ASCENT OF THE SAl'. 23 



quently exerted a force of propulsion considerably 

 greater than the pressure of an additional atmos- 

 phere. The velocity, as well as the force of ascent, 

 must, however, be liable to great variation ; being 

 much influenced by evaporation, and other changes, 

 which the sap undergoes in the leaves. Various 

 opinions have been entertained as to the agency 

 by which the motion of the sap is effected ; but 

 although it seems likely to be resolved into the 

 vital movements of the cellular structure already 

 mentioned, the question is still enveloped in con- 

 siderable obscurity. There is certainly no evi- 

 dence 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 vegetable 

 tissue, and exerted, under certain circumstances, 

 and in conformity to certain laws, which wc have 

 not yet succeeded in determining. 



§ 3. Exlialation. 



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 imme- 

 diately, by the medium of the stomata, or orifices 

 which abound in the surface of those organs, sub- 

 jected to the \n^oce^soi 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 



