134 THE 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. 92. Mountain palm (Gaussia sp.), growing on a steep, west-facing 
cliff. The stems show a phototropic curvature toward the source of 
most abundant light. 
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 
