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CHAPTER XIII.

OF THE CIRCULATION.

PLANTS have no circulation of their fluids analogous to that of the blood in the higher animals; that is to say, departing and returning incessantly from and to one common point. But that their fluids have a motion may be inferred from their nature; and that it is often of extreme rapidity is proved by the great quantity of water which they perspire; all of which must be replenished by aqueous particles in rapid motion along the tissue from the roots. A young vine leaf, in a hot day, perspires so copiously, that, if a glass be placed next its under surface, it is presently covered with dew, which, in half an hour, runs down in streams. Hales computed the perspiration of plants to be seventeen times more than that of the human body. He found a sunflower lose one pound four ounces, and a cabbage one pound three ounces, a day by perspiration. By some contrivances of glass tubes and a mercurial apparatus, he found means to measure the force of suction in particular trees, which will of course be in proportion to the amount of evaporation; and he ascertained that an apple branch 3 feet long would raise a column of mercury 5 inches in half an hour; a nonpareil branch 2 feet long, with 20 apples on it, 12 inches in 7 minutes; and the root of a growing pear tree 8 inches in 6 minutes. In short, he computed that the force of motion of the sap is sometimes five times greater than that which impels the blood in the crural artery of the horse. Guettard asserts that the young shoots of Cornus mascula lose twice their own weight a day. This perspiration is regulated in part by the number of the stomates, and in part by the thickness of the epidermis: hence evergreens, in which the stomates are small, and less numerous

than in deciduous or herbaceous plants, and the epidermis thicker and harder, perspire much less than other plants.

M. Biot has succeeded in injecting the red colouring matter of Phytolacca decandra into the flowers of white byacinths. He learned from a paper by De la Baisse, in Recueil des Prix de l'Académie de Bordeaux, vol. iv., that the juice of this plant is free from all the objections usually found to the red colouring matter used for such experiments, and that he had succeeded in injecting it into all sorts of white flowers, and even green leaves. Biot found, however, that although he did in many cases succeed, yet the practice was attended with peculiar difficulties. Many plants refused the injection altogether, others took it up with rapidity. A few minutes sufficed to vein with a multitude of red lines all the petals of a white monthly rose, while a white musk rose was not affected. He even found that the flowers of the same species resisted the entrance of the colouring matter in an unequal degree.

That a general motion of fluids really exists in plants is, therefore, undoubted. It is most rapid in the spring and early summer, and most languid in winter; but never actually suspended, unless under the influence of frost. This has been demonstrated by Biot, who, by means of an apparatus described in the Institut Newspaper, succeeded in measuring the power of motion in the sap of plants, in witnessing the phenomena which regulated it, and in determining the causes that brought them about.

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Atmospherical circumstances," he says, " and especially the absence or presence of solar light, exercise a marked influence upon these phenomena; but it is exceedingly difficult to ascertain their exact nature. Nevertheless, among them is one, the effects of which are so constant and undoubted, that they appear susceptible of being defined. This consists in the sudden appearance of frost immediately succeeding mild weather, and lasting for some time. Mild weather either favours or brings about the ascent of the sap; but, if a sudden frost supervenes, it seizes upon the part of the trunk swollen with fluid, and forces the latter to fall back again: should the frost continue and increase in severity, the earth at the foot of the tree freezes; and, whether at that time the roots are mecha

nically compressed by it, or whether the duration of the cold causes contraction by a vital action, the roots commence causing a considerable discharge of fluid from the lower part of the apparatus. This goes on night and day, except when the pipes to carry off the sap are frozen. As soon as a thaw comes on and the earth is relaxed, the roots, emptied of their juices, find themselves below their point of saturation; they then emit nothing, but on the contrary absorb the descending juices. I satisfied myself of this not only by my apparatus, but in sawing through the trunk of a large poplar tree, a yard from the ground. The surface of the section of the stump was dry, but that of the trunk itself dripped with water."

The motion of the sap appears to be of two kinds; 1. general, and 2. special: these must be carefully distinguished. The former is what has been alluded to in the preceding observations; the latter is altogether of a different nature, and exists in two entirely different conditions, generally confounded with each other, till distinguished by Professor Schultz. Of these, the first is called Rotation, the latter Cyclosis; the two are said never both to occur in the same species.

Of Rotation.

This kind of motion is confined to plants of a low organisation, but not entirely to flowerless or cellular families. It, however, forms for Professor Schultz an important physiological means of separating the vegetable kingdom into two primary classes, namely, Homorgana and Heterorgana: the former of which, consisting wholly or in great measure of cellular tissue, contains all the cellular flowerless, and some flowering plants of a low organisation; the latter all the higher flowering plants, and the vascular flowerless. It consists in a special circulation of the fluid contained in the interior of each cell, and is always so limited; the rotation in one cell never interposing or mixing with that in another cell. The rotating sap of such plants is said by Schultz to have the power of absorbing coloured fluids, while the cinenchymatous vessels, in which cyclosis goes on, either do not take

up any coloured fluid, or, at least, not till they receive it in an altered state from other forms of tissue.

Corti, in 1774, Fontana, L. C. Treviranus, and especially Amici, made the earliest observations upon rotation. It was found that if a portion of Nitella flexilis, or even of the crustaceous Charas, their opake cuticle being first scraped away, be examined, a current of sap will be distinctly seen in each cellule, setting from joint to joint, flowing down one side and returning up the other, without any membrane intervening to separate the opposing currents; each cellule has a movement of its own, independent of that of the cellules above and below it; sometimes the movement stops, and then goes on again after a brief interval; if a cellule is divided into two by a ligature passed round it, a separate movement is seen in each of the divisions; this motion is rendered distinctly obvious by the numerous minute green granules which float in the transparent fluid, and which follow the course of the currents.

The observations of Amici have been verified and much extended by subsequent investigations.

Among other things, it has been ascertained that in Nitella the currents have always a certain relation to the axis of growth, the ascending current uniformly passing along the side of the cell most remote from the axis, and the descending current along the side next the axis.

Mr. Varley considers (Trans. Soc. Arts, xlix. p. 20.) that, in addition to the principal current, which he finds setting up one side and down the other within the green interior granular sac of each joint of Chara, there are two others, of which one takes place between the side of the interior sac and the side of the outer transparent coating, the other current is said to occur in the centre of the interior cell, and to be very sluggish.

A further and very detailed examination of the Chara fragilis has been made by M. Dutrochet, the general results of which are to be found in the Ann. des Sciences, n. s., vol. ix. p. 73. It appears from them, among other things, that experiments, expressly instituted by M. Becquerel, show the motion not to be owing to a voltaic action of the green globules lining the cells, nor to any known form of electrical agency,

but to vital force; and, also, that the rapidity of the movement is increased by an elevated, and diminished by a lowered, temperature, the mean rate of motion of the swimming granules being a millimetre (of a line) in 35 or 36 seconds.

Similar motions have been seen in several other plants. In the cells of Hydrocharis Morsus-Ranæ the fluid has been observed to move round and round their sides in a rotatory manner, which, however, has not been seen to follow any particular law.

Pouchet and Meyen (Ann. Sc., n. s., iv. 257.) have remarked it in the longer cells of the stem of Zannichellia palustris, and the latter in Vallisneria, Stratiotes, Potamogeton, and the radical hairs of Marchantia. It may be distinctly seen in Equisetum. According to Schultz (Arch. Bot. ii. 425.), it is also visible in Podostemaceæ, Ceratophyllum, Naiadaceæ, Zosteraceae, Lemna, Mosses, Hepaticæ, Lichens, Algæ, and Fungi. The rotation in Vallisneria canadensis is most beautiful. In large cylindrical cells filled with a transparent fluid, there float large brilliantly green spherules, which rotate up one side and down another with a slow motion, sometimes crowding together, sometimes distant, and occasionally stopping. There is, moreover, among the woody tubes, a more rapid movement of very minute oval bodies, which goes on in lines upwards and downwards.

According to Meyen, the granules seen moving in the rotating currents are of different kinds (Ann. Sc., n. s., iv. 261.), the larger being grains of starch, others vesicles slightly coloured by chlorophyll, and some being drops of oil. I find but little trace of fæcula in Vallisneria, tincture of iodine chiefly producing a brown colour upon the granules, but here and there a blue nucleus was visible; perhaps the result would have been different, had the watery infusion of iodine been employed.

Of Cyclosis.

At page 35. a particular kind of tissue, called cinenchyma, or vessels of the latex, has been mentioned. It is in this

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