IxNCREASING DURATION III 



given in fig. 57, which shows that the retardation of growth 

 is at first rapid and then tends to reach a hmit. This 

 must also be true when hght acts only on one side of the 

 organ, the retardation of growth of the directly stimulated 

 proximal side of the organ contributing to bring about the 

 positive curvature. 



Fig. 58. Tropic effect of increasing intensity of light, 1:2:3, 



on growing organ (Crinum). 



Experiment 64. Tropic curvature of a growing organ. — 

 The flower-bud of Crinum was used for this experiment. 

 The intensity of light acting on one side of the organ was 

 increased by bringing the source of light nearer to it, 

 the duration of exposure being in all cases kept the same, 

 namely, i minute. Increasing intensity of light in the ratio 

 of I : 2 : 3 gave rise to increasing positive curvature (fig. 58) 

 in the ratio of 1:2-5:5. 



Effect of Increasing Duration of Exposure 



Experiment 65. — The specimen of Crinum was in a 

 slightl}^ subtonic condition ; the responses therefore were 

 a short-lived negative preceding the normal positive. The 

 duration of successive exposures was for i, 2, and 3 



