ADJUSTMENT TO SURROUNDINGS 
133 
to the stimulus of light may be shown by a very simple 
experiment. A young seedling of mustard, or any other 
convenient plant, is allowed to develop in complete dark- 
ness in ordinary tap-water, until its root and stem are 
FIG. 91. Seedling of white mustard (Brassica alba) showing the com- 
bined effect of light and gravity on the direction of growth of root and 
shoot. The dotted line figure indicates the position of root and shoot at 
the beginning of the experiment. The change indicated was accomplished 
in about 48 hours. (Cf. Fig. 84.) 
each about i inch long. Being subjected to only the 
influence of gravity, the root grows vertically downward, 
the stem vertically upward. If the bottle and plant are 
