VEGETABLE EXHALATION. 27 



3. Exhalation. 



THE nutrient sap, which, as we have seen, rises in the 

 stem, and is transmitted to the leaves without any change in 

 its qualities or composition, is immediately, by the medium of 

 the stomata, or orifices which abound in the surface of those 

 organs, subjected to the process of exhalation. The propor- 

 tion of water which the sap loses by exhalation in the leaves, 

 is generally about two-thirds of the whole quantity received; 

 so that it is only the remaining third that returns to nourish 

 the organs of the plant. It has been ascertained that the 

 water thus evaporated is perfectly pure ; or, at least, does not 

 contain more than a 10,000,000th part of the foreign mat- 

 ter 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 daily loss of the plant by exha- 

 lation was twenty ounces ; and this, he computes, is a quanti- 

 ty seventeen times greater than that lost by insensible per- 

 spiration 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 tem- 

 perature, 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 arti- 

 ficial 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 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 



* Physiologic Vegetale, i. 112. 



