138 



GROWTH HORMONES IN PLANTS 



side. By unilateral illumination, as will be shown later, the 

 growth substance in the tip is moved to the back; therefore, less 

 growth substance flows upon the front side than is normally 



Fig. 45. — Transmission of phototropic stimulus in the Avena coleoptile; A, 

 incision on the darkened side. B, incision on the lighted side. C, control 

 plants. {From Purdy, 1921.) 



the case, and the growth rate of the front side is retarded. The 

 "stimulus transmission" on the front side, therefore, is condi- 

 tioned by the decrease of the growth-substance supply from the 

 tip to the basal region. 



