USES OF LEAVES. 139 



ready saturated with vapour, we prevent its further 

 escape from the leaves, which must of necessity cease 

 to transpire. They retain the same fluids with which 

 they are already supplied, and though they perform 

 none of the actions, they exhibit the appearance of per- 

 fect health. Thus they may be preserved for weeks, 

 and thus the botanical traveller, who expects to de- 

 rive every advantage from his journey, will collect 

 and preserve the plants that meet his eye, till he has 

 leisure to examine them. 



The quantity of vapour thus exhaled, though various, 

 is commonly greater than at first view we should be 

 prepared to expect. When the sun is bright, and the 

 atmosphere dry and warm, leaves perspire more copi- 

 ously, than when Clouds obscure the brightness of the 

 one, and diffuse a moisture through the other. In the 

 former case, the mown grass is converted into hay in a 

 few hours, in the latter, it is so tenacious of its fluids 

 as to baffle every effort of the farmer to hasten the 

 process. 



In general, succulent plants exhale more sparingly 

 than others. It seems to have been the design of Na- 

 ture, that they should inhabit the burning sands of the 

 Torrid Zone, and the peculiarity of their native situa- 

 tion, makes it necessary for them to preserve the 

 fluids, which, with so much difficulty they procure. 

 But plants with thin membranaceous leaves which 

 generally occupy moist situations, where they are sup- 

 plied with an abundance of water, perspire very copi- 

 ously. The Sunflower, which is very frequently met 

 with in the United States, was found to exhale two 

 pounds in the course of a day ; and in the same space 

 of time the Cornelian Cherry, a shrub with thin and 

 almost transparent leaves, growing in the hedges of 



