132 
THE VEGETATIVE FUNCTIONS OF PLANTS 
the fact that its axis is not parallel to the plumb-line? 
Have the root and the shoot a nervous system, or a brain? 
How is the bending accomplished? These questions can- 
not be discussed here, but they should be given careful 
thought. They lead us into one of the most fascinating 
realms of plant study. 
FIG. 90. Phototropic response of a seedling of white mustard (Brassica 
alba), b, box, admitting light only through a narrow slit at the right; 
v, vial; w, water-surface; cl, cheese-cloth; sh, shoot; r, root. 
131. Effect of Light on Direction of Growth. The 
fact that stems ordinarily grow toward, and roots away 
from the light, as mentioned above, is common knowl- 
edge. That this is the normal response of roots and stems 
