Cornus mascula lose twice their own weight a day. This perspiration is regulated by the number of the stomata: hence evergreens, in which they are small, and less numerous than in deciduous or herbaceous plants, perspire much less.

With this function are connected all the phenomena that attend transplantation. If a growing plant is removed from one situation to another in the summer, it will die; because its spongioles will be so much destroyed as to be incapable of absorbing fluid from the soil as fast as it is given off by the leaves; and hence the system will be emptied of fluid. But if a plant is growing in a pot, it may be transplanted at any season of the year; because its spongioles, being uninjured, will be able to counterbalance the loss caused by perspiration, as well after transplantation as before, if not better.

With regard to the vessels through which this universal diffusion of the sap takes place, it has already been stated that its upward course is always through the woody fibre, and probably also through the ducts; and that it passes downwards through the woody fibre. But there can be no reasonable doubt that it is also dispersed through the whole system by means of some permeable quality of the membranes of the cellular tissue, which is invisible to our eyes, even aided by the most powerful glasses. It has also been suggested that the sap finds its way upwards, downwards, and laterally through the intercellular passages which exist at the points of union of every individual elementary organ. That such a channel of communicating the sap is employed by Nature to a certain extent I do not doubt, especially in those plants in which the intercellular passages are very large; but whether this is an universal law, or has only a partial operation, is quite unknown, and is not perhaps susceptible of absolute proof. Link seems disposed to deny any conveyance of fluids through the intercellular passages.

The accumulation of sap in plants appears to be attended with very beneficial consequences, and to be deserving of the especial attention of gardeners. It is well known how weak and imperfect is the inflorescence of the turnip tribe, forced to flower before their fleshy root is formed; and how vigorous it is after that reservoir of accumulated sap is completed. Mr.

Knight, in a valuable paper upon this subject, remarks that the fruit of melons, which sets upon the plant when very young, uniformly falls off; while, on the contrary, if not allowed to set until the stem is well formed, and much sap accumulated for its support, it swells rapidly, and ripens without experiencing any deficiency of food in the course of its growth. In like manner, if a fruit tree is by any circumstance prevented bearing its crop one year, the sap that would have been expended accumulates, and powerfully contributes to the abundance and perfection of the fruit of the succeeding year. And again, the plan recommended by Mr. Knight, of always planting large tubers of the potatoe, is another proof of the importance of plenty of accumulated sap to the vigorous growth of all plants.

The cause of the motion of the sap is a subject which has long excited great curiosity, and has given rise to numberless conjectures. It was for a long time believed that there was a sort of circulation of the sap of plants, to and from a common point, analogous to that of the blood of animals; but this was disproved by Hales, and is not now believed. This excellent observer, whose "Statics" are an eternal monument of his industry and skill, thought that the motion of the sap, the rapidity of which he had found to be greatly influenced by weather, depended upon the contraction and expansion of the air, which exists in great quantities in the interior of plants. Others have ascribed the motion to capillary attraction. Mr. Knight was once of opinion that it depended upon a hygrometrical property of the plates of silver grain (medullary rays), which traverse the stem in all directions. A number of other theorists have called to their aid a supposed irritability of the vessels; but no contraction of the vessels has ever yet been noticed, and certainly does not take place in Chara, where the motion has been most distinctly observed. Du Petit Thouars suggests that it arises thus:-in the spring, as soon as vegetation commences, the extremities of the branches and the buds begin to swell: the instant this happens a certain quantity of sap is attracted out of the circumjacent tissue for the supply of those buds; the tissue, which is thus emptied of its sap, is filled instantly by that beneath or

about it: this is in its turn replenished by the next; and thus the whole mass of fluid is set in motion, from the extremities of the branches down to the roots. Du Petit Thouars is therefore of opinion that the expansion of leaves is not the effect of the motion of the sap, but, on the contrary, is the cause of it; and that the sap begins to move at the extremities of the branches before it stirs at the roots. That this is really the fact, is well known to foresters and all persons accustomed to the felling or examination of timber in the spring. Some good observations upon this were communicated to Mr. Loudon's Gardener's Magazine, by Mr. Thomson, gardener at Welbeck; who, however, drew a wrong inference from them.

Amici is of opinion that the motion in Chara depends upon galvanic action; and Dutrochet has since formed a theory of all the motions of fluids in plants depending upon the same agency. 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 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, M. 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 most powerful: and that the fluids of plants being more dense than those which surround 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 M. 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, is 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: it will, perhaps, be found the most ready explanation of most of the phenomena connected with the movement of fluids.

CHAPTER IV.

OF THE PITH, WOOD, AND BARK.

VARIOUS are the notions that from time to time have been entertained about the PITH. The functions of brain, lungs, stomach, nerves, spinal marrow, have by turns been ascribed to it. Some have thought it the seat of fecundity, and have believed that fruit trees deprived of pith became sterile; others supposed that it was the origin of all growth; and another class of writers, we cannot say observers, have declared that it was the channel of the ascent of sap. It is, however, no part of the plan of this work to refute this and similar exploded speculations.

It is probable that its real and only use is to serve in the infancy of a plant for the reception of the sap, upon which the young and tender vessels that surround it are to feed when they are first formed; a time when they have no other means of support. M. Dutrochet considers it to act not only as a reservoir of nutriment for the young leaves, but also to be the place in which the globules, which he calls nervous corpuscles, are formed out of the elaborated sap. (L'Agent Immédiat, &c., p. 44, &c.)

The MEDULLARY SHEATH seems to perform a far more important part in the economy of plants; it diverges from the medulla whenever a leaf is produced, and, passing through the petiole, ramifies among the cellular tissue of the lamina, where it appears as veins: hence veins are always composed of bundles of woody fibre and spiral vessels. So situated, the veins are in the most favourable position that can be imagined for absorbing the fluid that, in the first instance, is conducted to the young pith, and that is subsequently impelled upwards through the woody fibre. So essential is the medullary sheath to vegetation in the early age of a branch, that, as is well known, although the pith and the bark, and even the young wood, may be destroyed, without the life of a young