I06 CHAP. XL PHOTOTROPIC EFFECT OF LIGHT 



once in lo seconds. The moment of application of light 

 is indicated by the up-pointing arrow, and its removal by 

 the horizontal arrow within circle, the total duration of 

 exposure being 60 seconds. It will be noted that the 

 response occurred within a short time of the exposure, and 

 that the tropic movement persisted for nearly 3 minutes 

 after the cessation of light, when there was a recovery 

 which was practically complete (fig. 53). 



It may be said in general that there is a quick recovery 

 after stimulation of moderate intensity and duration. But 

 the curvature induced under stronger stimulation remains 

 more or less persistent ; in extreme cases it becomes fixed 

 by permanent growth. 



Determination of the Latent Period 



The latent period of phototropic response is usually 

 regarded as of long duration, from several minutes to an 

 hour or so. The shortest latent period for the cotyledon 



Fig. 54. Latent period of response to light in, a pulvinated 

 organ. Response occurred after 14 dots which were at 

 intervals of 2 seconds. (Erythrina.) 



of Avena was found by Czapek to be 7 minutes. Little 

 is known about the relation of the latent period to the 

 intensity of light. 



Experiment 61. Latent period of Erythrina. — The re- 

 cording plate was made to move at a fast rate, the successive 



