DIA-PHOTOTROPISM AND NEGATIVE PHOTOTROPISM 329 



effect of unilateral stimulus of light on the responsive 

 movement of main pulvinus of Alifnosa piidica. The 

 results will be found of much theoretical interest, since a 

 single experiment will give an insight to all possible types 

 of phototropic response. Before describing the experiment 

 I shall demonstrate the tropic reactions of the two halves 

 of the pulvinus of Jlitnosa. 



UNEQUAL EXCITABILITY OF UPPER AND LOWER HALVES OF 

 PULVINUS TO PHOTIC STIMULATION. 



I have by method of selective amputation shown that 

 as regards electric stimulation the excitability of the upper 

 half of the pulvinus is very much less than that of the 

 lower half (p. 85). I have obtained similar results with 

 photic stimulation. 



Tropic effect of light acting from above : Experiment 

 124. — Light of moderate intensity from an incandescent 

 electric lamp was applied on the upper half of the pul- 

 vinus of Mimosa for 4 minutes ; this induced a contraction 

 of the stimulated upper half and gave rise to an up or 

 erectile response. On the stoppage of light recovery took 

 place in the course of ten minutes. The phototropic 

 curvature is thus seen to be positive. A series of such 

 positive responses of the upper half of the pulvinus is 

 given in figure 118. 



9 



Effect of light acting from below : Experiment 125. — 

 Light was now applied from below ; this also induced a 

 contraction of the lower half of the pulvinus, causing a 

 down-movement (Fig. 119). As the responsive movement is 

 towards light, the phototropic effect must be regarded as 

 positive. The greater excitability of the lower half of the 



2(3 k 



