ADJUSTMENT TO SURROUNDINGS 135 



curvature in the direction of the source of Hght are 

 positively phototropic; those which respond by a curva- 

 ture in the opposite direction are negatively phototropic. 

 Like geotropism, phototropism is a special kind of irri- 

 tabiHty. Organs growing in the light are, of course, 

 subject to the influence of both light and gravity at the 

 same time. 



Fig. 93. — Seedlings of t\iQ whittlM^me. {Liipinus alb us). At the left, 

 grown under normal illumination; at the right, grown in darkness. Both 

 cultures are of the same age, 



Phototropic response on a large scale, in nature, is 

 shown in Fig. 92. 



133. Effect of Light on Rate of Growth. — Every one is 

 famihar with the fact that stems grown in darkness, or 

 in reduced light, are commonly much elongated, and 



