3/6 COMPARATIVE ELECTRO-PHYSIOLOGY 



evaporation, in the leaves. That this osmotic effect is, 

 however, merely secondary, and that the ascent is chiefly 

 due to excitatory action, is seen in certain experiments 

 which may be mentioned here, in which the ascent takes 

 place with even greater vigour than before, when it is opposed 

 by an osmotic influence. 



The various solutions of salts are very unequal in their 

 physiological action : some, like potassium nitrate, are neutral, 1 

 but others, as strong solutions of sodium chloride, are ex- 

 citatory. Thus the action of 

 a strong solution of potassium 

 nitrate is physiologically more 

 or less neutral, while its osmotic 

 action, at the same time, is 

 pronounced. A strong solution 

 of common salt, on the other 

 hand, is both excitatory and 

 osmotic. 



If then we apply KNO ;3 

 solution to the cut end of a 

 stem, water will be osmotically 

 withdrawn from the plant, in 

 opposition to the normal ascent 

 ,..._ of sap. There will thus be a 



ric. 221. Effect of Strong KNO S 



Solution depression of the rate of suction 



The first record shows the normal after the application, as seen in 



and the second the depressed rate r .. ir 



of suction caused by the reagent. the following record (fig. 22 l), 



which I obtained with a cut 



branch of Croton. If, however, in a similar experiment, 

 a strong solution of NaCl be applied, two antagonistic 

 reactions will be set up. One, due to the osmotic action, 

 will oppose suction, and the other, due to the excitatory 

 nature of the reagent, will accelerate it, while the resultant 

 effect will be modified by the excitability of the experimental 

 plant. In fig. 222 is shown the effect of NaCl solution in 



1 It should IK- mentioned here, that even such neutral salts, in strong solution, 

 induce physiological depression. 



