GROWTH SUBSTANCES FOR PHOTOTROPISM 141 



quantity of light necessary for excitation, positive curvatures are 

 evoked; i.e., the stimulated organ bends toward the light over a 

 considerable range of stimulus values. These curvatures become 

 more pronounced and disappear less rapidly with increasing 

 amounts of light. If the amount of light is increased still more 

 within a certain range, a stage of indifference appears, and after 

 some time an opposing movement may take place so that weak 

 negative reactions can be observed. With still larger amounts of 

 light, positive curvatures are again brought about. Pringsheim 

 presented graphically the influence of the amount of applied light 

 upon the manner of reaction of the seedlings, and these conclu- 

 sions were verified by Clark (1913). It is clear from the investi- 

 gations of Pringsheim and Clark that three different modes of 

 response can be distinguished in the Avena coleoptile: the first 

 positive curvature, the first negative curvature, and the second 

 positive curvature. 



Arisz (1915) investigated carefully the manner of reaction of the 

 Avena coleoptile to different amounts of light in an attempt to 

 determine at what quantities of light the various curvatures 

 appeared. The results of his experiments (Table 7) indicate that 



Table 7. — Size of Curvature in the Avena Coleoptile in Response to 

 Different Amounts of Unilateral Light 

 Amount of Light, m.c.s. Size of Maximum Curvature* 



7.6 0.7 



12.4 1 



18.1 1.6 



26.4 2.3 



45 3 



65 3.3 



75 4 



100 5 



140 4.7 



237 5.4 



560 4 



1,500 3 



2,800 1-2 



* Curvature in millimeters deviation from the vertical position. Coleoptiles were about 

 25 mm. in length. 



the magnitude of curvature increases with the amount of light 

 applied until a maximum curvature is reached with about 100 

 meter-candle seconds. After this point, curvature decreases with 

 increasing amounts of light. With about 6,000 to 40,000 meter- 



