212 GROWTH HORMONES IN PLANTS 



curvatures in Avena coleoptiles, hypocotyls of Helianthus, and 

 roots of Pisum, in agreement with expectations. 



Growth substance accumulates in the under side of both roots 

 and stems when they are placed in a horizontal position. The 

 subsequent opposite response of these organs has been pointed 

 to as the explanation of positive and negative geotropism in the 

 two ends of the plant axis. It remains to be shown how the same 

 growth hormone can retard growth in roots, bringing about 

 downward curvature, and promote the rate of growth in shoots, 

 causing upward bending. 



