425 Dr, Bell on the Vhyfiology of Plants, 



l*]jrface, it is natural to fuppofe, that the quantity 

 of thefe will very materially affedl the quantity 

 pf the perfpiration. Accordingly, the experi- 

 ments of Dr. Hales have afcertained, that the 

 perfpiration of vegetables is increafed or dimi- 

 niftied, chiefly, in proportion to the increafe or 

 diminution of their foliage.* The degree of 

 heat in which the plant was kept, according 

 to the fame author, varied the quantity of matter 

 perfpired ; this being greater, in proportion to 

 the greater heat of the furrounding atmofphere. 

 The degree of light has likewife confiderable 

 influence in this refpefl : for Mr. Fhilip Miller % 

 c;<periments prove, that plants uniformly per- 

 ipire moft in the forenoon, though the tempe- 

 rature of the air, in which they are placed, 

 Jhould be unvaried. Mr. Guettard likewife in- 

 forms us, that a plant, expofed to the rays of the 

 fun, has its perfpiration increafed to a much 

 greater degree, than if it had been expofed to 

 the fame heat, under the fhade. Finally, the 

 perfpiration of vegetables is increafed in pro- 

 portion as the atmofphere is dry, or in other 

 words, diminiflied in proportion as the atmo- 

 fphere is humid. 



The more vigorous and healthy the plant, the 

 more copious the perfpiration j this fundion, 

 like the reft, depending much on the vital 

 energy. ExcelTive perfpiration feems to hurt, 



* Statical EfTays, vol. I, p. 29. 



