ADJUSTMENT TO SURROUNDINGS 135 
curvature in the direction of the source of light 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- 
tability. 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 the white lupine (Lupinus albus). 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 
familiar with the fact that stems grown in darkness, or 
in reduced light, are commonly much elongated, and 
