696 



LIGHT AND LIFE 



Light 



Dark 



8 Hr. Pholopd. 

 16 Hr. Photopd. 

 24 Hr. Photopd. 



-log M lAA -log M lAA 



Fig. 8. Effect of the photoperiod of the parent plant on the indole acetic acid 

 (lAA) optimum of green pea stem sections grown in light and dark. 



to apical 5 mm sections excised from these treated stem pieces. While 

 Piirves and Hillman (17) could find no such induced synergism in 

 etiolated pea stems, we have consistently been able to induce GA-IAA 

 synergism in 16-hour and 24-hour photoperiod green stems of ap- 

 propriate length by such manipulation (Table 5) . Thus, 10 cm long 

 stem pieces from 16-hour photoperiod plants show GA-IAA synergism, 

 while shorter pieces do not; conversely, 2 cm stem pieces from 24-hour 



4.0- 



-log M GA 



Fig. 9. Effect of the photoperiod on tlie gibbercllic acid (G.\) optimum for 

 growth of green pea stem sections. 



