Insectivorous Plants

Of the six genera, Drosera has been incomparably the most successful in the battle for life; and a large part of its success may be attributed to its manner of catching insects. It is a dominant form, for it is believed to include about 100 species, which range in the Old World from the Arctic regions to Southern India, to the Cape of Good Hope, Madagascar, and Australia; and in the New World from Canada to Tierra del Fuego. In this respect it presents a marked contrast with the five other genera, which appear to be failing groups. Dionaea includes only a single species, which is confined to one district in Carolina. The three varieties or closely allied species of Aldrovanda, like so many water-plants, have a wide range from Central Europe to Bengal and Australia. Drosophyllum includes only one species, limited to Portugal and Morocco. Roridula and Byblis each have (as I Bentham and Hooker, 'Genera Plantarum.' Australia is the metropolis of the genus, forty-one species having been described from this country, as Prof. Oliver informs me.

hear from Prof. Oliver) two species; the former confined to the western parts of the Cape of Good Hope, and the latter to Australia. It is a strange fact that Dionaea, which is one of the most beautifully adapted plants in the vegetable kingdom, should apparently be on the high-road to extinction. This is all the more strange as the organs of Dionaea are more highly differentiated than those of Drosera; its filaments serve exclusively as organs of touch, the lobes for capturing insects, and the glands, when excited, for secretion as well as for absorption; whereas with Drosera the glands serve all these purposes, and secrete without being excited.

By comparing the structure of the leaves, their degree of complication, and their rudimentary parts in the six genera, we are led to infer that their common parent form partook of the characters of Drosophyllum, Roridula, and Byblis. The leaves of this ancient form were almost certainly linear, perhaps divided, and bore on their upper and lower surfaces glands which had the power of secreting and absorbing. Some of these glands were mounted on pedicels, and others were almost sessile; the latter secreting only when stimulated by the absorption of nitrogenous matter. In Byblis the glands consist of a single layer of cells, supported on a unicellular pedicel; in Roridula they have a more complex structure, and are supported on pedicels formed of several rows of cells; in Drosophyllum they further include spiral cells, and the pedicels include a bundle of spiral vessels. But in these three genera these organs do not possess any power of movement, and there is no reason to doubt that they are of the nature of hairs or trichomes. Although in innumerable instances foliar organs move when excited, no case is known of a trichome having such power.74 We are thus led to inquire how the so-called tentacles of Drosera, which are manifestly of the same general nature as the glandular hairs of the above three genera, could have acquired the power of moving. Many botanists maintain that these tentacles consist of prolongations of the leaf, because they include vascular tissue, but this can no longer be considered as a trustworthy distinction. The possession of the power of movement on excitement would have been safer evidence. But when we consider the vast number of the tentacles on both surfaces of the leaves of Drosophyllum, and on the upper surface of the leaves of Drosera, it seems scarcely possible that each tentacle could have aboriginally existed as a prolongation of the leaf. Roridula, perhaps, shows us how we may reconcile these difficulties with respect to the homological nature of the tentacles. The lateral divisions of the leaves of this plant terminate in long tentacles; and these include spiral vessels which extend for only a short distance up them, with no line of demarcation between what is plainly the prolongation of the leaf and the pedicel of a glandular hair. Therefore there would be nothing anomalous or unusual in the basal parts of these tentacles, which correspond with the marginal ones of Drosera, acquiring the power of movement; and we know that in Drosera it is only the lower part which becomes inflected. But in order to understand how in this latter genus not only the marginal but all the inner tentacles have become capable of movement, we must further assume, either that through the principle of correlated development this power was transferred to the basal parts of the hairs, or that the surface of the leaf has been prolonged upwards at numerous points, so as to unite with the hairs, thus forming the bases of the inner tentacles.

The above named three genera, namely Drosophyllum, Roridula, and Byblis, which appear to have retained a primordial condition, still bear glandular hairs on both surfaces of their leaves; but those on the lower surface have since disappeared in the more highly developed genera, with the partial exception of one species, Drosera binata. The small sessile glands have also disappeared in some of the genera, being replaced in Roridula by hairs, and in most species of Drosera by absorbent papillae. Drosera binata, with its linear and bifurcating leaves, is in an intermediate condition. It still bears some sessile glands on both surfaces of the leaves, and on the lower surface a few irregularly placed tentacles, which are incapable of movement. A further slight change would convert the linear leaves of this latter species into the oblong leaves of Drosera anglica, and these might easily pass into orbicular ones with footstalks, like those of Drosera rotundifolia. The footstalks of this latter species bear multicellular hairs, which we have good reason to believe represent aborted tentacles.

The parent form of Dionaea and Aldrovanda seems to have been closely allied to Drosera, and to have had rounded leaves, supported on distinct footstalks, and furnished with tentacles all round the circumference, with other tentacles and sessile glands on the upper surface. I think so because the marginal spikes of Dionaea apparently represent the extreme marginal tentacles of Drosera, the six (sometimes eight) sensitive filaments on the upper surface, as well as the more numerous ones in Aldrovanda, representing the central tentacles of Drosera, with their glands aborted, but their sensitiveness retained. Under this point of view we should bear in mind that the summits of the tentacles of Drosera, close beneath the glands, are sensitive.

The three most remarkable characters possessed by the several members of the Droseraceae consist in the leaves of some having the power of movement when excited, in their glands secreting a fluid which digests animal matter, and in their absorption of the digested matter. Can any light be thrown on the steps by which these remarkable powers were gradually acquired?

As the walls of the cells are necessarily permeable to fluids, in order to allow the glands to secrete, it is not surprising that they should readily allow fluids to pass inwards; and this inward passage would deserve to be called an act of absorption, if the fluids combined with the contents of the glands. Judging from the evidence above given, the secreting glands of many other plants can absorb salts of ammonia, of which they must receive small quantities from the rain. This is the case with two species of Saxifraga, and the glands of one of them apparently absorb matter from captured insects, and certainly from an infusion of raw meat. There is, therefore, nothing anomalous in the Droseraceae having acquired the power of absorption in a much more highly developed degree.

It is a far more remarkable problem how the members of this family, and Pinguicula, and, as Dr. Hooker has recently shown, Nepenthes, could all have acquired the power of secreting a fluid which dissolves or digests animal matter. The six genera of the Droseraceae very probably inherited this power from a common progenitor, but this cannot apply to Pinguicula or Nepenthes, for these plants are not at all closely related to the Droceraceae. But the difficulty is not nearly so great as it at first appears. Firstly, the juices of many plants contain an acid, and, apparently, any acid serves for digestion. Secondly, as Dr. Hooker has remarked in relation to the present subject in his address at Belfast (1874), and as Sachs repeatedly insists,75 the embryos of some plants secrete a fluid which dissolves albuminous substances out of the endosperm; although the endosperm is not actually united with, only in contact with, the embryo. All plants, moreover, have the power of dissolving albuminous or proteid substances, such as protoplasm, chlorophyll, gluten, aleurone, and of carrying them from one part to other parts of their tissues. This must be effected by a solvent, probably consisting of a ferment together with an acid. Now, in the case of plants which are able to absorb already soluble matter from captured insects, though not capable of true digestion, the solvent just referred to, which must be occasionally present in the glands, would be apt to exude from the glands together with the viscid secretion, inasmuch as endosmose is accompanied by exosmose. If such exudation did ever occur, the solvent would act on the animal matter contained within the captured insects, and this would be an act of true digestion. As it cannot be doubted that this process would be of high service to plants growing in very poor soil, it would tend to be perfected through natural selection. Therefore, any ordinary plant having viscid glands, which occasionally caught insects, might thus be converted under favourable circumstances into a species capable of true digestion. It ceases, therefore, to be any great mystery how several genera of plants, in no way closely related together, have independently acquired this same power.

As there exist several plants the glands of which cannot, as far as is known, digest animal matter, yet can absorb salts of ammonia and animal fluids, it is probable that this latter power forms the first stage towards that of digestion. It might, however, happen, under certain conditions, that a plant, after having acquired the power of digestion, should degenerate into one capable only of absorbing animal matter in solution, or in a state of decay, or the final products of decay, namely the salts of ammonia. It would appear that this has actually occurred to a partial extent with the leaves of Aldrovanda; the outer parts of which possess absorbent organs, but no glands fitted for the secretion of any digestive fluid, these being confined to the inner parts.

Little light can be thrown on the gradual acquirement of the third remarkable character possessed by the more highly developed genera of the Droseraceae, namely the power of movement when excited. It should, however, be borne in mind that leaves and their homologues, as well as flower-peduncles, have gained this power, in innumerable instances, independently of inheritance from any common parent form; for instance, in tendril-bearers and leaf-climbers (i.e. plants with their leaves, petioles and flower-peduncles, &c., modified for prehension) belonging to a large number of the most widely distinct orders, – in the leaves of the many plants which go to sleep at night, or move when shaken, – and in the irritable stamens and pistils of not a few species. We may therefore infer that the power of movement can be by some means readily acquired. Such movements imply irritability or sensitiveness, but, as Cohn has remarked,76 the tissues of the plants thus endowed do not differ in any uniform manner from those of ordinary plants; it is therefore probable that all leaves are to a slight degree irritable. Even if an insect alights on a leaf, a slight molecular change is probably transmitted to some distance across its tissue, with the sole difference that no perceptible effect is produced. We have some evidence in favour of this belief, for we know that a single touch on the glands of Drosera does not excite inflection; yet it must produce some effect, for if the glands have been immersed in a solution of camphor, inflection follows within a shorter time than would have followed from the effects of camphor alone. So again with Dionaea, the blades in their ordinary state may be roughly touched without their closing; yet some effect must be thus caused and transmitted across the whole leaf, for if the glands have recently absorbed animal matter, even a delicate touch causes them to close instantly. On the whole we may conclude that the acquirement of a high degree of sensitiveness and of the power of movement by certain genera of the Droseraceae presents no greater difficulty than that presented by the similar but feebler powers of a multitude of other plants.

The specialised nature of the sensitiveness possessed by Drosera and Dionaea, and by certain other plants, well deserves attention. A gland of Drosera may be forcibly hit once, twice, or even thrice, without any effect being produced, whilst the continued pressure of an extremely minute particle excites movement. On the other hand, a particle many times heavier may be gently laid on one of the filaments of Dionaea with no effect; but if touched only once by the slow movement of a delicate hair, the lobes close; and this difference in the nature of the sensitiveness of these two plants stands in manifest adaptation to their manner of capturing insects. So does the fact, that when the central glands of Drosera absorb nitrogenous matter, they transmit a motor impulse to the exterior tentacles much more quickly than when they are mechanically irritated; whilst with Dionaea the absorption of nitrogenous matter causes the lobes to press together with extreme slowness, whilst a touch excites rapid movement. Somewhat analogous cases may be observed, as I have shown in another work, with the tendrils of various plants; some being most excited by contact with fine fibres, others by contact with bristles, others with a flat or a creviced surface. The sensitive organs of Drosera and Dionaea are also specialised, so as not to be uselessly affected by the weight or impact of drops of rain, or by blasts of air. This may be accounted for by supposing that these plants and their progenitors have grown accustomed to the repeated action of rain and wind, so that no molecular change is thus induced; whilst they have been rendered more sensitive by means of natural selection to the rarer impact or pressure of solid bodies. Although the absorption by the glands of Drosera of various fluids excites move- ment, there is a great difference in the action of allied fluids; for instance, between certain vegetable acids, and between citrate and phosphate of ammonia. The specialised nature and perfection of the sensitiveness in these two plants is all the more astonishing as no one supposes that they possess nerves; and by testing Drosera with several substances which act powerfully on the nervous system of animals, it does not appear that they include any diffused matter analogous to nerve-tissue.

Although the cells of Drosera and Dionaea are quite as sensitive to certain stimulants as are the tissues which surround the terminations of the nerves in the higher animals, yet these plants are inferior even to animals low down in the scale, in not being affected except by stimulants in contact with their sensitive parts. They would, however, probably be affected by radiant heat; for warm water excites energetic movement. When a gland of Drosera, or one of the filaments of Dionaea, is excited, the motor impulse radiates in all directions, and is not, as in the case of animals, directed towards special points or organs. This holds good even in the case of Drosera when some exciting substance has been placed at two points on the disc, and when the tentacles all round are inflected with marvellous precision towards the two points. The rate at which the motor impulse is transmitted, though rapid in Dionaea, is much slower than in most or all animals. This fact, as well as that of the motor impulse not being specially directed to certain points, are both no doubt due to the absence of nerves. Nevertheless we perhaps see the prefigurement of the formation of nerves in animals in the transmission of the motor impulse being so much more rapid down the confined space within the tentacles of Drosera than elsewhere, and somewhat more rapid in a longitudinal than in a transverse direction across the disc. These plants exhibit still more plainly their inferiority to animals in the absence of any reflex action, except in so far as the glands of Drosera, when excited from a distance, send back some influence which causes the contents of the cells to become aggregated down to the bases of the tentacles. But the greatest inferiority of all is the absence of a central organ, able to receive impressions from all points, to transmit their effects in any definite direction, to store them up and reproduce them.