counted for by Dr. Darwin, on the principle of their not collapsing when emptied of their sap; which is owing to their rigidity, and the elastic nature of their coats. When a portion of a stem or branch is cut off, the sap soon exhales from it, or rather is pushed out by the action of the vessels themselves: hence they are found empty; and for the same reason the arteries of animals were formerly thought to contain air only. When the sap-vessels have parted with their natural contents, air and even quicksilver will readily pass through them, as is shown by various experiments. Arguments in support of any theory must be very cautiously deduced from such experiments, or from any other observations not made on vegetables in their most natural state and condition; and, above all, that great agent the vital principle must always be kept in view, in preference to mere mechanical considerations.
These to which I give the common name of sap-vessels, comprehending the common tubes of the alburnum, and the central vessels, of Mr. Knight, may be considered as analogous to the arteries of animals; or rather
