Dutrochet has formed a theory of all the motions of fluids in plants depending upon the agency of galvanism. He found that small bladders of animal and vegetable membrane, being filled with a fluid of greater density than water, securely fastened, and then thrown into water, acquired weight; he also remarked, that if the experiment was reversed, by filling them with water and immersing them in a denser fluid, the contrary took place, and that the bladders lost weight. He took a small bladder, and filled it with milk, or gum arabic dissolved in water; to the mouth of this bladder he adapted a tube, and then plunged the bladder in water: in a short time the milk rose in the tube, whence he inferred that water had been attracted through the sides of the bladder. This experiment was also reversed, by filling the bladder with water, and plunging it in milk: the fluid then fell in the tube, whence he inferred that water had been attracted through the coat of the bladder into the milk. From these and other experiments, Dutrochet arrived at the inference, that, if two fluids of unequal density are separated by an animal or vegetable membrane, the denser will attract the less dense through the membrane that divides them: and this property he calls endosmose, when the attraction is from the outside to the inside; and exosmose, when it operates from the inside to the outside. In pursuing this investigation, he remarked that, if an empty bladder is immersed in water, and the negative pole of a galvanic battery introduced into it, while the positive pole is applied to the water on the outside, a passage of fluid takes place through the membrane, as had previously happened when the bladder contained a fluid denser than water; by reversing the experiment, the reverse was found to take place: from all which Dutrochet deduces the following theory: -That, when two fluids of unequal density are separated by an intervening membrane, the more dense is negatively electrified, and the less dense positively electrified; in consequence of which, two electric currents of unequal power set through the membrane, carrying fluid with them; that which sets from the positive pole, or less dense fluid, to the negative pole, or more dense fluid, being much the more powerful: and that the fluids of plants being more dense than those which sur

round them, a similar action takes place between them and the water in the soil, by means of which the latter is continually impelled into their system. Philosophers do not seem disposed to admit the legitimacy of Dutrochet's conclusion, that this transmission takes place by means of galvanic agency; but that the phenomenon is correctly described by the ingenious author, and that it is constantly operating in plants, are beyond all dispute. It is by endosmose that vapour is absorbed from the atmosphere, and water from the earth; that sap is attracted into fruits by virtue of their greater density; and, probably, that buds are enabled to empty the tissue that surrounds them, when they begin to grow.

But, although endosmose will be found a ready explanation of many of the phenomena connected with the ordinary movement of fluids, it throws no light upon rotation or cyclosis, which, so far as we at present know, are motions inexplicable upon any principle yet discovered.

CHAPTER XIV.

OF THE DIRECTIONS TAKEN BY THE ORGANS OF PLANTS.

THE substance of all that is known upon this subject has been combined with some excellent observations of his own, by Dutrochet, in a memoir, of which I shall avail myself in the following remarks.

"The general phenomena of nature," says this writer, "which are daily before our eyes, are often those which mankind considers the least attentively. Those who are unaccustomed to reflect upon such subjects can scarely believe that there is any very extraordinary mystery in the ascent of the stems of vegetables, or in the descent of their roots; and yet this is one of the most curious circumstances connected with vegetable life. The downward direction of the roots may appear easy of explanation: it may be said that, like all other bodies, they have a tendency towards the centre of the earth, in consequence of the known laws of gravity (as is the opinion of Knight, in Phil. Trans. for 1806); but on what principle, then, is to be explained the upward tendency of the stem, which is in direct opposition to those laws? And here lies the difficulty. Dodart is the first who appears to have paid attention to this circumstance; he pretends to explain the turning backwards of seeds sown in an inverted position by the following hypothesis: - He assumed that the root is composed of parts that contract by humidity; and that the stem, on the contrary, contracts by dryness. For this reason, according to him, it ought to happen that, when a seed is sown in an inverted position, the radicle will turn back towards the earth, which is the seat of humidity; and that the plumula, on the contrary, turns to the sky, or rather atmosphere, a drier medium than the earth. The experiments of Du Hamel are well known, in which he at

tempted to force a radicle upwards and a plumula downwards, by enclosing them in tubes which prevented the turning back of these parts. It was found that, as the radicle and plumula could not take their natural direction, they became twisted spirally. These experiments, while they prove that the opposite tendencies of the radicle and plumula cannot be altered, still leave us in ignorance of the cause of such tendencies. We are equally ignorant of the cause of the directions of the leaves. Bonnet believed that he could explain that phenomenon upon the hypothesis of Dodart just referred to, with respect to the radicle and plumula. According to him, the lower surface of the leaves is, like the radicle, composed of fibres which contract by humidity; and the upper, like the plumula, of fibres that contract by dryness. As a proof of these assertions, Bonnet manufactured some artificial leaves: the upper surface of which was parchment, which contracts by dryness, and the lower of linen, which contracts by moisture. These leaves were submitted to the action of dryness and humidity; and Bonnet found they were affected much in the same way as true leaves, to find proofs to support a favourite hypothesis."

so easy is it

In consequence of the unsatisfactory nature of these and other theories, more modern physiologists have been satisfied with inscribing the particular directions taken by plants among the vital phenomena of vegetation. And this is, perhaps, as much as we are likely to ascertain relating to them, and all similar manifestations of the overruling power of nature. Dutrochet, however, being of opinion that some more direct explanation of such phenomena is to be found, instituted a variety of experiments of a novel kind. "Seeing," he remarks, "that the stem is always directed towards heaven, and the root towards the earth, we cannot but believe that there is some relation between the cause of gravitation and that of the life of vegetables: the constant direction of the stem towards the light leads us also to suppose that this agent performs some important part in determining the directions of the parts of plants. The stem must be placed in the midst of the atmosphere, in order to develope itself; the roots, on the contrary, require to lie within the earth. Hence, it may

be inferred that several causes concur to produce the pheno

mena in question."

Dutrochet filled with earth a box, the bottom of which was perforated with many holes: he placed seed of the Kidneybean in these holes, and suspended the box in the air, at about eighteen feet from the earth. Here the seeds, being placed in holes pierced through the bottom of the box, received the influence of the atmosphere and light from below; while the humid earth was placed above them. If the cause of the different directions of the radicle and plumule consisted in an affinity of the former for humidity, and of the latter for the atmosphere, the radicle ought to shoot upwards, and the plumule downwards; but this did not take place. The radicles, on the contrary, found their way downwards out of the box into the atmosphere, where they quickly dried up and perished; and the plumules forced their way backwards into the earth. This experiment was afterwards modified, by increasing the quantity of earth above the seeds, and by some other contrivances; but the result was always the same: it was uniformly found, that there was no affinity between the radicle and the seat of moisture sufficient to counteract the natural downward tendency of the roots. It was also inferred, that there existed no more positive affinity between the stems and the atmosphere than between the roots and water.*

There are certain parasitic plants which strike their roots into the stems of other plants, and which always grow at right angles with the stem to which they are fixed. The seed of the Mistletoe will germinate in any direction, either upwards, downwards, or laterally. The first movement made by this plant consists in an extension of its caulicule, which derives its support from the cotyledons, and which terminates

* Professor Schultz, however, succeeded in overcoming the tendency of the roots downwards and stems upwards. He planted seeds of Cabbages, Mustard, and Kidneybeans in moss, and so arranged them that the only light they could receive was from a mirror, which threw the solar rays upon them from below upwards; they sent their roots upwards, and their stems downwards. It would, therefore, appear that light is the great cause of the direction taken by the stem.