436 COMPARATIVE ELECTRO-PHYSIOLOGY 



cold. It is known that the continuous application of cold 

 diminishes or abolishes the excitability of a tissue. If, then, 

 it is by excitatory action on the upper side that curvature is 

 induced, it will be found that the local application of cold on 

 that side will retard or arrest this responsive curvature, the 

 application of cold on the lower side producing practically no 

 effect. If, on the contrary, this curvature should be mainly 

 due to some excitatory action on the lower side, we may then 

 expect to find that the application of cold on that side has 



FIG. 265. Effect on Apogeotropic Movement of Temporary Application 

 of Cold on Upper and Lower Surfaces respectively 



Application above is seen to produce arrest of movement, while application 

 below has no perceptible effect. Ordinate of curve represents up-move- 

 ment of tip of organ in mm ; abscissa represents time. 



the effect of arresting the growing curvature, its application 

 on the upper being more or less ineffective. On carrying out 

 these experiments it was found that cold had the effect of 

 arresting curvature only when it was applied on the upper 

 side of the shoot (fig. 265). This conclusively proved that 

 gravitational stimulus, acting on the horizontally-laid shoot, 

 induced response by excitatory contraction of the upper side. 

 This fact, of response by contraction, is fully concordant with 

 what we know of the effect on the plant of other forms of 

 stimulation. This question I intend, however, in the course 



