656 P. W. Brian and H. G. Hemming 



auxin and gibberellin in increasing tissue extension. Now, similar 

 irradiances will increase the ability of legume shoots to convert trypto- 

 phan to indoleacetic acid. Light will even more markedly enhance 

 the ability of Coleus roots to form auxin or indoleacetate from tryp- 

 tophan. Could we not infer, therefore, that light caused an enhanced 

 auxin biosynthesis in your experimental material, and that you were 

 simply measuring the interaction between the two growth substances? 

 Dr. Brian: 1 don't think that explanation is possible because the 

 effect of light on sections not supplied with exogenous auxin is very 

 small, and the effect of light is greatest when you have optimal exoge- 

 nous auxin. But even when you've gone beyond the optimal of ex- 

 ogenous auxin so that growth is not quite as good as it would be with 

 a somewhat lower concentration, light still has an increasing effect. I 

 don't think, therefore, that the production of further auxin in the 

 tissue itself could explain the results. 



