Aggregation, which was insisted upon in our remarks on the Sundews, may be seen to take place very quickly in the glands of the Dionæa, after contact with nitrogenous subjects, every cell having its contents aggregated, in a beautiful manner, into dark, or pale purple, or colourless, globose masses of protoplasm. The function of the little stellate projections, with eight radiating arms, not having been demonstrated can only be conjectured.
From these details of the structure of the leaves we are enabled to correlate them with their movements. When an insect touches one of the sentinel filaments, on an expanded leaf, the irritation is at once communicated, and the lobes close together, with the captured insect enclosed between them, its struggles, in so far as they touch the filaments, only serving to accelerate the closing. The interlocking marginal spines prevent any escape, except in the case of very minute insects. Contact with the glands causes them to absorb, and then the secretion of the acid-mucilaginous fluid commences, and proceeds as long as any material is left to stimulate the action of the glands. Under this treatment the insect becomes dissolved, as far as it is capable of dissolution, and is assimilated by the leaf, this action causing aggregation of the protoplasm in the cells of the glands. All these steps in the process have been determined, by means of careful experiment, which we have not deemed it necessary to recount. That the captured insects were in some way made subservient to the nourishment of the plant was conjectured from the first. Dr. Curtis found them enveloped in a fluid of mucilaginous consistence, which seemed to act as a solvent, the insects being more or less consumed by it. This was verified, and the digestive character of the liquid well-nigh demonstrated some years ago by Mr. Canby, of Wilmington, who, upon a visit to North Carolina, and afterwards at his own home, followed up Dr. Curtis's suggestions with some capital observations and experiments, which were published in 1868, although they did not seem to have attracted the attention which they deserved.
The points which Mr. Canby made out are, that this fluid is always poured out around the captured insect in due time; "if the leaf is in good condition and the prey suitable"; that it comes from the leaf itself, and not from the decomposing insect (for, when the trap caught a plum curculio, the fluid was poured out while he was still alive, though very weak, and endeavouring, ineffectually, to eat his way out); that bits of raw beef, although sometimes rejected, after awhile were generally acted upon in the same manner—i.e., closed down upon tightly, slavered with liquid, dissolved mainly, and absorbed; so that, in fine, the fluid may well be said to be analogous to the gastric juice of animals, dissolving the prey, and rendering it fit for absorption by the leaf. Many leaves remain inactive, or slowly die away, after one meal; others re-open for a second, and perhaps a third capture, and are at least capable of digesting a second meal.
When the lobes close together from irritation by inanimate substances, or touching, the inner surface remains concave until the lobes expand again; but, if an insect or a piece of meat is enclosed, each lobe gradually flattens, and that apparently with considerable force, thus pressing the enclosed object firmly against the secreting glands. When no object is caught the lobes soon expand again in from twenty-four to thirty-two hours, and even before fully expanded they are ready to act again, so that a leaf has been found to close and re-open alternately, but unsuccessfully, for four times during six days. Closing in this manner, from irritation by inanimate objects, does not, therefore, prevent the lobes from acting vigorously several times, until some suitable prey is caught, and then they remain closed for an indefinite period, or, if they open again at all, remain torpid and insensible for a considerable period. "In four instances leaves after catching insects never reopened, but began to wither, remaining closed—in one case for fifteen days over a fly; in a second, for twenty-four days, though the fly was small; in a third, for twenty-four days over a woodlouse; and in a fourth, for thirty-five days over a crane fly. In two instances, in which very small insects had been naturally caught, the leaf opened as quickly as if nothing had been caught." Dr. Canby says that the leaves remain closed for a longer period over insects than over meat.
In all cases where the leaves re-opened, after having remained a long time closed over insects, or meat, or similar substances, they were so torpid during many succeeding days that touching the sensitive filaments was followed by no response whatever. In their native country, where the plants grow with vigour, they appear to be more capable of repeating their operations than when transplanted here. Mrs. Mary Treat, who cultivated and watched these plants in New Jersey, which is not so far removed from their natural habitat, has stated that "several leaves caught successively three insects each, but most of them were not able to digest the third fly, but died in the attempt. Five leaves, however, digested each three flies, and closed over the fourth, but died soon after the fourth capture. Many leaves did not digest even one large insect." The capacity for digestion is not, therefore, unlimited in the Dionæa, more than it is in higher organisms. Apoplexy from over-feeding might even here be a reasonable verdict.
As to the kind of insects which are captured by this plant we have the record of the contents of fourteen leaves, sent, with their prey, from their native country. Four of these had caught rather small insects, of which three were ants, and the fourth a small fly, but the other ten had caught large insects, of which eight were beetles (two chrysomelas, five elaters, and a curculio), a thick broad spider, and a scolopendra. Of the whole there was only one flying insect, or, rather, usually and readily progressing by flight. This hardly seems to harmonise with the statement by Dr. Canby that "as a general thing beetles and insects of that kind, though always killed, seem to be too hard shelled to serve as food, and after a short time are rejected."
We may here allude to that phase of the subject which was so successfully investigated and illustrated by Dr. Burdon Sanderson, and which amounted to establishing the identity of the phenomena of muscular contraction and contractility in Dionæa. The property of contracting when irritated, which enables the Dionæa to catch insects, was the special phase of the subject to which Dr. Burdon Sanderson directed his attention. In this phenomenon, he says, "we have to do not merely with contractility but with irrito-contractility. The fact that the property requires two words to express it implies that there are two things to express, viz. (1) that contraction takes place, and (2) that it takes place in answer to irritation. As this is the case, not only here, but in all other instances of animal or vegetable active motion, we recognise in physiology these two properties as fundamental—irritability or excitability, and contractility, the former designating the property, possessed by every living structure whatever, of being excited into action (i.e. of having its stored-up force discharged) by some motion or disturbance from outside; the latter, that kind of discharge or action which results in change of form, and usually declares itself in the doing of mechanical work. This property of excitability, which, let me repeat, is common to all living structures, is, as we have seen, comparable in its simplest manifestations to that possessed by many chemical compounds (of explosiveness) and many mechanical contrivances (of going off or discharging when meddled with, as in the case of the rat-trap already referred to).
"In physiology, as in the other sciences of observation, the process of investigation is throughout one of comparison. Not only do we proceed, from first to last, from the known towards the unknown, but what we speak of as our knowledge, or understanding, of any new fact consists simply in our being able to bring it into relation with other facts previously well ascertained and familiar, just as the geographer determines the position of a new locality by ascertaining its topographical relation to others already on the chart.
"The comparison we have now to make is between the contractility displayed by the leaf of Dionæa, and the contractility of muscle. I choose muscle as the standard of comparison, because it is best known, and has been investigated by the best physicists of our time, and because its properties are easily illustrated and understood. I shall be able to show that the resemblance between the contraction of muscle and that of the leaf is so wonderfully complete, that the further we pursue the inquiry the more striking does it appear. Whether we bring the microscope to bear on the structural changes which accompany contraction, or employ the still more delicate instruments of research, which you have before you this evening, in order to determine, and measure, the electrical changes which take place in connexion with it, we find that the two processes correspond in every essential particular so closely, that we can have no doubt of their identity.
"Muscle, like every other living tissue, is the seat, so long as it lives, of chemical changes, which if the tissue is mature, consist entirely in the disintegration of chemical compounds and the dissipation of the force stored up in these compounds, in the form of heat or some other kind of motion. This happens when the muscle is at rest, but much more actively when it is contracting, in which condition it not only produces more heat than it produces at other times, but also may do—and under ordinary circumstances does—mechanical work; these effects of contraction of muscle are, of course, dependent in quantity on the chemical disintegration which goes on in its interior.
"Again, muscle, so long as it is in the living state, is electromotive. This property it probably possesses in common with other living tissues, for it is very likely that every vital act is connected with electrical change in the living part. But in muscle, as well as other irritable and contractile tissues in animals, the manifestation of electromotive force is inseparably connected with the special function of the tissue i.e., with contraction, the connexion being of such a nature, that the electromotive force expresses, not the work actually done at any given moment, but the capacity for work. Thus, so long as the muscle lives, its electromotive force is found to be on the whole proportioned to its vigour. As it gradually loses its vitality, its power of contracting and its electromotive force disappear pari passu. When it contracts, the manifestation of electromotive force diminishes in proportion to the degree of contraction. But it is to be borne in mind that, although, when the muscle or the leaf contracts, electromotive force disappears and work is done, there is no reason for supposing that there is any conversion of the one effect into the other, or that the source of the force exercised by the organ in contracting is electrical."
Dr. Burdon Sanderson then proceeded by a series of experiments to demonstrate the correspondence between the electrical phenomena which accompany muscular contraction and those which are associated with the closing of the Dionæa leaf.
With this brief and rapid summary of the main features relating to the carnivorous propensities of the Venus's Fly-trap, we may casually refer to a few other plants belonging to the same natural order as the Sundews and Dionæa, which possess similar propensities, but to a less interesting degree, or do not differ greatly from the two preceding types.