Page:Encyclopædia Britannica, Ninth Edition, v. 13.djvu/148

 136 INSECTIVOROUS PLANTS 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 check. Solution soon took place in the former cases ; and, just as in animal digestion, the edges of the cubes of egg were first rounded oif, 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., but could not digest epidermic productions (nails, hairs, feathers), fibro-elastic tissue, mucin, pepsin, urea, chitin, chlorophyll, cellu lose, gun-cotton, oil, fat, and starch, thus completing the analogy with the gastric digestion of animals. Pollen-grains had their pro toplasmic contents dissolved, and seeds were usually killed. Irritability and Movcm&nts. 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 dis tance below them, since when the glands are 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 all ; in all other cases all the tentacles, even those of the centre, bend towards the point whence the stimulus comes. 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 fibro-vascular 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 the 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 im pulse 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 colls, 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 hemi spherical cells, filled with brownish protoplasm and bearing papillse, which sometimes rise above the surface of the leaf, or the filaments of the tentacles. He finds similar organs in Dioneea and Nepenthes, but 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 with 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. Dionxa Muscipula, L. This plant, the well-known Venus s Fly-trap, was first described in 1768 by Ellis in a remarkable letter to Linnaeus, in which he gave a substan tially correct account of the structure and functions of its leaves, and even suggested the probability of their carni- vorisin. Linnaeus declared it the most wonderful of plants (miracuhmi naturse), 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 into spike-like processes. The upper surface of each lobe is covered with minute circular sessile glands, each consist ing of from 20 to 30 cells filled with purplish fluid. It bears also three fine-pointed sensitive filaments arranged Fio. 5. Leaf of Venus s Fly-trap (Dionxa museiptiJa), viewed laterally in its expanded state. (After Darwin.) in a triangle. These contain no 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 mid rib, the spikes inter lock, and the insect is imprisoned. If very minute, and so not worth digesting, it is able to escape between the inter locked Spines ; more FIG. 6. Leaf of D. muscipula closed over insect, usually, however, it A &amp;gt; viewcd from the sid : B - from abQve - 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 contain ing 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 secre tion continued to run down the footstalk during the whole time nine days during which the plant was kept under observation. Aggre gation may be observed in the glands, and, at least on treatment with carbonate of ammonia, the aggregative pro cess may be watched ascend ing the sensitive hairs. Though the filaments arc exquisitely sensitive to the ii.. rf, -,i T i FIG. 7. A, sensitive filament and glands slightest contact with solid O f D. mu scipui, x so ; B, giamis, bodies, yet they are far less sensitive than those of Droscra to prolonged pressure, a singular difference in evident relation to the habits of the two plants. Like the leaves of Drosera, however, those