temperature of the air, the time of the 

 day, and the humidity of the atmosphere. 

 Leaves form the greatest part of the sur- 

 face, and it is found that the quantity of 

 these very materially affect the quantity of 

 perspiration ; and this process is increas- 

 ed or diminished, chiefly, in proportion 

 to the increase or diminution of the fo- 

 liage of vegetables. The degree of heat 

 in which the plant is kept also varies the 

 quantity of matter perspired ; this being 

 greater, in proportion to the greater heat 

 of the surrounding atmosphere. The 

 degree of light has likewise considerable 

 influence in this respect ; for plants uni- 

 formly perspire most in the forenoon, 

 though the temperature of the air, in 

 which they are placed, should be unvari- 

 ed. A plant also exposed to the rays of 

 the sun, has its perspiration increased to 

 a much greater degree than if it had 

 been exposed to the same heat under the 

 shade. Finally, the perspiration of veg- 

 etables is increased in proportion as the 

 atmosphere is dry, or, in other words, 

 diminished in proportion as the atmos- 

 phere is humid. The more vigorous 

 and healthy the plant, the more copious 

 the perspiration j this function, like the 



