Developmental Effects of Auxins 105 



the two are added together more growth is obtained, but neither type 

 of differentiation dominates. 



The concentration of adenine required to obtain a given number 

 of buds was found to be higher if more auxin was added and lower 

 with less auxin. This type of interaction holds not only for buds but 

 for callus and root formation as well. 



A simplified way of looking at Skoog and Tsui's concept might be 

 to assume that the concentration ratio of auxin to adenine will deter- 

 mine to a large extent whether buds, callus, or roots will be formed. 

 At low auxin to adenine ratios, buds will be produced; and at high 

 ratios (i.e. high auxin: low adenine), roots will be produced: 



Buds Callus Roots 



Auxin/adenine ratio: 



Investigating the specificity of this function, Skoog and Tsui 

 found that adenine, guanine, and adenylic acid were all highly effec- 

 tive in producing buds on excised plant material. Organic nitrog- 

 enous compounds other than the purines were found to be much 

 less effective in this control of differentiation. An interaction between 

 these compounds and phosphates was found. 



Further evidence of the intimate relationship between adenine 

 and auxin is found in experiments with the purine inhibitor, 2,6-dia- 

 minopurine (Miller, 1953). This compound inhibits bud formation in 

 tobacco stem sections, and the inhibition is relieved by further addi- 

 tions of adenine. The auxin stimulation of growth in straight-growth 

 tests is likewise inhibited, and again the inhibition is relieved by addi- 

 tion of adenine. 



The impact of these studies upon the understanding of auxin 

 action in morphology is great indeed. They bring together previously 

 heterogeneous observations of the effects of auxin upon callus and root 

 formation and correlation phenomena in general. For years we have 

 recognized that callus formation in whole plants or in cuttings from 

 plants occurs at places where auxin content would be expected to be 

 high due to its basipetal movement. Girdling of woody twigs results 

 in callus formation followed generally by root formation above the 

 girdle where auxins accumulate. Buds are commonly formed below the 

 girdle, where the auxin content would be expected to be low in 

 proportion to the nitrogenous food materials moving up from the 



