EXCITATORY CHARACTER OF SUCTION AL RESPONSE 39! 



theories brought forward to account for the ascent of sap 

 were admittedly inadequate. The further objections, urged 

 against the fundamentally excitatory nature of the processes 

 involved, on the ground of the important part played in the 

 ascent by sap-wood, generally regarded as dead, I have also 

 shown to be untenable. The sap-wood I have shown to be 

 not dead, but living, and to exhibit the normal response of 

 living tissues to excitation. The long persistence of suction, 

 when the roots are killed with hot water, or the cut 

 specimen placed in poison, was shown to be accounted for 

 by the fact that the death of any individual zone does not 

 arrest suction in those above. I have shown, moreover, that 

 those agents, such as rising temperature, which exalt -the 

 general physiological activity of the tissue, enhance suctional 

 activity also. Those which, like cold or anaesthetics, act, on 

 the other hand, to depress the general physiological activity, 

 will depress and arrest its suctional activity also. And, 

 finally, the fact that the water-movement is a form of 

 excitatory response has been fully demonstrated by the 

 experiments described and the records given in the course 

 of the present chapter. The physiological theory of the 

 ascent of sap may thus be regarded as established. 



