GRADATION OF EXCITABILITY 163 



towards the light (positive curvature). This slow down- 

 movement continued for 40 seconds or more (left record, 



fig- 90)- 



Experiment 96. Effect of strong light. — The leaf 



responded by a fall, that is, positive curvature, after 

 a latent period of 4 seconds. This fall was slow for about 

 7 seconds, after which it became very abrupt (right record, 

 fig. 90). 



I would here draw special attention to the following 

 statement of results regarding the effect of strong light 

 applied on the upper and lower halves of the pulvinus : 



1. Strong light applied above induces a preliminary and 



short-lived up-response, detectable only by a sensitive 

 method of record. This up-response is followed by 

 a pronounced down-response, that is, with the fall 

 of the leaf. 



2. Strong light applied below induces a fall of the leaf and 



a down-response from the beginning, without any 

 other change. 



3. The effect apparently is the same, whether light be 



applied above or below, the direction of the responsive 

 movement being finally determined by the contraction 

 of the more excitable half of the organ. 



Physiological Detection of Gradation of 

 Excitability of Different Layers 



Some of the characteristics of the response led to the 

 important discovery of a gradation of excitability in the 

 different layers of tissue in the lower half of the pulvinus. 

 An inspection of the curve of response of strong Hght 

 applied on the lower half will make this clear {cf. right 

 record, fig. 90). The responsive fall of the leaf, due to 

 percolation of excitation upwards from layer to layer, was 

 at first slow. It became very abrupt as soon as the exci- 

 tation reached a certain tissue of the cortex intermediate 

 between the lowermost and uppermost layers in the lower 



