A. W. (RALSTON AND RAVJNDAR KAVR 



695 



6 5 



og M lAA 



Apical 5 mm 



" 4 mm 



O « 3 mm 



« 2 mm 



Fig. 7. Effect of indole acetic acid (lAA) concentration on the increase in 

 fresli weight of etiolated pea stem sections of various lengths. 



10-^ M. These facts seem to be independent of the photoperiod 

 under which the plant is grown. 



Gibberellic acid (GA) optimum. The GA optimum for etiolated 

 pea sections was found by Purves (15) to be ca. 10-'^ to 10-*^ M. In 

 contrast, the GA optimum for green sections is 10-^ M or higher, de- 

 pending on the photoperiodic treatment of the plants from which 

 the sections were derived (Fig. 9) . 



GA-IAA synergism. It has been demonstrated that GA and lAA 

 promote growth non-synergistically in etiolated sections (9, 16, 17) 

 but synergistically in green sections (2) . Table 4 demonstrates the 

 effect of photoperiod on the degree of synergistic interaction. The 

 synergism (seen in the difference between the observed and calculated 

 values for growth in GA + lAA) is readily apparent in sections from 

 the 8-hour photoperiod plant, but the differences observed in sections 

 from the 16-hour plants are insignificant, and in those from the 24- 

 hour plants barely significant. 



Galston and Warburg (9) demonstrated that passage of GA from 

 base to apex through excised stem pieces of etiolated and green stems 

 resulted in synergistic interaction with lAA subsequently applied 



