Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/147

G TI V U U S PLAN T 8 135 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 tracheae, 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 (1) 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 only ^-g^-^ 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 absorp tion 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 num ber, 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 proto plasm commences at the base of the tentacles and proceeds upwards. Aggregation is a vital process : the cells must O FIG. 4. Diagram of the same cell of a tentacle of D. rotundifolia, showing the various forms successively assumed by the aggregated masses of protoplasm. (After Darwin.) ba 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 " plas- molysis," or shrinking away of the protoplasm from the cell-wall, which takes place on treating a portion of vege table 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 Dtonxa, 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 assorts that plants are killed l>y immersion for ten minutes in water at 45 to 46 C. , and that their protoplasm coagulates at 50" or 60. Darwin, however, found that the im mersion of leaves of Drosera for ten minutes in water at 50, instead of killing the leaves, excited the tentacles into quick movement, that a tem perature of 54 - 4 paralysed the leaves without killing them, and that some 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. Action of Ammonia Salts. All the salts of ammonia produce in flexion, 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 innnnnnnr f a g rain > * s 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 ani mals, must be infinitely smaller than this, as Mr Darwin remarks, the fact remains truly wonderful that the absorptiQii 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 180, 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 sodium 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 analo gous to nerve. Camphor in solution acts as a stimulant ; the vapours however, of camphor, chloroform, alcohol, ether, and car bonic acid have a narcotic or antesthetic 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 (gum-arabic, sugar, starch, dilute alcohol, olive-oil, tea). The ten tacles were not 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 experiment 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 in flected. We arc 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 be 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 pro positions all hold good of Drosera. Frankland analysed the secre tion 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 Rces and Will prepared a glycerin extract which when acidulated rapidly digested fibrin.