84 



TROPISMS 



for the blackening of photographic paper by light, and it 

 can be shown that the same law holds for heliotropic 

 reactions of plants as well as animals. 



Blaauw 46 . 47 established this fact for the etiolated seed- 

 lings of Avena sativa. These organisms were exposed to 

 lights of a definite candle power for some time and then 

 left in the dark. After a certain time the seedlings began 

 to bend, becoming concave on that side which had pre- 

 viously been illuminated. By varying the candle power 

 of light (i) and the duration of illumination (t), he found 

 that the value of it required to cause 50 per cent, of the 

 seedlings to bend was always the same. Table III gives 



TABLE III 



Time required for different intensities of light to produce heliotropic curvatures in 50 per 



cent, of the seedlings of Avena 



