24 THE VITAL FUNCTIONS. 



evaporated is perfectly pure ; or at least does not 

 contain more than a 10,000,00()th part of the foreign 

 matter with which it was impregnated when first 

 absorbed by the roots. The water thus exhaled, 

 being dissolved by the air the moment it escapes, 

 passes off in the form of invisible vapour. Hales 

 made an experiment with a sun flower, three feet 

 high, enclosed in a vessel, which he kept for fifteen 

 days ; and inferred from it that the weight of the 

 fluid daily exhaled by the plant was twenty ounces ; 

 and this he computes is a quantity seventeen times 

 greater than that lost by insensible perspiration 

 from an equal portion of the surface of the human 

 body. 



The comparative quantities of fluid exhaled by 

 the same plant at different times are regulated, not 

 so much by temperature, as by the intensity of the 

 light to which the leaves are exposed. It is only 

 during the day, therefore, that this function is in 

 full activity. De Candolle has found that the 

 artificial light of lamps produces on the leaves an 

 effect similar to that of the solar rays, and in a 

 degree proportionate to its intensity.* As it is 

 only through the stomata that exhalation proceeds, 

 the number of these pores in a given surface must 

 considerably influence the quantity of fluid ex- 

 haled. 



By the loss of so large a portion of the water 

 which, in the rising sap, had held in solution 

 various foreign materials, these substances are ren- 

 dered more disposed to separate from the fluid, and 

 to become consolidated on the sides of the cells 



* Physiologic Vegetale, i. 1 I '2. 



