550 /. A. Lockhart 



30 1 — 



00003 0.001 0.003 0.01 0.03 



DOSE OF GA/PLANT (yug) 



Fig. 2. The height of dwarf Pisum ('Morse's Progress #9') 4 days after treatment 

 with gibberellic acid (at the doses indicated) and transfer of certain plants to con- 

 tinuous red radiation (150 ergs-cm'=-sec"^). Plants 6 days old at time of treatment 

 (ca. 4 cm. above soil level). Also indicated are the standard deviation of the means. 

 Twelve plants per treatment. 



conditions was equal. That is, the amount of gibberellic acid (or 

 gibberellic A^) recjuired to give a threshold response, a half-maximal 

 response, or a saturating response is equal in irradiated and dark- 

 grown plants (Figure 2). This indicates the effective level of en- 

 dogenous gibberellin is reduced by irradiation, while responsiveness 

 of the plant to gibberellin remains unchanged. Thus, radiation either 

 reduces synthesis, or diverts or destroys existing gibberellin. Phillips 

 et al. (30) have recently shown that the level of extractable, endogenous 

 growth substances in the 'Alaska' pea is reduced by irradiation. Gib- 

 berellins and auxins were not completely distinguished, but all acidic 

 growth factors showed a general correlation with growth rate. These 

 analyses provide further support for the conclusion that visible radia- 

 tion acts on stem growth through a decrease in endogenous gibberellin. 



Cell Wall Plasticity 



It was pointed out above that inhibition of growth by radiation 

 acts through a reduction in the effective level of endogenous gibberel- 

 lin. Furthermore, it was shown (p. 545) that cell elongation of irradi- 



