210 CONTROL MECHANISMS IN CELLULAR PROCESSES 



later it was returned to auxin alone, and after the usual lag its 

 growth resumed and attained 7.8 [jl per minute, so this time the 

 growth did not 'catch up." In this case we seem to have shown, 

 with a single section, that stored growth is obtained previous to 

 auxin treatment but not during auxin treatment, which is consistent 

 with the explanation suggested above. 



It may be noted that the suggested explanation of auxin promo- 

 tion of growth involves an indirect action on the growth of the cell 

 wall, that is, auxin gives indication of acting directly only on some 

 aspect of metabolism within the cell which supplies the growth 

 process with "substrate." This material, whatever its nature, may 

 be imagined to be capable of supporting kinds of activity other than 

 cell enlargement in cells equipped with enzymes other than the 

 growth enzyme, and in this way the different effects of auxin on 

 different tissues and organs might be explicable. 



Recently Carr and Ng ( 1959 ) investigated the "residual effect" 

 of auxin, using wheat coleoptile sections, and concluded that the 

 promotive effects caused by auxins and inhibitors could best be 

 interpreted as due to rate effects on various metabolic reactions, in 

 directions which cause a shift in the balance of cell metabolism in 

 favor of syntheses. A similar explanation for promotive effects of 

 certain inhibitors had been put forward by Slocum and Little ( 1957 ) . 

 The suggestion made above may be considered an extension of 

 these views which indicates somewhat more specifically the rela- 

 tionship between processes acted on by auxin and the other agents 

 studied, and the growth process itself. We would emphasize again, 

 however, that it is a tentative interpretation of the observations, 

 and other possible explanations of them have by no means been 

 excluded. Also, it undoubtedly oversimplifies growth by formally 

 reducing it to those steps indicated by the kinetic evidence. 



Summary 



Current views on the mechanism of plant cell growth, with some 

 of their experimental basis, are discussed. Experiments on oat cole- 

 optile sections are described which suggest that the immediate 

 mechanism of growth is not a passive plastic stretching, indirectly 

 maintained by metabolism; instead it appears that growth rate is 

 controlled by the rate of some metabolic reaction, most likely acting 

 on the cell wall. Experiments on the effect of inhibitory treatments 



