134 



THK VEGETATIVE FUNCTIONS OF PLANTS 



then placed inside a covered box, to which the light is ad- 

 mitted only through a narrow slit in one side, the stimulus 

 of the one-sided illumination will be followed by a curva-' 

 ture of the root away from the light, and of the stem 

 toward the light, as shown in Fig. 90. 



Fig. q2. — Mountain palm (Gaussia Sp.), growing on a steep, west-facing 

 cliff. The stems show a phototropic curvature toward the source of 

 most abundant light. (Photo by the author.) 



The result of the simultaneous stimulation of root and 

 shoot by light and gravity is illustrated in Fig. 91. 



132. Phototropism. — The property of an organ by virtue 

 of which it may detect the direction of the source of incident 

 light rays is phototropism. Organs which respond by a 



