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 

 coinvenient 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 



