182 PHYSIOLOGICAL BOTANY. PART II. 



residing in the lowest extremities of the roots by which 

 the propulsion of the sap is regulated. Although these 

 results so closely resemble those of endosmose, there 

 still exists a difficulty in connecting the two phenomena ; 

 for whilst we may admit the possibility of an inter- 

 change between the contents of the vesicles composing 

 the spongioles, and the water in the soil which sur- 

 rounds them, by the ordinary operation of endosmose, 

 it is difficult to explain how the sap may be propelled 

 forward so violently as it appears to be, in the open 

 channels through the centre of the stem, which contain 

 crude sap of nearly the same specific gravity as water 

 itself. It would be further necessary to account for the 

 manner in which a continued supply of fresh materials 

 is obtained for carrying on the endosmose, which must 

 otherwise soon cease when the fluid within has become 

 much diluted. We shall find, however, that a constant 

 supply of fresh material is actually provided by the 

 direct action of the vital force, during a subsequent 

 period in the function of nutrition ; and hence it is not 

 impossible, though it has not been proved, that both 

 the propulsion as well as the absorption of the sap may 

 principally if not entirely be owing to the operation 

 of mechanical causes ; "dependent however for their 

 lengthened continuance upon the existence of the vital 

 energy by which those conditions are perpetually re- 

 newed, and without which the endosmose would of neces- 

 sity soon cease. Although therefore it is quite evident 

 that the immediate effects of the vital force must be some- 

 where present, and co-operative with the two pheno- 

 mena we have described, these themselves may be only 

 the secondary results, and not the direct effects of its 

 action. 



(ifi?.) Adfluxion. Another cause which promotes 

 the rise of the sap, is the continued discharge of moisture 

 which takes place from the surface of the leaves and 

 other parts, by a process to be described presently (art. 

 l68.). This effect produces a constant absorption from be- 

 low ; and thus a branch placed in water gradually imbibes 



