50 NUTRITION. 



which surrounds them, by the ordinary operation 

 of endosmosis, 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 

 material is obtained for carrying on the endos- 

 mosis, 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 renewed, and without 

 which the endosmosis would of necessity soon 

 cease." (Henslow's Principles of Botany, pp. 

 181-2.) 



40. It would seem natural here to observe 

 what course the sap takes in its rise in the plant, 



