THE PHOTOTROPIC CURVE AND ITS CHARACTERISTICS 35;i 



The maxim -iin tropic curvature was, in the present case, 

 reached in the course of nine minutes. The attainment of 

 this maximum depends on the excitability of the tissue, and 

 the intensity of incident stimulus. The characteristics that 

 have been described are not confined to the phototropic 

 curve but exhibited by tropic curves in general. Similar 

 characteristics have been found in the c irve for electric 

 stimulus (Fig. liK'a), and will also be m^t with in the curve 

 for geotropic stimulus (Fig. 161). 



I may here reftsr incidentally to the three types of 

 responses exhibited by an organ to successive stimuli of 

 uniform intensity ; these appear to correspond to the three 

 dilferent regions of tropic curve ; in the first stage, the 

 plant exhibits a tendency to exhibit a 'staircase' increase of 

 response ; in the intermediate stage, the response is uniform ; 

 and in the lust stage, the responses show a 'fatigue' decline. 



For purpose of simplicity, I first selected the simple 

 type of phototropic curve, where ,the specimen employed 

 was in a favourable tonic condition, and the stimulus was, 

 from the beginning, above the minimal. Transverse con- 

 duction, which induces neutralisation or reversal into 

 negative, was moreover absent in the specimen. I shall now 

 take up the more complex cases : (1) where the condition of 

 the specimen is slightly sub-tonic, (2) where the stimulus 

 is gradually increased from the sub-minimal, and (3) where 

 the specimen possesses the power of transverse conduction. 



EFFECT OF SUB-MINIMAL STIMULUS. 



It is unfortunate that the terms in general use for des- 

 cription of effective stimulus should be so very indefinite. 

 A stimulus which is just sufficient to evoke excitatory con- 

 traction is termed the minimal, stimulus below the thres- 

 hold being tacitly regarded as ineffective. The employ- 

 ment of sensitive recorders has, however, enabled me to 



