Potentials of Auxins and Auxin Research 305 



concentration may promote one phenomenon and inhibit another. A 

 particularly clear case of this is found in the effects of auxin concen- 

 trations on growth of different organs. As shown in figure 43, the 

 optimal auxin concentration for growth of stems is higher than that 

 for buds, which is in turn higher than that for roots. Consequently a 

 given auxin application may promote stem growth and inhibit the 

 growth of the other two types of organs. 



The quantitative effects of auxins are further complicated by the 

 interactions of auxin with other plant constituents. In the discussion 

 of differentiation it was shown that the effect of a given amount of 

 auxin might produce quite opposite effects depending upon the 

 presence or absence of other growth materials. Thus Skoog and Tsui 

 (1948) found that the addition of auxin to tobacco stem sections in- 

 hibited bud formation, but in the presence of adenine the auxin 

 inhibition disappeared. Leopold and Guernsey (1953) demonstrated 

 that auxins applied to pea seeds inhibited flowering, but that the addi- 

 tional application of carbohydrates reversed the inhibition. These 

 latter workers also found that a given auxin application could have 

 quite opposite effects on flowering, depending upon the subsequent 

 temperature experience, probably because of the effects of temperature 

 in altering the plant constituents. 



Still a further complication in the quantitative responses to 

 auxin is found in the form and nature of the auxins indigenous to 

 the plant. It is well known that auxins may exist in either free or 

 bound forms in the plant, and there are probably several bound 

 forms of auxin yet to be discovered. In discussing responses of plants 

 to different quantities of auxin it is difficult to determine whether 

 one should speak in terms of free auxin, bound auxin, or both. 



Different degrees of sensitivity are found not only between differ- 

 ent tissues and organs but also between different ages of the same 

 tissue or organ. For example, a tissue which is weakly differentiated 

 is much more sensitive to auxin than a mature, well-differentiated 

 tissue. 



The multiplicity of these complicating factors points up the neces- 

 sity for a strong background in auxin physiology by the research 

 worker. Even with such a background it is difficult enough to interpret 

 variations in effects and to be intelligently alert for the subtle dif- 

 ferences that may be brought about by a given treatment. Without a 

 sound understanding of auxin physiology the research worker is 

 certainly at a disadvantage in knowing what to look for and what 

 potentialities and limitations auxin treatments may have. 



Perhaps one of the most remarkable things about research on 



