CHAPTER XXXVII 



THE RESPONSIVE PECULIARITIES OF THE TIPS OF 

 GROWING ORGANS 



Difference between shoot and root in their response to stimulus of gravity — 

 •Difference in character of response between tip and growing region of root — 

 Scope of the investigation — Electrical investigation — Responsive results of: I. 

 Longitudinal transmission of effect of stimulus from tip ; {a) Moderate uni- 

 lateral stimulation ; (b) effect of stronger unilateral stimulation— 2. Direct 

 unilateral stimulation of growing region— Moderately strong stimulus— 3. 

 Transverse transmission of effect of stimulus ; (a) moderate stimulation ; (b) 

 stronger stimulation — Mechanical response inferred from observed electrical 

 response — Tabular statement. 



We have seen in the last chapter that the responsive effect 

 of gravitational stimulus in an apogeotropic organ is of the 

 same nature as that of any other form of stimulation. In 

 positively geotropic organs like roots, however, this would 

 seem not to be the case, for here the responsive curvature 

 is in the opposite direction. Thus, a root placed horizontally 

 bends in the direction of gravity, and not away from it. 



It may be urged that there is some polar difference 

 between shoot and root, on account of which, if the response 

 of the one be regarded as positive, that of the other must be 

 negative ; but I have shown that, so far from this being the 

 case, the response of the root to stimulus is precisely the 

 same as that of all other organs, the shoot included, for all 

 alike under stimulation exhibit' contraction (p. 76). 



We shall first see, then, whether in the root there is any 

 difference, as regards the action of gravity, from, for instance, 

 the stem. We know that the growing stem with regard to 

 the gravitational stimulus is both the perceptive and respond- 

 ing organ ; for we may cut and isolate any portion of it, 



