THE PHOTOTROPIC CURVE AND ITS CHARACTERISTICS 347 



excitation ; but it became effective on being repeated 

 four times in rapid succession. 



(2) The same specimen was next subjected to a feebler 

 stimulus of intensity of 0*1 unit, and it required a re- 

 petition of 20 times for the stimulus to become effective. 



The total stimulus in the first case was 0'5x4=2, and 

 this was found to be the same as 0'lx20=2 in the 

 second case. Thus the intensity of stimulus is increased 

 by repetition ; in the limiting case where the interval 

 between successive stinmlus is zero, the stimulus becomes 

 continuous. Bearing in mind the additive effects of sti- 

 mulus we see that its effective intensity increases with the 

 duration of applica*^ion. This important conclusion found 

 independent support from the results of Experiment 133 

 given in the last chapter. 



We shall now take up the general question of the 

 characteristic^ of the phototropic curve, which gives the 

 relation between increasing stimulus and the resulting exci- 

 tation. As regards stimulus we found that its effective- 

 ness increases with the duration of application. The 

 induced excitation in growing organs may be measured by 

 concomitant retardation of growth caused by stimulus. In 

 the excitation curves which will be presently given, the 

 abscissae represent increasing stimulus and ordinates the 

 resulting excitation. This excitation curve may be ob- 

 tained by making the plant record on a moving plate its 

 retardation of growth by means of the High Magnification 

 Crescograph. I reproduce below two records of the effects 

 of continuous photic and electric stimulation. The ordinate 



