K. V. THIMANN 



is necessary to consider the distinction between direct and indirect 

 effects on growth. Some of the observed effects of auxin are in part 

 at least indirect. When swelUngs are produced, or fruits induced to 

 grow, under the stimulus of auxin, there must be a continuous supply 

 of carbohydrates, amino acids, and especially water to the growing 

 zone. The plant, therefore, must have some mechanism whereby 

 materials for growth are accumulated at the place at which they are 

 being used. To take the simplest case, that of the carbohydrates, it 

 could be considered that, as the sugars are converted to polysaccharides 

 and deposited as such in the cell wall, the concentration of soluble 

 sugars decreases and hence more carbohydrates would flow there in 

 consequence of the concentration gradient. However, accumulation 

 in plant tissues does not always follow the gradient. Thus, salts are 

 concentrated in young roots by a factor many times the concentration 

 of the external nutrient solution, and sugars are sometimes accumulated 

 in quite high concentrations in storage organs. Auxin itself, as men- 

 tioned on page 324, is freely transported against its gradient. The 

 accumulation of materials in the growing organs, and the transport of 

 materials there, consequently do not necessarily constitute a "gradient" 

 process; and numerous suggestions have been made that auxin exerts 

 its growth-promoting activity through the accumulating or "mobiliz- 

 ing" of food materials. Auxin is envisaged as in some way controlling 

 transport, so that, wherever auxin is, other materials will be collected. 

 This view has been expanded to include mobilizing effects on hypo- 

 thetical special organ-forming substances, such as factors for root 

 growth, stem growth, and leaf growth (9). The evidence for such 

 organ-forming substances is indirect only; evidence for a mobilization 

 phenomenon as the cause of the growth-promoting activity of auxin is 

 at present by no means convincing. 



It remains true, of course, that when auxin produces growth 

 there is an accompanying accumulation of materials. It has been 

 shown, for instance, that application of concentrated auxin paste to 

 decapitated bean plants produces localized swelling which is associated 

 with a large increase in soluble carbohydrates and total dry weight. 

 It has also been shown in ingenious experiments (9) on oat seedlings 

 that a zone of the plant which has been treated with auxin and later 

 cut off is subseqixently able to grow in solution better than a control 

 zone. Hence it is not the occurrence of such accumulations, but the 



328 



