Page:An introduction to physiological and systematical botany (1st edition).djvu/229

 Rh spiral vessels were believed to convey all through the plant, in order that it might act on the sap as it does on the animal blood. The analogy thus understood was not correct, because air is conveyed no further than the lungs of animals; but without this hypothesis no use could be found for the supposed longitudinal air-vessels.

The observations of Dr. Hales come next in order to those of Grew and Malpighi. By means of the air-pump, an instrument much in use in his time, Hales obtained abundance of air from every part of the vegetable body, as well as from recently extracted sap. Plants were found to perish very soon in an exhausted receiver. Some of this great man's experiments, however, require to be received with caution. He rightly remarked that air was not only taken in by plants very copiously along with their food, but also imbibed by their bark; see Veg. Staticks, chap. 5. But when, from observing that it would freely from the bark pervade the longitudinal vessels of a branch, he concluded that Malpighi and Grew were right in their ideas of longitudinal air-vessels, he was misled by