62 KENNETHV.THIMANN 



auxin concentrations, or the rooting of fragments of Helianthus hypo- 

 cotyls in tissue culture containing auxin, are difficult to explain on this 

 basis. The influence of leaves in promoting the rooting of cuttings after 

 auxin treatment, brought out especially clearly by Rappaport, was at 

 first thought to be due to the contribution of the specific hormone by 

 the leaves, but in the light of several more recent studies this effect is 

 almost certainly nutritional. In at least one plant which is very hard to 

 root with auxin treatment, the White Hibiscus studied by van Overbeek 

 and coworkers (1946), the entire effect of the leaves could be replaced 

 by nutrients, leaving no evidence for a hormonal factor. 



Similarly the inhibition of lateral bud development by auxin applied 

 at the tip was explained by the mobilization of bud growth factors at 

 the point of auxin application, thus starving the buds of their essential 

 factors. But inhibition has been shown to occur when auxin is applied 

 to the lateral buds themselves ; and although its mechanism is certainly 

 not clear, it does seem to be essentially a direct effect of auxin on the 

 lateral buds rather than a diversion of other hormones away from these 

 buds. It is, however, possible in this case that the action is due to the 

 intermediate formation of an inhibiting substance by the auxin. Then 

 again, the inhibition of root growth, a phenomenon exhibited by all 

 auxins and in very low concentrations, is observed on isolated roots 

 and is definitely exerted on the root itself. 



All these considerations very strongly support the idea which was 

 tentatively put forward more than ten years ago (Thimann, IQ35) that 

 auxin controls some fundamental master reaction in the cell. From this 

 reaction, growth in size dififerentation, or even inhibition may follow 

 according to the abilities of the cells concerned and their supply of 

 water and other factors. It remains true that during the occurrence of 

 the actual growth process there is some mobilization of materials, i.e. 

 of water and nutrients. But this is a part of the growth process itself, 

 rather than its primary cause. Its part will be referred to later. 



Before dealing with this fundamental master reaction and the evi- 

 dence for it, it will be worthwhile to discuss one of the fields which has 

 been most active in the last few years, namely the relation between free 

 and hound auxin. 



In many standard test objects growth is a simple function of the 

 auxin applied, otherwise bio-assay would not be possible. From this it is 

 but a short step to the concept that in the intact plant, or rather in a 

 part of an intact plant, growth is a simple function of the auxin made 

 available to that part. Of course nutrient materials have to be available 



