THE SIMPLE PHOTOTROPIC CURVE I49 



I use the necessary new term susceptibility to indicate 

 the relation between the stimulus and the resulting 

 excitatory contraction ; this latter will often be designated 

 by the shorter term excitation : 



.-.,., Excitatory contraction 



Susceptibility = -^rr- \ 



•^ -^ Stimulus 



Different organs of plants exhibit unequal suscepti- 

 bihties ; some undergo excitation under feeble stimulation, 

 while others require more intense stimulation to induce it. 

 Even in one identical organ the susceptibility will be found 

 to undergo a characteristic variation, being feeble at the 

 beginning, considerable in the middle, and becoming feeble 

 once more towards the end. 



The Simple Phototropic Curve 



The simple phototropic curve is obtained, as already 

 explained, by making the plant-organ record its movement 

 under continuous action of light applied on one side. Curves 

 were obtained in this way, of both pulvinated and growing 

 organs. 



Characteristic Curve of Erythrina indica 



Experiment 90.— A parallel beam of light from a Nernst 

 lamp was thrown on the upper half of the pulvinus of 

 a leaf of Erythrina, and the increasing positive curve was 

 recorded on a smoked-glass plate which was moved past the 

 writer by a clockwork at a uniform rate. The pulvinus, it 

 should be remembered, does not possess any transverse con- 

 ductivity. The record (fig. 84) is exactly reproduced from 

 the original by photomechanical process. The successive 

 horizontal dot-intervals of 20 seconds represent equal 

 increments of stimulation during successive equal lengths 

 of exposure. The vertical distances between the dots 

 represent, on the other hand, the corresponding increase 



