l62 CHAP. XV. PHOTONASTIC PHENOMENA 



The velocity of transmission of excitation through the 

 upper half of the pulvinus can be found from the data 

 given by the record. The thickness of the pulvinus was 

 2 mm., that of the upper half was i mm. approximately. 

 The time of vertical transmission can be found from the 

 record, being the interval between the application of 

 stimulus and the moment of reversal from up to down 

 movement of the leaf. In the present case it was found 

 to be i8 seconds. From this the rate of transmission across 

 the upper half of the organ can be ascertained : 



Velocity of transmission = — = o -05 mm. per second. 



This velocity I find to vary from o-oi to o-i mm. per 

 second, for the following reasons : 



The velocity is increased {a) when the thickness of the 

 pulvinus is small ; {b) when the temperature is near the 

 optimum ; (c) when the season is favourable ; {d) when 

 the tissue is in a favourable tonic condition ; and (e) when 

 the stimulus is strong. The velocity is decreased under 

 opposite conditions. 



Effect of Light on the Lower Half of the 



Pulvinus 



Experiment 95. Effect of moderate light. — After a 

 latent period of 7 seconds the leaf began to fall downwards 



Fig. 90, Effect of feeble (left record) and of strong light (right record) 

 on lower half of pulvinus (Mimosa). Dot-intervals i second. 



