58o 



GROWTH HORMONES 



that the auxin present in roots is entirely or largely a result of downward 

 translocation from aerial organs. 



The recent suggestion by Thimann (1937) that roots, buds, and stems all 

 react in a comparable way to auxin, their growth being inhibited by relatively 

 high and promoted by relatively low auxin concentrations, is a possible ex- 

 planation of the contrasting effects of auxins upon elongation in roots and 

 aerial organs (Fig. 129). Elongation of roots is favored only at very low 

 concentrations; at all higher concentrations their growth is checked. Stems 

 and coleoptiles show a similar behavior except that the optimum range of 

 concentrations for elongation is much higher than for roots. The same con- 

 centrations of auxins which favor stem elongation result in a retardation of 

 root elongation. The effect of auxins on bud development is considered in 

 Chap. XXXIV; for the moment we need only note that they seem to occupy 



10 



10' 10" 10' :o' 10' 10 10 



MOLAR AUXIN CONCENTRATION 



10 



10 



Fig. 129. Relation of auxin concentration to elongation of roots, buds and stems 



according to Thimann (1937). 



an intermediate position between roots and stems with respect to their response 

 to different auxin concentrations. Briefly, therefore, whether auxin will exert 

 an accelerating or an inhibiting effect upon growth seems to depend in part 

 upon its concentration and in part upon the specific tissue involved. 



Nothing is known of the mechanism of the inhibiting effects of the auxins 

 beyond the not very helpful suggestion that in relatively high concentrations 

 they may exert a "toxic" effect. The elongation-promoting influence of the 

 auxins, on the other hand, has been the subject of considerable experimental 

 study but as yet very little conclusive evidence of the mechanism of this effect 

 has been forthcoming. A reasonable presumption is that the auxins in some 

 manner influence the intercalation of new molecules during the areal extension 

 of cell walls. Some authorities believe, however, the auxin acts merely in 

 such a manner as to increase the plasticity (irreversible extensibility) of the 



