67O PLANT RESPONSE 



the stimulus. Now, in a naturally torsioning organ, such an j 

 induced torsional movement must obviously be opposite in 

 direction to the natural movement caused by internal energy. \ 

 This will be found to be illustrated in the modification in- 

 duced in autonomous torsion by the unilateral application of 

 light, as shown in fig. 272, where the first part of the curve 

 shows the normal negative torsion of the plant. At the 

 point marked with the upward arrow, light from a thirty-two 

 candle-power electric lamp was allowed to strike the stem 

 from one side. It will be seen that the excitatory effect of 

 this external stimulus is first exhibited in the retardation of 



Fig. 272. Retardation and Reversal of Normal Torsional Movement 

 by the unilateral Action of Light in Porana panicvlaia 



Light applied at f retards, and in the course of two minutes reverses, the 

 normal movement of negative torsion, making it positive. On cessation 



of light at { * \ there is recovery of the original negative torsion. 



the normal rate of torsion, culminating in its abolition, suc- 

 ceeded by actual reversal of direction. The normal negative 

 was thus converted into positive torsion. On the cessation 

 of the external stimulus the normal torsion was gradually 

 resumed. The effect described is best observed in vigorous 

 specimens in which the natural torsional movement is fairly 

 strong. 



In studying the action of light on autonomous longi- 

 tudinal growth, we found that when a series of responses to 

 the action of light were recorded, the first effect of incident 



