Insectivorous Plants. Darwin Charles
exquisitely sensitive to momentary touches from any solid body.
When the pedicel of a tentacle is cut off by a sharp pair of scissors quite close beneath the gland, the tentacle generally becomes inflected. I tried this experiment repeatedly, as I was much surprised at the fact, for all other parts of the pedicels are insensible to any stimulus. These headless tentacles after a time re-expand; but I shall return to this subject. On the other hand, I occasionally succeeded in crushing a gland between a pair of pincers, but this caused no inflection. In this latter case the tentacles seem paralysed, as likewise follows from the action of too strong solutions of certain salts, and by too great heat, whilst weaker solutions of the same salts and a more gentle heat cause movement. We shall also see in future chapters that various other fluids, some vapours, and oxygen (after the plant has been for some time excluded from its action), all induce inflection, and this likewise results from an induced galvanic current.6
CHAPTER III
Nature of the contents of the cells before aggregation – Various causes which excite aggregation – The process commences within the glands and travels down the tentacles – Description of the aggregated masses and of their spontaneous movements – Currents of protoplasm along the walls of the cells – Action of carbonate of ammonia – The granules in the protoplasm which flows along the walls coalesce with the central masses – Minuteness of the quantity of carbonate of ammonia causing aggregation – Action of other salts of ammonia – Of other substances, organic fluids, &c. – Of water – Of heat – Redissolution of the aggregated masses – Proximate causes of the aggregation of the protoplasm – Summary and concluding remarks – Supplementary observations on aggregation in the roots of plants.
I WILL here interrupt my account of the movements of the leaves, and describe the phenomenon of aggregation, to which subject I have already alluded. If the tentacles of a young, yet fully matured leaf, that has never been excited or become inflected, be examined, the cells forming the pedicels are seen to be filled with homogeneous, purple fluid. The walls are lined by a layer of colourless, circulating protoplasm; but this can be seen with much greater distinctness after the process of aggregation has been partly effected than before. The purple fluid which exudes from a crushed tentacle is somewhat coherent, and does not mingle with the surrounding water; it contains much flocculent or granular matter. But this matter may have been generated by the cells having been crushed; some degree of aggregation having been thus almost instantly caused.
If a tentacle is examined some hours after the gland has been excited by repeated touches, or by an inorganic or organic particle placed on it, or by the absorption of certain fluids, it presents a wholly changed appearance. The cells, instead of being filled with homogeneous purple fluid, now contain variously shaped masses of purple matter, suspended in a colourless or almost colourless fluid. The change is so conspicuous that it is visible through a weak lens, and even sometimes by the naked eye; the tentacles now have a mottled appearance, so that one thus affected can be picked out with ease from all the others. The same result follows if the glands on the disc are irritated in any manner, so that the exterior tentacles become inflected; for their contents will then be found in an aggregated condition, although their glands have not as yet touched any object. But aggregation may occur independently of inflection, as we shall presently see. By whatever cause the process may have been excited, it commences within the glands, and then travels down the tentacles. It can be observed much more distinctly in the upper cells of the pedicels than within the glands, as these are somewhat opaque. Shortly after the tentacles have re-expanded, the little masses of protoplasm are all redissolved, and the purple fluid within the cells becomes as homogeneous and transparent as it was at first. The process of redissolution travels upwards from the bases of the tentacles to the glands, and therefore in a reversed direction to that of aggregation. Tentacles in an aggregated condition were shown to Prof. Huxley, Dr. Hooker, and Dr. Burdon Sanderson, who observed the changes under the microscope, and were much struck with the whole phenomenon.
The little masses of aggregated matter are of the most diversified shapes, often spherical or oval, sometimes much elongated, or quite irregular with thread- or necklace-like or club-formed projections. They consist of thick, apparently viscid matter, which in the exterior tentacles is of a purplish, and in the short distal tentacles of a greenish, colour. These little masses incessantly change their forms and positions, being never at rest. A single mass will often separate into two, which afterwards reunite. Their movements are rather slow, and resemble those of Amoebae or of the white corpuscles of the blood. We may, therefore, conclude that they consist of protoplasm. If their shapes are sketched at intervals of a few minutes, they are invariably seen to have undergone great changes of form; and the same cell has been observed for several hours. Eight rude, though accurate sketches of the same cell, made at intervals of between 2 m. or 3 m., are here given (fig. 7), and illustrate some of the simpler and commonest changes. The cell A, when first sketched, included two oval masses of purple protoplasm touching each other. These became separate, as shown at B, and then reunited, as at C. After the next interval a very common appearance was presented – D, namely, the formation of an extremely minute sphere at one end of an elongated mass. This rapidly increased in size, as shown in E, and was then re-absorbed, as at F, by which time another sphere had been formed at the opposite end.
The cell above figured was from a tentacle of a dark red leaf, which had caught a small moth, and was examined under water. As I at first thought that the movements of the masses might be due to the absorption of water, I placed a fly on a leaf, and when after 18 hrs. all the tentacles were well inflected, these were examined without being immersed in water. The cell here represented (fig. 8) was from this leaf, being sketched eight times in the course of 15 m. These sketches exhibit some of the more remarkable changes which the protoplasm undergoes. At first, there was at the base of the cell 1, a little mass on a short footstalk, and a larger mass near the upper end, and these seemed quite separate. Nevertheless, they may have been connected by a fine and invisible thread of protoplasm, for on two other occasions, whilst one mass was rapidly increasing, and another in the same cell rapidly decreasing, I was able by varying the light and using a high power, to detect a connecting thread of extreme tenuity, which evidently served as the channel of communication between the two. On the other hand, such connecting threads are sometimes seen to break, and their extremities then quickly become club-headed. The other sketches in fig. 8 show the forms successively assumed.
Shortly after the purple fluid within the cells has become aggregated, the little masses float about in a colourless or almost colourless fluid; and the layer of white granular protoplasm which flows along the walls can now be seen much more distinctly. The stream flows at an irregular rate, up one wall and down the opposite one, generally at a slower rate across the narrow ends of the elongated cells, and so round and round. But the current sometimes ceases. The movement is often in waves, and their crests sometimes stretch almost across the whole width of the cell, and then sink down again. Small spheres of protoplasm, apparently quite free, are often driven by the current round the cells; and filaments attached to the central masses are swayed to and fro, as if struggling to escape. Altogether, one of these cells with the ever changing central masses, and with the layer of protoplasm flowing round the walls, presents a wonderful scene of vital activity.
[Many observations were made on the contents of the cells whilst undergoing the process of aggregation, but I shall detail only a few cases under different heads. A small portion of a leaf was cut off, placed under a high power, and the glands very gently pressed under a compressor. In 15 m. I distinctly saw extremely minute spheres of protoplasm aggregating themselves in the purple fluid; these rapidly increased in size, both within the cells of the glands and of the upper ends of the pedicels. Particles of glass, cork, and cinders were also placed on the glands of many tentacles; in 1 hr. several of them were inflected, but after 1 hr. 35 m. there was no aggregation. Other tentacles with these particles were examined after 8 hrs., and now all their cells had undergone aggregation; so had the cells of the exterior tentacles which had become inflected through the irritation transmitted from the glands of the disc, on which the transported particles rested. This was likewise the case with the short tentacles round the margins of the disc, which had not as yet become inflected. This latter fact shows that the process of aggregation
My son Francis, guided by the observations of Dr. Burdon Sanderson on Dionaea, finds that if two needles are inserted into the blade of a leaf of Drosera, the tentacles do not move; but that if similar needles in connection with the secondary coil of a Du Bois inductive apparatus are inserted, the tentacles curve inwards in the course of a few minutes. My son hopes soon to publish an account of his observations.