Page:An introduction to physiological and systematical botany (1st edition).djvu/86

that gives us any adequate or satisfactory notion. In those organs the sap is exposed to the action of light, air and moisture, three powerful agents, by which it is enabled to form various secretions, at the same time that much superfluous matter passes off by perspiration. These secretions not only give peculiar flavours and qualities to the leaf itself, but are returned by another set of vessels, as Mr. Knight has demonstrated, into the new layer of bark, which they nourish and bring to perfection, and which they enable in its turn to secrete matter for a new layer of alburnum the ensuing year. It is presumed that one set of the returning vessels of trees may probably be more particularly destined to this latter office, and another to the secretion of peculiar fluids in the bark. See Phil. Trans, for 1801, p. 337. In the bark principally, if I mistake not, the peculiar secretions of the plant are perfected, as gum, resin, &c., each undoubtedly in an appropriate set of vessels. From what has just been said of the office of leaves, we readily perceive why all the part of a branch above a leaf or leaf-bud dies when cut, as each portion receives nou-