CHAPTER XL 



ON CHEMO-TROPISM AND GALVANO-TROPISM 



General difficulties of the investigation — How to overcome these difficulties— 

 Three distinct methods of testing results : ( i ) by variation of longitudinal 

 growth ; (2) by responsive movement of pulvinus ; and (3) by growth-curva- 

 ture — Method of application of chemical reagent — Effect of alkali — Effect of 

 acid — Effect of copper sulphate — Action of sugar solution — Chemo-tactic 

 movements— Explanation of anomalous osmotic or plasmolytic action — Excita- 

 tory versus plasmolytic reaction in pulvinus of Mimosa : ( i ) Favourable tonic 

 condition— (2) Ordinary tonic condition — Polar effects of currents inducing 

 growth-curvatures — Localised polar effects on pulvinus — Anodic and kathodic 

 effects on longitudinal growth — Generalised law of polar excitation in plants — 

 GalVano-tropic response — The indirect effect of polar excitation — The effect on 



. growth of ' electrification ' of soil. 



It will be understood that growth-curvatures take place so 

 slowly, that the effects of various external agents in inducing 

 them can only be observed visually after the lapse of con- 

 siderable intervals of time, which may be a matter of hours, 

 or even of days. This fact is sufficient of itself to introduce 

 elements of complication into the problem ; for (i) the 

 plant may in that time undergo unknown spontaneous 

 variations ; (2) the fundamental effect of any given stimulus 

 is subject, when too long continued, to reversal, as we have 

 seen in the case of the leaf of Mimosa, which from normal 

 contraction passes into fatigue-relaxation, under the long- 

 continued action of stimulus ; (3) the subjection of a plant 

 during too long a period to an abnormal condition may 

 sometimes cause derangement of its general functions ; and 

 (4) there is the further element of variation which depends 

 upon the point of application of stimulus itself, since we have 

 seen that stimulation, acting directly on the responding organ, 

 may produce one effect, and indirectly exactly the opposite. It 



