SKOOG AND TSUI 283 



complexes as mediators in energy transfer reactions in cellular syntheses, 

 it appears that auxin also must be concerned in these phosphorylation 

 systems. It seems, however, that its role in morphogenesis can hardly 

 be restricted to the regulation of the rate of a single reaction leading to the 

 formation of a single final product, but it must at least include the regula- 

 tion of the relative rates of coordinated synthetic reactions so as to 

 modify both the quantities and proportions of several products. 



The correlative and morphogenetic actions of auxin which are well 

 established in bud inhibition and root formation must be extended to 

 include the formation of buds. For even though the addition of lAA 

 in our experiments results in suppression rather than promotion of bud 

 formation, the marked effect of auxin on cell proliferation together with 

 its properties of high physiological activity and unique mode of transport 

 must be decisive in furnishing new cells, in setting the stage at which 

 these become differentiated into primordia, and in enabling the latter 

 to develop as organized units. 



The plan of the present work has been based in part on the early 

 observations indicating that the conditions leading to induction of bud 

 formation are similar to the conditions required for the subsequent 

 growth of the buds. Some results have been obtained which show that 

 separate factors may become limiting in these two processes. Neverthe- 

 less, the general conclusion from all the results is that the correlative 

 mechanism which determines the relative growth of organs, and hence 

 the final form of the plant, also functions at the level of cellular dif- 

 ferentiation into tissues and primordia. 



Keeping in mind the fact that in our present state of ignorance, growth 

 of any kind is far too complex a phenomenon to be interpreted in detail, 

 the formulation of simple points of view on the nature of growth and 

 developmental processes may be useful in guiding further inquiry. For 

 example, the concept of a growth substance regulating the rate of growth 

 of tissues has been extremely fruitful. It has been a prerequisite step in 

 obtaining the evidence, which is now accumulating, to show that sub- 

 stances of many kinds may be preferentially produced or concentrated 

 in certain tissues or organs and that any one of a number of such 

 substances may become a limiting factor for growth. However, recently 

 Sachs' old concept of organ-forming substances has been revived. It 

 has appeared in various new forms (2,5), but to the general effect that 

 a specific substance is required for the formation and/or growth of each 



