MODIFICATION OF SUCTIONAL RESPONSE 383 



the strong mechanical stimulus of a cut. I therefore devised 

 an experiment which appears to show that this run-down 

 of energy does in such a case constitute a factor in the 

 abolition of suction. Without in any way disturbing the 

 mucilaginous end of the stem, which had ceased to exhibit 

 its suctional activity, I supplied it with water somewhat above 

 the ordinary temperature. This thermal stimulation at once 

 initiated renewed suctional activity with great vigour, just as 

 its rhythmic mechanical activity was renewed by Desmodium 

 on the application of similar stimulus. 



Osmotic versus excitatory action.— Though, under the 

 co-operation of a favourable disposition of osmotic substances, 

 the suctional activity of the tissue may be increased, yet I 

 have shown that suction is normally maintained even with- 

 out the co-operation of this factor (p. 376). I shall now 

 proceed to show that this suction may increase even in 

 opposition to osmotic action. And such a demonstration will 

 further prove the excitatory physiological nature of the 

 processes which bring about the ascent of sap. Among 

 various solutions of salt, some are physiologically neutral in 

 their effects ; l of these, potassium nitrate may be taken as 

 an example. Others, again, like strong solutions of sodium 

 chloride, act as excitatory agents. The application of this 

 last reagent is known to initiate rhythmic excitation in animal 

 tissues. Similar effects have been shown to be brought about 

 by this reagent, in the case of Biophytum and Desmodium. 



Thus, in a strong solution of potassium nitrate, we have 

 a reagent whose physiological action is more or less neutral 

 while its osmotic action is pronounced, and in a strong 

 solution of common salt we have an agent which is both 

 excitatory and osmotic at the same time. If, then, we 

 apply KNO3 solution to the base of a cut stem, placed 

 in the Shoshungraph, water will be osmotically withdrawn 

 from the plant, in opposition to normal suction, and the 



1 It should, however, be remembered that solutions, even of inactive salts, 

 above 'a certain strength, will induce physiological depression, and thus bring 

 about diminution of transpiration. 



