The causes which produce the ascent of the sap are of a very powerful nature. The celebrated Hales ascertained that a vine branch, in a few days, sucked up water with a force equal to the weight of sixteen pounds on the square inch: this was a power greater than atmospheric pressure; and when it is recollected that the pressure of the atmosphere is capable of lifting thirty-three or thirty-four feet of water in a common pump, some estimate may be formed of the force with which the sap ascends. This ascent appears to be produced by the influence of two causes: the one, a quality peculiar to living beings, by which the buds in common with all growing organs are capable of attracting or sucking towards them the juices necessary for their nutrition; and in agreement with this, the sap is found to ascend in the first instance near the buds: the other, a general property of all matter which has been but lately discovered. This latter property, which has been called endosmose, is found to operate when two fluids of different densities are separated by a membrane. Under these circumstances, and in obedience to an attraction for each other, both fluids pass through the membrane, and mix together; but the denser and thicker fluid finding a greater difficulty to penetrate the membrane than the lighter and thinner, consequently passes through in less quantity. To illustrate this, let us suppose a bladder containing a little syrup, and placed in a vessel of water, and we will have the conditions necessary for endosmose: the syrup and water will both pass through the bladder in opposite directions, but a greater quantity of water will pass into the syrup, than of the latter into the water. It will be evident to the reader that this excess of thin liquid passing into the denser will constitute a force or power which will require an equal force to neutralise it; and it has been ascertained that the tendency of water to penetrate a membrane for the purpose of mixing with a syrup of once and a third its own specific weight, required a force equal to sixty-three pounds on the square inch to overcome it. Now, a plant growing in the ground is similarly circumstanced to the bladder in this experiment: its roots furnished with extremities of spongy membrane are interposed between thin water and carbonic acid externally, and a syrupy solution of sugar and gum internally. Now, under those circumstances we need not be surprised if an endosmose should operate, abundantly sufficient to elevate the sap with a force even greater than that determined by Hales.