132 



THE VEGETATIVK FUNCTIONS OF Pl.ANTS 



the fact that its axis is not parallel to the plumb-hne? 

 Have the root and the shoot a nervous system, or a brain? 

 How is the bending accomphshed? 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. — Pliototroijic response of a seedling of white mustard {Brassier 

 alba), b, box, admitting light only through a narrow slit at the right; 

 V, vial; w, water-surface; d, 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 Hght, as mentioned above, is common knowl- 

 edge. That this is the normal response of roots and stems 



