28 THE VITAL FUNCTIONS. 



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 daily 

 loss of the plant by exhalation was twenty ounces ; 

 and this he computes is a quantity seventeen 

 times greater than that lost by insensible perspi- 

 ration 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 regu- 

 lated, 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 activity. De Candolle 

 has found that the artificial light of lamps pro- 

 duces on the leaves an efl'ect 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 in- 

 fluence the quantity of fluid exhaled. 



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 

 rendered more disposed to separate from the 

 fluid, and to become consolidated on the sides 



* Physiologie Vegetale, i. 112. 



