Page:Proceedings of the Royal Society of London Vol 1.djvu/139

Rh bark of the root, is carried up by the albumum, or white wood of the root, the trunk, and the branches; that it passes through what .he calls the central vessels into the succulent part of the annual shoot, the leaf-stalk, and the leaf; and that it thence returns to the bark through the returning vessels of the leaf-stalk. The principal object of the present paper is to point out the causes of the descent of the sap through the bark, and the consequent formation of wood.

The causes to which he ascribes this descent are: l. Gravitation; 2. Motion communicated by winds or other agents; and 3. Capil- lary attraction, and perhaps some peculiar circumstances in the con- formation of the vessels themselves, which renders them better cal- culated to carry ﬂuids in onc'direction than in another.

Before he proceeds upon the experiments from which he has de- duced these conclusions, he premises a few observations on the func- tions of the leaf, from which all the descending ﬂuids in the tree ap- pear to be derived. He describes an experiment he made on a leaf of a vine, in which its lower surface being placed in contact with a clean piece of plate glass, this glass was soon found to be covered with a strong dew, which had evidently exhaled from the leaf; and at the end of half an hour so much water was found to have been discharged from the leaf, that it ran from the glass when it was held obliquely. The position of the leaf being then inverted, and its upper surface being brought in contact with the glass, not the slightest portion of moisture appeared, although the leaf was for some time exposed to the full inﬂuence of the meridian sun. Hence it is in- ferred, that the vessels intended for perspiration are conﬁned to the under surface of the leaf, and that these, like the cutaneous lym- phatics of the animal (economy, are also capable of absorbing mois- ture when the plant is in a state to requir, whereas the upper surface seems chieﬂy formed for absorbing light; and if anything exhale from it, it is probably vital air, or some other permanently elastic ﬂuid,

Reverting now to the principal object of his paper, the author de- scribes an experiment on a shoot of a vine, which he bent downwards nearly in a perpendicular direction. After it had grown some time in this position, and acquired a ligneous texture, he stripped the bark from a part of it, and thus cut off all communication through the bark between the shoot and the parent stem. Former experiments have shown, that had this shoot grown in its erect position, the lip of the bark above the wound would have shown an accumulation of fresh wood and bark ; but in this instance the contrary was found to be the case; the lip next to the stem, which by its position was now uppermost, gave evident signs of this accumulation. This is ascribed to the gravitation of the sap, from the curvature of the shoot down to the lip. The result of this experiment seems to point out one of the causes why perpendicular shoots grow with much greater vigour than those which are inclined or horizontal, they having probably a more perfect and rapid circulation.

The effect of motion on the circulation of the sap was deduced