Groivtli of Roots, Steins, and Leaves 



21 



more apparent by thrusting into the soil two slender wire rods, one on 

 either side of the stem. 



Growth Curvature affected by Water. — ^\'e have 

 been studying growth curvatures affected by gravity. While 

 the tendency of the main root is downward and that of the 



Fig. 29. — Leaves of }'Oung castor-oil plant looking toward the light. 



stem is upward, both are sensitive to other influences. Food 

 material which roots absorb must be dissolved in water, in 

 search of which they often go a long distance. The roots of 

 a tree growing by a stream will reach far out on the side 

 toward the water. Young plants show this curvature. 



