VEGETABLE EXHALATION. 27 



the question is still enveloped in considerable 

 obscurity. There is certainly no evidence to 

 prove that it has any analogy to a muscular 

 power; and the simplest supposition we can 

 make is that these actions take place by means 

 of a contractile property belonging to the vege- 

 table tissue, and exerted, under certain circum- 

 stances, and in conformity to certain laws, which 

 we have not yet succeeded in determining. 



'§ .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 compo- 

 sition, 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 proportion 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 matter 

 with which it was impregnated when first ab- 

 sorbed by the roots. The water thus exhaled, 



