CHAPTER XXVI 



THE EXCITATORY CHARACTER OF SUCTIONAL. RESPONSE 



Propagation of excitatory wave in plant attended by progressive movement of 

 water Hydraulic response to stimulus The Shoshungraph Direct and photo- 

 graphic methods of record Responsive variations of suction under physiological 

 modifications induced by various agents Effects of lowering and raising of 

 temperature Explanation of maintenance of suction, when root killed 

 Effect of poison influenced by tonic condition Effect of anaesthetics on 

 suctional response Excitatory versus osmotic action Stimulation by alter- 

 nating induction-shocks Terminal and sub-terminal modes of application 

 Three modes of obtaining response-records, namely (i) the unbalanced, 

 (2) the balanced, (3) the over-balanced Renewal of suction previously at 

 standstill, by action of stimulus Reponsive enhancement of suction by 

 stimulus After-effect of stimulus Diminution of latent period as after-effect 

 of stimulus Response under over-balance Response under sub-terminal 

 stimulation Variation of response under seasonal changes. 



IN the last chapter it was shown that the various objections 

 hitherto urged against the excitatory nature of the ascent of 

 sap were not justified. In the course of the present chapter, 

 therefore, I shall adduce proofs that the water- movement in 

 the plant is the result of stimulatory action. Instead of 

 vaguely referring the phenomenon to physiological activity, 

 moreover, we shall attempt, proceeding from the basis 

 of other excitatory reactions, already clearly established, 

 first to see whether inferences based on these are capable of 

 explaining the present problem, and secondly, to subject 

 those inferences themselves to the test of experimental in- 

 vestigation. 



We cut a certain length of the stem of Mimosa, and keep 

 it immersed for some time in a very dilute solution of common 

 salt, until the tissue has become charged with this. The 

 specimen is then taken out, and thoroughly rinsed with clean 

 water. It is now held vertically, with the lower end dipped 



