they are the stomach, lacteals and arteries all in one, for I conceive it to be a great error in Dr. Darwin to call by this name the vessels which contain the peculiar secretions of the plant. These sap-vessels, no doubt, absorb the nutritious fluids afforded by the soil, in which possibly, as they pass through the root, some change analogous to digestion may take place; for there is evidently a great difference, in many cases, between the fluids of the root, at least the secreted ones, and those of the rest of the plant; and this leads us to presume that some considerable alteration may be wrought in the sap in its course through that important organ. The stem, which it next enters, is by no means an essential part, for we see many plants whose leaves and flowers grow directly from the root.
Part of the sap is conveyed into the flowers and fruit, where various fine and essential secretions are made from it, of which we shall speak hereafter. By far the greater portion of the sap is carried into the leaves, of the great importance and utility of which to the plant itself Mr. Knight's theory is the only one
- Phytologia, sect. 2.