EFFECTS OF AIR ON LEAVES. 153 



directions; and perhaps the only point they correctly 

 establish is, that air is imbibed through the bark, a 

 part known to be full of air-vessels. But the seventh 

 chapter of the Vegetable Statichs contains some re- 

 marks much more to our purpose. Dr. Hales there 

 clearly anticipates by conjecture, what succeeding phi- 

 losophers, more enlightened chemists, have ascertained. 

 His words are remarkable : 



" We may therefore reasonably conclude, that one 

 great use of leaves is what has been long suspected by 

 many, vis. to perform in some measure the same office 

 for the support of the vegetable life, that the lungs of 

 animals do, for the support of the animal life ; plants 

 very probably drawing through their leaves some part 

 of their nourishment from the air.' 1 p. 326. A little 

 further on he adds, " And may not light also, by freely 

 entering the expanded surfaces of leaves and flowers, 

 contribute much to the ennobling the principles of 

 vegetables ?" p. 328. 



Next in order of time to those of Hales follow the 

 experiments of Bonnet. We have already detailed his 

 observations on the power of leaves to imbibe moisture; 

 whence it is ascertained that plants are furnished with 

 a system of cuticular absorbents, which carry fluids 

 into their sap-vessels, so as to enable them in some 

 degree to dispense with supplies from the root. With 

 respect to the effects of air upon leaves, this ingenious 

 philosopher has not been equally successful. He is re- 

 corded as the discoverer of the expiration of plants; but 



