164 AIR-VESSELS OP THE LEAVES. 



abundance of air from every part of the vegetable 

 b()d\ , as well as from recently extracted sap. Plants 

 were found to perish very soon in an exhausted receiv- 

 er. Some of this great man's experiments, however, 

 require to be received w-ith caution. He rightly re- 

 marked that air was not only taken in by plants very 

 copiously along with their food, but also imbibed by 

 their bark ; see Feg. Staticks, chap. 5. But when, 

 from observing that it would freely from the bark per- 

 vade the longitudinal vessels of a branch, he concluded 

 th tt Mjlpighi 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 gather- 

 ed, 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 unquestionably pass 

 through theni, especially when the whole w^eight of the 

 aimosphere is acting, as in Dr. Hales' experiments with 

 the air-pump, upon so delicate a fabric as the internal 

 vascular structure of a plant, forcing its way through 

 pores or ment>branes not naturally 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 judgment can be formed, by any experiments, con- 

 cerning the movements of its fluids in life and vigour^ 

 See Chapter 1, These experiments of Dr. Hales there- 

 fore prove no more than that the vegetable body is per- 

 vious in various directions ; and perhaps the only point 

 they correctly establish is, that air is imbibed through 

 tfii- baik, a part known to be full of air-vessels. But 

 tilt L^eventh chapter of the Vegetable staticks contains 



