5H PLANT RESPONSE 



directly, producing a movement towards, while in the other 

 the effect of stimulus is transmitted from a distance, produc- 

 ing a responsive movement in the opposite direction. As 

 against this assumption, however, we are confronted with the 

 fact, which we shall find later, that in some instances of 

 transmitted stimulus of light, the responding organ bends 

 towards, and not away from, the source of stimulation. 



Scope of the investigation.— We have therefore to 

 determine (i) whether or not the tips of all growing organs 

 behave alike ; (2) why the behaviour of the tip is different 

 from that of the responsive zone of growth ; and (3) under 

 what circumstances the transmitted effect of stimulus causes 

 an organ to move towards, and under what, to move away 

 from, the source of stimulation, at the same time ascertaining 

 clearly the mechanics of such movements. For the purpose 

 of this investigation I shall first use the electrical method of 

 inquiry, which I have already fully described, since it has 

 the unique advantage of offering unerring indications under 

 the most difficult experimental conditions ; and shall study by 

 its means the characteristic differences of response as between 

 the tip and growing region of a single organ, in this case the 

 shoot. 



Electrical investigation. — It will be shown that growth- 

 curvature is produced by unequal variations of turgidity on 

 the two sides of the responding organ. This variation can 

 be detected with great certainty by electrical means. I have 

 already explained how a positive turgidity-variation gives a 

 concomitant electrical variation of galvanometric positivity ; 

 and that the true excitatory effect of negative turgidity- 

 variation gives rise to a concomitant electrical change of 

 galvanometric negativity. For the present experiments 

 I took specimens of various growing plants — such as Bryo- 

 pkyllum, Cucurbita, and others — and the results obtained 

 from all were alike. In order to obtain unmistakable indica- 

 tions of the effect produced in the responding zone, one 

 electrical connection was made at a point in the growing 

 region, A (fig. 212, a), and the other with a point so distant 



