lower ends of some cuttings of the Apple-tree and Horse-chesnut into an infusion of the skins of a very black grape in water, an excellent liquor for the purpose. The result was similar. But Mr. Knight pursued his observations much further than Dr. Darwin had done; for he traced the coloured liquid even into the leaves, "but it had neither coloured the bark nor the sap between it and the wood; and the medulla was not affected, or at most was very slightly tinged at its edges." Phil. Trans, for 1801, p. 335.
The result of all Mr. Knight's experiments and remarks seems to be, that the fluids destined to nourish a plant, being absorbed by the root and become sap, are carried up into the leaves by these vessels, called by him central vessels, from their situation near the pith. A particular set of them, appropriated to each leaf, branches off, a few inches below the leaf to which they belong, from the main channels that pass along the alburnum, and extend from the fibres of the root to the extremity of each annual shoot of the plant. As they approach the leaf to which they are destined, the central vessels become more numerous,
