RESPONSE OF TIPS OF SHOOT AND ROOT 517 



wire, heated by electrical current, is placed in the neighbour- 

 hood of the point to be stimulated. The intensity of stimulus 

 in this case is regulated by varying either the strength of the 

 heating current, or the distance of the heating wire from the 

 point to be excited. All the different forms of stimulation 

 will be shown to produce the same results. 



Responsive results of : i. Longitudinal transmission of 

 effect of stimulus from tip : (a) Moderate unilateral stimula- 

 tion. — Using the thermal mode of stimulation applied at T (fig. 

 212, a), I obtained positive electric-variation at A, the record 

 of which is given in fig. 213. A similar result was obtained 



Fig. 213. Record of Positive 

 Electrical Variation, indicating 

 Positive Turgidity- Variation 

 (represented by Down Curve), 

 induced in Growing Region 

 by Moderate Stimulation on 

 same side of Tip. Time- 

 marks = minutes 



Fig. 214. Record showing 

 Galvanometric Positivity 

 subsequently Neutralised 

 under Transmission of 

 True Excitatory Effect, 

 due to Continuance of 

 Moderate Stimulation of 

 the Tip 



with the mechanical stimulation of a pin-prick. The same 

 was again observed on effecting stimulation by dilute acid. 



(b) Effect of stronger unilateral stimulation. — I next pro- 

 duced a somewhat stronger thermal stimulation by suitably in- 

 creasing the heating current. This gave rise to the electrical 

 indication of the preliminary positive ttirgidity-variation, as the 

 immediate effect. But the long-continued action of the stimulus 

 caused the transmission of the true excitatory, which, reaching 

 A, caused the neutralisation of the previous effect (fig. 214). 

 With another specimen, I next applied to the tip a still stronger 

 unilateral stimulus. This caused a brief positive electrical 

 and turgidity variation, followed by a reversed response of 



