152 AIR-VESSELS OF THE LEAVES. 



pump, an instrument much in use in his time, Hales 

 obtained abundance of air from every part of the ve-. 

 getable 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, how- 

 ever, 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. Statick$$ chap. 5. But when, 

 from observing that it would freely from the bark per- 

 vade 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 appearances. 

 We cannot but be aware that, when a branch is 

 gathered, the sap must soon flow out of those spiral 

 coated tubes, which are large, elastic, and, no doubt, 

 irritable. After they are emptied, air may unquestion- 

 ably pass through them, especially when the whole 

 weight of the atmosphere is acting, as in Dr. Hales's 

 experiments with the air-pump, upon so delicate a 

 fabric as the internal vascular structure of a plant* 

 forcing its way through pores or membranes not na- 

 turally designed to admit it. We must also recollect 

 that a plant, cut even for a short time, begins to lose 

 its vital principle, after which no just judgement can 

 be formed, by any experiments, concerning-the move- 

 ments of its fluids in life and vigour. See Chapter 1 . 

 These experiments of Dr. Hales therefore prove no 

 more than that the vegetable body is pervious in various 



