320 LIFE MOVEMENTS IN PLANTS 



It is evident that there is a critical intensity of stimu- 

 lus, above which there is a retardation, and below which 

 there is the opposite reaction of acceleration. This criti- 

 cal intensity, I have found to be low in vigorous speci- 

 mens, and high in sub-tonic specimens. Thus the same 

 intensity of stimulus may induce a retardation of growth in 

 specimens the tonic condition of which is ahovi par, and 

 an acceleration in others, in which it is heloiv par. The 

 following experiments will demonstrate the immediate and 

 after-efifect of light of increasing intensity and duration. 



Effect of light of moderate intensity : Experiment 120. — 

 The source of light was a small arc lamp placed at a 

 distance of 50 cm., the intensity of incident light was 

 increased or decreased by bringing the source of light nearer 



Fig'. 115. — Immediate and after-effect of stimulus of light on ejrowth. (a) shows 

 immediate effect of moderate light to be a transitory acceleration (down-curve) 

 followed by retardation (up-curve). The after-effect on cessation of light is an 

 acceleration (down-curve) followed by restoration to normal. (6) Immediate and 

 after-effect of stronger light : immediate effect, a retardation ; after-effect, recovery 

 to normal rate without acceleration. 



or further away from the plant. Two inclined mirrors 

 were placed behind the plant so that the specimen was 



