Page:An introduction to physiological and systematical botany (1st edition).djvu/81

Rh or subdivided. "To these vessels," says Mr. Knight, "the spiral tubes are every where appendages." p. 336. By this expression, and by a passage in the following page, 337, this writer might seem to consider the spiral line, which forms the coats of these vessels, as itself a pervious tube, or else that he was speaking of other tubes with a spiral coat, companions of the sap-vessels; but the plate which accompanies his dissertation, and the perspicuous mode in which he treats the subject throughout, prevent our mistaking him on the last point. In order to conceive how the sap can be so powerfully conveyed as it is through the vessels in which it flows, from the root of a tall tree to its highest branches, we must take into consideration the action of heat. We all know that this is necessary to the growth and health of plants; and that it requires to be nicely adjusted in degree, in