688 PLANT RESPONSE 



The fact that these periodic forces in leaves and in plant 

 act concordantly causes the periodic movements to be of large 

 amplitude. 



The periodic diurnal variations of internal hydrostatic 

 tension are also exhibited by periodic variations in the rate 

 of growth. The curves of the diurnal periodicity of growth 

 and of the nyctitropic movement of the leaf are therefore 

 similar. 



This forced diurnal vibration, being often repeated, gives 

 rise to periodic after-effects, which persist for a time even on 

 the cessation of the periodically exciting cause. 



