, that a minute insect has fallen or alighted upon one, or more, of the outer tentacles, it will in the course of ten seconds be moving towards the centre, whither it will ultimately be carried, whilst the surrounding tentacles will also follow in the bending movement, until all are closed over the captive insect. But, it may be asked, are we assured that the first inward movement of the tentacles will not alarm the insect and cause it to take flight? It might do so if this were not provided against by the viscid secretion with which the glands are covered, and which increases in quantity with the inward movement of the tentacles. This secretion is so tenacious that it may be drawn out in strings, and if once a small insect alights upon it, it struggles in vain to get free. It is, in fact, a kind of birdlime, prepared naturally, and exposed systematically, for the capture of little flies. The club-shaped summit of each tentacle is a manufactory and storehouse for this sticky substance, which is exuded and exposed on the surface. Although these drops glisten and sparkle in the sun, they have another and more important function to perform than only to justify the cognomen of the plant.
Not only does the response of the tentacles to irritation remind us of sensibility in the animal kingdom, but the apparent power of discrimination which the tentacles possess seems surprising. It is an undoubted fact that the power does exist of distinguishing not only between inorganic and organic substances, as between a piece of glass and a piece of boiled egg, but also between a hard-skinned beetle and a soft fly, and even between different kinds of fluids. Mr. Darwin's experiments give abundant evidence of this, and his book, like all other of his works, is a complete cyclopedia of authentic facts. For instance, drops of pure water were tried on thirty or forty leaves, but no effect whatever was produced. Drops of milk were placed on sixteen leaves, and the tentacles of all became greatly inflected. Ten leaves were tried with drops of cold tea, but the tentacles did not respond. Whereas eight were tested with dissolved isinglass, as thick as milk, and all of them recognised it by inflecting the tentacles. Nor was the treatment of solids less remarkable. "Minute flies were placed on the discs of several leaves, and on others balls of paper, bits of moss and quill, of about the same size as the flies, and the latter (the flies) were well embraced in a few hours, whereas, after twenty-five hours, only a very few tentacles were inflected over the other objects. The bits of paper, moss and quill were then removed from these leaves, and bits of raw meat placed on them, and now all the tentacles were soon energetically inflected." 1 Yet another mode of recognition was manifested. Over and over again, in the work from whence the above is quoted, it is demonstrated that the tentacles remained for a much longer period inflected over what we should term digestible substances than over such indigestible things as bits of glass and paper. The inference to be drawn from this fact is that the plant recognised the latter as indigestible, and hence that the tentacles let go their hold and returned to their previous position of expectancy, whilst in the former they remained closed in the act of digestion. It may be remarked here that as the tentacles, whilst becoming inflected, exude a larger drop of secretion than when erect, so in recovering from inflection they become drier, with little or none of the secretion exuded, until after they have again resumed their erect position. By the first action the capture and digestion of the prey has to be provided for; by the last any adhering legs or wings of dead insects are got rid of.
We have demonstrated the perfectibility of our fly-catching plant in all that relates to the securing of its prey, and, within certain limits, to its power of selection. The next question is, "What will he do with it?" and this naturally leads us to investigate its powers of digestion and absorption. If the phenomena exhibited by the plant are analogous to those of animals during digestion, we may fairly conclude that the motive is the same. We have stated the fact, which may be repeated in Mr. Darwin's own language, "that the glands of the disc when irritated transmit some influence to the glands of the exterior tentacles, causing them to secrete more copiously, and the secretion to become acid, as if they had been directly excited by an object placed on them. The gastric juice of animals contains, as is well known, an acid and a ferment, both of which are indispensable for digestion, and so it is with the secretion of Drosera. When the stomach of an animal is mechanically irritated it secretes an acid, and when particles of glass or other such objects were placed on the glands of Drosera, the secretion and that of the surrounding and untouched glands was increased in quantity and became acid." It is well known how easy it is to test the presence of an acid by the application of litmus paper, and this test has been applied to the secretion of the glands of the sundew in innumerable instances. The same author says,—"I have tried, indeed, hundreds of times, the state of the secretion on the discs of leaves which were inflected over various objects, and never failed to find it acid." And this observation has been corroborated by others both in this plant and in the Dionæa. When the leaves have not been excited the viscid secretion is not acid, or but very slightly so, but, after the tentacles have commenced bending over any object, the secretion becomes more or less acidulated.
Another property which this secretion possesses has also been alluded to—namely, its antiseptic quality. It checks the appearance of mould and minute animalcules, and for a time prevents the discolouration and decay of substances over which it has been transfused. This, again, is analogous to the gastric juice of animals, which is known to arrest the putrefaction of substances under its influence. And here we have another singular coincidence, even if nothing more, which must have its weight in determining whether the glands of the Sundew possess the power of digestion. If it can be shown that, in addition to the power of catching insects and holding them, and also of discriminating between digestible and indigestible substances, these leaves secrete a fluid possessing all the attributes of a digestive fluid, dissolving without putrefaction just such substances as an animal would dissolve in its stomach by the ordinary process of digestion, we furnish very strong presumption in favour of their being called "insectivorous."
Although his remarks were illustrative of another plant, we may better quote here the observations of Dr. Burdon Sanderson, as they apply with equal force to the sundew as to the Venus's fly-trap. In his lecture at the Royal Institution, after describing the plant and its mechanism, he referred to its power of digestion. "When," he says, "we call this process digestion we have a definite meaning. We mean that it is of the same nature as that by which we ourselves, and the higher animals in general, convert the food they have swallowed into a form and condition suitable to be absorbed, and thus available for the maintenance of bodily life. We will compare the digestion of Dionæa with that which in man and animals we call digestion proper, the process by which the nitrogenous constituents of food are rendered fit for absorption. This takes place in the stomach. It also is a fermentation, i.e., a chemical change, effected by the agency of a leaven or ferment which is contained in the stomach juice, and can be, like the ferment of saliva, easily separated and prepared. As so separated it is called pepsin. Consequently, having the ferment, we can easily imitate digestion out of the body. For this experiment there are three things necessary (1) That our liquid should contain pepsin; (2) That it should be slightly acid; (3) That it should be kept at the temperature of incubation (about 97° Fahr.). We select for the experiment a substance which, although nutritious and containing nitrogen, is not easily digested—such, for example, as boiled white of egg. In water containing a small percentage of hydrochloric acid, and a trace of pepsin, it is gradually dissolved; but chemical examination of the liquid shows us that it has not been destroyed, but merely transformed into a new substance called peptone, which is afterwards absorbed, i.e., taken into the circulating blood."
"Between this process and the digestion of the Dionæa leaf the resemblance is complete. It digests exactly the same substances in exactly the same way, i.e., it digests the albuminous constituents of the bodies of animals just as we digest them. In both instances it is essential that the body to be digested should be steeped in a liquid, which in Dionæa is secreted by the red glands on the upper surface of the leaf; in the other case by the glands of the mucous membrane. In both the act of secretion is excited by the presence of the substance to be digested. In the leaf, just as in the stomach, the secretion is not poured out unless there is something nutritious in it for it to act upon; and, finally, in both cases the secretion is acid. As regards the stomach we know what the acid is,—it is hydrochloric acid. As regards the leaf we do not know precisely as yet, but Mr. Darwin has been able to arrive at very probable conclusions."