leaves leaf glands tentacles secretion acid cells nitrogenous protoplasm absorption
INSECTIVOROUS PLANTS. Insectivorous or, as they are sometimes more correctly termed, carnivorous plants are, like the parasites, the climbers, or the succulents, a physiological assemblage belonging to a number of distinct natural orders. They agree in the extraordinary habit of adding to the supplies of nitrogenous material afforded them in common with other plants by the soil and atmosphere, by the capture and consumption of insects and-other small animals. The curious and varied mechanical arrangements by which these supplies of animal food are obtained, the ways and degrees in which they are utilized, and the remarkable chemical, histological, and electrical phenomena which accompany these processes of prehension and utilization, can only be understood by a separate and somewhat detailed examination of the leading orders and genera. It is convenient to follow the order adopted by Mr Darwin in his work on Insectivorous Plants (Lund., 1875), to which our knowledge of as subject is mainly due, incorporating, however, as far as possible the leading observations of other writers on the subject. We must preface this, however, by a brief summary of the facts of taxonomy and distribution.
Taxonomy. - The best known and most important order - the Droseracex - is placed among the calycifloral exogens, and has obvious affinities with the Saxifragaces. It includes six genera - Byblis, Roridula, .Drosera, Drosophyllunt, Aldrovanda, and Dionwa, of which the last three are monotypic, i.e., include only one species. The curious pitcher-plant, Cephalotus follicularis, is usually raised to the dignity of a separate natural order Cephalotem, though Bentham and Hooker (Gen. Plant.) place it among the Ribesiaceae. The Sctrraceniacex are thalamiflorals, and contain the genera Sarracenia, Darlingtonia, Ileliccmr phora, while the true pitcher plants or Nepenthaceir, consisting of the single large genus Nepenthes, are placed near the Aristolochiacew among the Apetalaz. Finally the genera Pinguicula, Utricularia, Genlisea, and Polypompholix belong to the gamopetalous order Utricularix. Thus all the four leading divisions of the exogenous plants are represented by apparently unrelated orders ; certain affinities, however, are alleged between Droseraceoe, Sarraceniaccx, and Nepenthacex, Distribution. - While the large genus Drosera has an all but world-wide distribution, its congeners are restricted to well-defined and usually comparatively small areas. Thus Drosophyllunt occurs only in Portugal and Morocco, Byblis in tropical Australia, and, although Aldrovanda is found in Queensland, in Bengal, and in Europe, a wide distribution explained by its aquatic habit, Dionxa is restricted to a few localities in North and South Carolina, mainly around Wilmington. Cephalotus occurs only near Albany in Western Australia, Reliant phora on the Roraima Mountains in Venezuela, Darlingtonia on the Sierra Nevada of California, and these three genera too are as yet monotypic; of Sarracoila, however, there are six or eight known species scattered over the eastern States of North America. The 36 species of Nepenthes are mostly natives of the butter parts of the Indian Archipelago, but a few range into Ceylon, Bengal, Cochin China, and some even occur in tropical Australia on the one hand, and in the Seychelles and Madagascar on the other. Pinguicula is abundant in the north temperate zone, and ranges down the Andes as far as Patagonia; the 150 species of Utricularia are mostly aquatic, and sonic, are found in all save polar regions; their unimportant congeners, Genlisea and Polypompholir, occur in tropical America and south-western Australia respectively. It is remarkable that all the insectivorous plants agree in inhabiting damp heaths, bogs, marshes, and similar situations where water is abundant, - a peculiarity perhaps due to their habit of copious secretion and censequent need of water.
Drosera. - The Common Sundew (D. rotundifolia) has extremely small roots, and bears five or six radical leaves horizontally extended in a rosette around the flowerstalk. The upper surface of each leaf is covered with gland-bearing filaments or "tentacles," of which there are on an average about two hundred. Each gland is surrounded by a large dew-like drop of a viscid but transparent and glittering secretion, and the popular names (Sundew, French Rossolis, German Sonnenthau) as well as the Linrman (from Spdo-ov, dew) have been thus suggested. The stalk of the tentacle has the essential structure of a leaf. A small fibro-vascular bundle, consisting mainly of spiral vessels, runs up through the stalk and is surrounded by a layer of elongated parenchyma cells lined by a thin layer of colourless circulating protoplasm, and filled with a homogeneous fluid, tinted purple by a modification of chlorophyll (erythrophyl], Sorby). The epidermis bears small multi cellular prominences. The glandular head of the tentacle contains a central mass of spirally thickened cells in im mediate contact with the upper end of the fibro-vascular bundle. Around these (but separated from them by a layer of much elongated cells, Warming) there is a layer of cells filled with purple fluid, and outside these lies a similar series of cells, whose contents differ slightly in tinge, and in behaviour when treated with reagents.
Insects seem to be attracted by the leaves of Drosera, but whether by their colour, their glittering secretion, their odour, or by all three, remains as yet unsettled. A fly alighting on the disk, or even only touching one or two of the exterior tentacles, is immediately entangled by the viscid secretion ; the tentacles to, which it is adhering begin to bend, and thus pass on their prey to the tentacles next succeeding them inwards, and the insect is thus carried by a curious rolling movement to the centre of the leaf. The tentacles on all sides become similarly inflected; the blade or the leaf may even become almost cup-shaped; and the insect, bathed in the abundant secretion which soon closes up its trache, is drowned in about a quarter of an hour. The leaves clasp also, but for a much shorter time, over inorganic bodies.
The bending of the tentacle takes place near its base, and may be excited (I) by repeated touches, although not by gusts of wind or drops of rain, thus saving the plant from much useless movement ; (2) by contact with any solid, even though insoluble and of far greater minuteness than could be appreciated by our sense of touch, - a morsel of human hair weighing onlyfia s of a grain, and this largely supported too by the viscid secretion, sufficing to induce movement ; (3) by the absorption of a trace of certain fluids, mostly nitrogenous. During the inflexion of the tentacle, and even before it touches the stimulating object, the secretion of the gland increases in quantity, and, instead of remaining neutral, becomes acid.
The stalk of a tentacle whose gland has been stimulated by repeated shocks, continuous pressure, or the absorption of any nitrogenous fluid, particularly a solution of ammonic carbonate, shows a mottled appearance; and, when examined under the microscope the formerly homogeneous fluid contents of its constituent cells are seen to have separated into purple masses of constantly varying number, shape, and size, suspended in a colourless fluid, and the layer of colourless circulating protoplasm which lines the cells thus becomes much more distinctly visible. This process, which is termed by Darwin " aggregation of the protoplasm," commences in the glands and gradually travels down the tentacles, being temporarily arrested at each cell-wall. The process of redissolution of the protoplasm commences at the base of the tentacles and proceeds upwards. Aggregation is a vital process : the cells must be alive, uninjured, and oxygenated ; if they are crushed or treated with carbonic acid the phenomenon does not take place. It is not necessarily related to inflexion, for one may be induced without the other ; it is totally unlike the " plasmoly-sis," or shrinking away of the protoplasm from the cell-wall, which takes place on treating a portion of vegetable tissue with any dense fluid, and which is simply due to exosmose ; and it does not depend upon increased secretion. Darwin has also observed aggregation in the sensitive hairs of Dionxa, and in the roots of various plants ; it seems indeed to be of wide distribution and profound importance in the physiology of the vegetable cell.
Effects of Heat. - Sachs asserts that plants are killed by immersion for ten minutes in water at 45° to 46° C., and that their protoplasm coagulates at 50° or 60g. Darwin, however, found that the immersion of leaves of Drosera for ten minutes in water at 50°, instead of killing the leaves, excited the tentacles into quick movement, that a temperature of 540.4 paralysed the leaves without killing them, and that sonic even survived a temperature of 62° C. Some of the lowest plants have frequently been described as living in hot springs, but that so highly organized a native of temperate and even almost arctic regions should withstand so high a temperature is very remarkable.
Adion of Ammonia Salts. - All the salts of annnonia produce inflexion, the carbonate strongly, the nitrate even more so, and the phosphate most of all. The immersion of a leaf in a solution of the last-mentioned salt, so weak that each gland could only absorb about 20001000o of a grain, is sufficient to produce complete inflexion of the tentacles. Though the particles of solid. matter which stimulate the olfactory nerves, and so produce the sensation of odour in animals, must be infinitely smaller than this, as Mr Darwin remarks, the fact remains truly wonderful that the absorption of so minute a quantity by a gland should induce some change in it, which leads to the transmission of a motor impulse down the entire length of the tentacle, causing the whole mass to bend, often through an angle of more than 1800, and this too in the absence of any specialized nervous system.
Action of various Salts and Acids. - In the case of salts the nature of the base seems to be of much more importance than that of the acid, a conclusion already arrived at by animal physiologists. Thus nine salts of sodhun caused inflexion, and were not poisonous; seven of the corresponding salts of potassium did not cause inflexion, and some were poisonous. This is interesting in connexion with the fact that large doses of sodium salts may be introduced into the circulation of mammals with impunity, whereas small doses of potassium salts speedily cause death. Of twenty-four acids tried, nineteen caused inflexion, and the majority, even including most of the organic acids, were poisonous, which is the more remarkable since juice of many plants seems much more strongly acid than the solutions which were employed. The poisonous action, however, is not improbably connected with the negative osmose which is known to be induced by dilute acids.
Action of Alkaloid Poisons, of other Substances, and of Vapours. - Acetate and sulphate of quinine, citrate of strychnine, nicotine, digitaline, act more or less strongly on the glands and kill them ; on the other hand, nitrate of quinine, atropine, veratrine, colchicine, theine, are quite harmless. Curare is not poisonous, and cobra poison, which kills animals by paralysing their nerve centres, causes "strong and rapid inflexion of the tentacles, and soon discharges all colour from the glands," stimulating also the movements of their protoplasm. Since alkaloids which act strongly on the nervous system of animals are without effect on Drosera, it seems probable that the sensibility of its glands, and their power of transmitting a stimulus to other parts of the leaf, are not due to elements analogous to nerve. Camphor in solution acts as a stimulant ; the vapours however, of camphor, chloroform, alcohol, ether, and carbonic acid have a narcotic or anaesthetic action, and kill the plants after a time.
Effects of Organic Fluids. - Digestive Power of Secretion. - Darwin treated sixty-one leaves of Drosera with non-nitrogenous solutions (gnm-arabic, sugar, starch, dilute alcohol, olive-oil, tea). The tentacles were 11 ot in a single case inflected. He then applied to sixty-four other leaves various nitrogenous fluids (milk, urine, albumen, infusion of meat, mucus, saliva, isinglass), and sixty-three had the tentacles and often the blades well inflected. Finally, taking twenty-three of the leaves which had served for the first expeiiment and treating them with bits of meat or drops of nitrogenous fluids, all save a few, apparently injured. by exosmose caused by the density of the former solution of gum, sugar, &c., were distinctly inflected.
A.Ve are thus led to inquire whether the leaves have only the power of absorbing matter already in solution or whether they can render nitrogenous matter soluble, that is, whether they have the power of true digestion. The digestion of albuminous bodies by animals is effected by means of a ferment, pepsin, acting in presence of weak hydrochloric acid, - neither the acid nor the ferment having the power of digesting in the absence of the other, though almost any other acid may 1e substituted for hydrochloric. When the stomach is mechanically excited, acid is secreted, but not pepsin ; this requires for its production the absorption of a minute quantity of already soluble animal matter (peptogene of Schiff). These propositions all hold good of Drosera. Frankland analysed the secretion obtained by stimulating four hundred and forty-five leaves with particles of glass, and came to the conclusion that its acidity was due to some acid of the acetic series, apparently either propionic or a mixture of acetic and butyric acids. Analysis of larger quantities enabled Will to show that the secretion contained formic as well as probably butyric and propionic acid, and Rees and Will prepared a glycerin extract which when acidulated rapidly digested fibrin.
Lawson Tait also separated a substance possessing the property of a digestive ferment.
Darwin fed numerous plants with roast meat and minute cubes of boiled white of egg, and placed other cubes in wet moss as a cheek. Solution soon took place in the former cases ; and, just as in animal digestion, the edges of the cubes of egg were first rounded off, and the striation of muscle was replaced by dark points, while the bits of egg left in moss putrefied. On neutralization of the acid by alkali, digestion stops ; on reacidification, it goes on again. Neither the watery nor the glycerin extract of leaves stimulated by fragments of glass was able to digest, showing that the ferment is not secreted until the glands have absorbed a trace of animal matter. The leaves digested fibrin, connective tissue, cartilage, bone, enamel, and dentine, gelatin, chondrine, casein of milk, &c., hut could not digest epidermic productions (nails, hairs, feathers), fibro-elastic tissue, mucin, pepsin, urea, chitin, chlorophyll, cellulose, gun-cotton, oil, fat, and starch, thus completing the analogy with the gastric digestion of animals. • Pollen-grains had their protoplasmic contents dissolved, and seeds were usually killed.
Irritability and Movements. - Cutting and pricking the leaf does not induce movement ; the petiole is quite insensible, nor do the pedicels of the glands bend when rubbed or stimulated by contact with food. Only the glands remain, and these at once respond to stimuli, yet their irritability seems to extend for a very slight distance below them, since when the glands arc cut off their pedicels often become inflected. When a tentacle receives an impulse either from its own gland or from the central tentacles, it bends towards the middle of the leaf, the short tentacles on which do not bend at ; in all other cases all the tentacles, even those of the centre, bend towards the point whence the stimulus conies. Thus all the tentacles of a leaf may be made to converge into two symmetrical groups by placing a fragment of phosphate of ammonia in the middle of each half of the blade. Contrary to the opinion of Ziegler, vivisection shows that the motor impulse is not transmitted through the libro-vaseular bundles, but through the cellular tissue. An impulse thus travels more rapidly along than across the leaf, since, from the elongated shape and the position of tho cells, fewer cell-walls have to be crossed in a given distance. Thus, when the central glands are excited, they send centrifugally some influence to the exterior glands, where aggregation of the protoplasm is set up, which may be watched descending their tentacles, and the whole process is not without analogy to a reflex action. The motor impulse seems to be allied to the aggregating process, and it has been attempted to explain the bending which takes place at the base of the tentacles by assuming either (1) a rapid passage of fluid out of the cells in that region, which would thus contract, at least if we suppose them to be previously in a state of high tension and to possess great elasticity, (2) a contraction of the protoplasm of these cells, (3) the contraction of the cell-walls as well as the protoplasm, or (4) a shrinkage of the fluid contents of the cells, owing to a change in their molecular state with the subsequent closing in of the walls.
Absorption. - Bennett has described what he terms absorptive glands beneath the epidermis, consisting of two nearly hemispherical cells, filled with brownish protoplasm and bearing papilin, which sometimes rise above the surface of the leaf, or the filaments of the tentacles. He finds similar organs in Di072XCI and Nepenthes, lint in no plants other than carnivorous, except Callitriche. Clark fed. Droscra with flies soaked in chloride of lithium, and after several days found that all parts of the plant when burned showed the characteristic spectrum of lithium ; and Tait, by cultivating plants with roots cut off and leaves buried in pure sand watered wail an ammoniacal solution, showed that the sundew can not only absorb nutriment from its leaves, but can actually live and thrive by their aid alone, if supplied with small quantities of nitrogenous material.
D;on-e3a ifuscipula, L. - This plant, the well-known Venus's Fly-trap, was first described in 1768 by Ellis in a remarkable letter to Linnmus, in which he gave a substantially correct account of the structure and functions of its leaves, and even suggested the probability of their earthvorism. Linnams declared it the most wonderful of plants (miraculum naturx), yet only admitted that it showed an extreme case of sensitiveness, supposing that the insects were only accidentally captured and subsequently allowed to escape. Two American botanists, Curtis and Canby, successively advanced our knowledge of the mode of capture and digestion, which has also been investigated by Mrs Treat, T. A. G. Balfour, and others, and most fully by Darwin.
The leaves are all radical, with broad foliaceous foot-stalks. Each leaf has two lobes, standing at rather less than a right angle to each other, their edges being produced in a triangle. These contain 110 fibro-vascular bundles, but present an articulation near their bases, which enables them to bend parallel to the surface of the leaf when the lobes close, When the filaments are touched by an insect, the lobes close very sharply upon the hinge-like midrib, the spikes interlock, and the insect is imprisoned. If very minute, and so not worth digesting, it is able to escape between the interlocked spines ; more usually, however, it is retained between the lobes, which gradually but firmly compress it, until its form is distinguishable from without. The leaf thus forms itself into a temporary stomach, and the glands, hitherto dry, commence, as soon as excited by the absorption of a trace of nitrogenous matter, to pour out an acid secretion containing a ferment, which_ rapidly dissolves the soft parts of the insect. This is produced in such abundance that, when Darwin made a small opening at the base of one lobe of a leaf which had closed over a large crushed fly, the secretion continued to run down the footstalk (luring the whole time - nine days - during which the plant was kept under observation. Aggregation may be observed in the glands, and, at least on treatment with carbonate of ammonia, the aggregative process may be watched ascending the sensitive hairs.
Though the filaments are exquisitely sensitive to the slightest contact with solid bodies, yet they are far less of Diontra are completely indifferent to wind and rain. The surface of the blade is very slightly sensitive ; it may be roughly handled or scratched without causing movement, but closes when its surface or midrib is deeply pricked or cut. Irritation of the triangular area on each lobe enclosed by the sensitive filaments causes closure. The footstalk is quite insensitive, Inorganic or non-nitrogenous bodies, placed on the leaves without touching the sensitive filaMonts, do not excite movement, but nitrogenous bodies, if in the least degree damp, cause after several hours the lobes to close slowly. So too the leaf which has closed over a digestible body applies a gradual pressure, which serves to bring the glands on both sides into contact with the body, and may also, as Balfour suggests, aid in absorption. Thus we see that there are two kinds of movement, adapted for different purposes, one rapid, excited mechanically, the other slow, excited chemically. Leaves made to close over insoluble bodies reopen in less than twenty-four hours, and are ready, even before being fully expanded, to shut again. But if they have closed over nitrogen-yielding bodies, they remain closely shut for many days, and after re-expanding are torpid, and never act again, or only after a considerable time. Even in a state of nature, the most. vigorous leaves are very rarely able to digest more than twice, or at most thrice, during their life. The secretion is a true gastric juice containing formic acid, and like gastric juice has remarkable antiseptic powers. Lindsay fed leaves with such quantities of meat as to kill them with indigestion, yet showed that the meat inside the leaf remained perfectly fresh while portions hanging outside putrefied.
While evidence is thus afforded of the absorption of the products of digestion by the complete disappearance of fibrin, albumen, &c., placed upon the leaf of Dion.xa, fraustadt was able, by feeding leaves with albumen dyed with aniline-red, to colour the contents and nuclei of the gland-cells.
The motor impulse, as in Drosera, is transmitted through the cellular tissue. Burden Sanderson has demonstrated the existence of a normal electric current in the leaf of Dioima, and the negative variation undergone by that current at the moment of closure of the leaf due to the conversion of electromotive force into mechanical work. This discovdry, which is of the highest importance as showing the profound resemblance between the closure of the leaf of Dioneea and the contraction of a muscle, has been followed up and extended by Munk. C. de Candolle ascribes the closure of the valves' to variations in the turgescence of the parenchyma of their upper surface.
A ldrovancla vesieulosa. - This " minute aquatic Dionwa" floats freely, and is destitute of roots. Its whorled leaves have two lobes, with slightly inflected margins, which open only about as much as the valves of a living mussel-shell, and thus capture the more easily the small crustaceans and mollusks which may get between them. Part of the upper surface of each lobe next the midrib bears colourless glands (like those of Die/ma, but stalked), together with numerous long sensitive filaments which have both median and basal articulations ; the outer thinner portion bears small quadrifid hairs. Darwin holds that the glands secrete and digest, while the quadrifids are destined to the absorption of decaying animal matter, the two regions of the leaf thus serving for very different purposes.
Drosophyllum, lusitanicum. - This plant catches such vast numbers of flies in a state of nature that the Portuguese cottagers call it the fly-catcher, and hang up branches of it in their houses for this purpose. Its linear leaves are thickly covered with stalked glands which resemble in the main the tentacles of Droserct, save in that they are incapable of movement, and that their secretion is acid before excitement. The secretion too is less viscid, and freely leaves the gland to wet the insect, which, creeping onward, soon clogs its wings and dies. There are, moreover, many minute colourless sessile glands which only begin to secrete when stimulated by the absorption of nitrogenous matter, with which they seem to be mainly concerned.
Roridula and Byblis resemble Drosophyllum, but their glands are of simpler structure than those of the latter, scarcely differing appreciably from the glandular hairs of other plants. Mr Darwin has thrown considerable light upon the question of how far the glands of plants not adapted for capturing insects share the power of absorption exhibited by those of the Droseracex. Choosing a number of plants at hazard, he found that the glands of two species of Saxifrage, a genus distantly allied to Drosera, of a Primula, and of Pelargonium, have the power of rapid absorption, and exhibit movements of aggregation in their protoplasm, whereas those of Erica, Mirabilis, and Anicotiana appear to have no such power. Heckel has made similar observations on the floral glands of Parnassia palustris, and on the leaf-glands of Geranium sparmannia, &c. The glandular hairs of at least some plants are known to be capable of absorbing ammonia, both in solution and in vapour, and probably some obtain animal matter from the insects which are occasionally entangled in the viscid secretion.
Pin g uieula or _Butterwort. - The large thick radical leaves of this genus have a very viscous surface and a pale colour, and bear two sets of glands, the larger borne on usually unicellular pedicels, the smaller almost sessile. When a fly is captured, the viscous secretion becomes strongly acid, the naturally incurved margins of the leaf