590 W. S. HUlman and W. K. Pwves 



tion should not be paraphrased "Do the physiological effects of gib- 

 berellin and auxin interact?" Several possible mechanisms of auxin- 

 mediated gibberellin action (cf. 2) can be summarized briefly as fol- 

 lows: (A) Gibberellin may protect native or exogenous auxin from 

 inactivation within the tissue. (B) It may act by increasing the syn- 

 thesis of native auxin, or its translocation or binding to active sites. 

 (C) It may increase the number of sites available with which auxin 

 molecules can react to cause growth. 



Such proposals all envisage gibberellin as acting by somehow in- 

 creasing net auxin activity. Actions in the reverse sense are not usually 

 considered, in view of the many similarities between gibberellin and 

 auxin activities, but would still constitute auxin-mediated actions. 



Most of the evidence available is derived from growth experiments, 

 while some comes from studies of the enzymatic activity or growth 

 substance content of extracts. Our own work has been entirely of the 

 former kind; it will be reviewed in the succeeding section and then 

 considered as part of the total data available. 



EXPERIMENTAL 



The etiolated pea epicotyl section test was the experimental sys- 

 tem chosen, since it responds to both GA and lAA. Sections were cut 

 from developing third internodes of 7- to 8-day seedlings of Pisum 

 sativum, 'Alaska,' grown in total darkness. They were incubated in 

 darkness for the next 20 to 24 hrs. in a basic medium consisting of 

 phosphate buffer plus 2 per cent sucrose (except as otherwise noted) 

 and further supplemented with GA or lAA as desired. For details of 

 the methods employed and of data described but not presented here see 

 (8) and (9). 



If sections are cut from various regions of the third internode and 

 their elongation in buffer-sucrose medium compared, the more apical 

 the section the greater the elongation. Thus SI sections (S standing 

 for short, i.e., 5 mm., and 1 standing for 1 mm. below the plumular 

 hook) elongate more than S4 sections, which in turn elongate more 

 than S7 sections. If lAA or GA is added to the medium, further dif- 

 ferences are found. SI sections under these conditions show a very 

 low lAA optimum (about lO-''' M), and the additional elongation over 

 the endogenous (buffer-sucrose only) caused by optimal lAA is quite 

 small; higher levels of lAA are inhibitory. The lower sections show 

 a higher lAA optimum (about lO" M), and the additional elongation 

 caused by optimal lAA is greater than for SI sections. The situation 

 is quite different with respect to GA. There is no pronounced op- 

 timum for GA activity, and a plateau is reached at about 10-« M. The 

 additional elongation caused by GA is greater in SI sections than in 



