140 



GROWTH HORMONES IN PLANTS 



Table 6. — The Quantity-of-stimulus Principle in the Phototropic 



Response of the Avena Coleoptile 



(Blaauiu, 1909) 



Along with a knowledge of the amount of light necessary to 

 produce a threshold curvature, it has been found equally profita- 

 ble to study the course of phototropic curvatures with greater 

 amounts of light. Different observers have found that as the 

 amount of light is increased progressively above the threshold- 

 stimulus value, various types of positive and negative bending 

 may occur. 



Positive and Negative Curvatures. — Pringsheim (1909) observed 

 that when an Avena coleoptile is unilaterally illuminated with 

 increasing amounts of light, no curvature is produced so long as 

 the amount of light is below the threshold. Above the minimum 



