222 PHYTOHORMONES 



dently comparable with the formation of cambium in the 

 stem; both must be ascribed to the ''sensitivity," or ''readi- 

 ness to divide," of a certain layer of cells. Cells which have 

 not this sensitivity can apparently not be stimulated to 

 divide by auxin, except perhaps in unphysiologically high 

 concentrations, and these cells therefore require the other 

 factor or factors discussed above. Thus Snow (1935a) 

 observed that an auxin concentration sufficient to produce 

 marked activation of the cambium produced no divisions 

 in the cortex or pith. We may perhaps conclude, though as 

 yet without experimental support, that cell division results 

 from the interaction of several factors, of which one is auxin; 

 the distribution of these factors differs for different tissues, 

 and those tissues, such as stem and root cambium and root 

 pericycle, which divide readily on the application of auxin, do 

 so because they already contain the other factors, and auxin 

 is therefore limiting. Cells like those of the Phaseolus 

 pericarp which do not respond to auxin must therefore be 

 considered as having the other factors limiting. Finally, 

 cells containing only traces of these other factors could 

 respond to them if extremely high concentrations of auxin 

 were added. The cambium or callus cultures of Gautheret 

 (1934, 1935), which continued to proliferate on culture 

 medium, presumably contained auxin and other factors in 

 storage. 



C. Callus and Stem Swellings 



One of the most obvious results of the application of 

 auxin to young tissues, especially if high concentrations are 

 used, is the swelling of the tissue. This aspect of auxin 

 activity has unfortunately not been studied very quantita- 

 tively, so that we cannot say just how the concentrations 

 needed to produce swelling compare with those present in 

 nature. The application of these findings to normal physio- 

 logical processes is therefore difficult, and more exact work 

 is needed, but it seems probable that they will have a bear- 

 ing on pathology and teratology. In woody cuttings these 



