150 PHYTOHORMONES 



roots already low in auxin giving an acceleration, roots high 

 in auxin a retardation. This would correspond approxi- 

 mately with Boysen Jensen's suggestion. Jost and Reiss 

 (1936) also tend to adopt this view. 



The other possibihty is that auxin is not necessary at all 

 for the growth of roots. The growth of isolated roots in 

 culture is known to be dependent on sugars, salts, and traces 

 of special substances, which are present in yeast extract 

 (White, 1933, 1934). These may perhaps include a special 

 root-growth promoting substance, but in any event addition 

 of auxin does not improve the growth, and such a medium 

 suffices for the indefinite growth of the roots of wheat and 

 tomato. The absence of detectable amounts of auxin from 

 such isolated roots (Fiedler, 1936) would certainly indicate 

 that it plays no part in their growth. The fact that in these 

 cultures roots continue to branch, however, would indicate 

 either that they produce traces of auxin or else that the 

 small amount of auxin in the yeast extract or other con- 

 stituent is sufficient to cause both growth and branching. 



Whether auxin plays any part in root elongation or not, 

 the mechanism whereby it causes inhibition of root growth 

 is, as stated above, not clear. Bonner (1935) has suggested 

 that instead of diminishing the forces between the micelles 

 of the cell wall, it increases them. However, Amlong (1936) 

 has shown that the decapitation of Vicia Faba roots causes a 

 decrease in their plasticity just as was shown by Heyn for 

 coleoptiles (VIII D). Not only does this contradict Bonner's 

 suggestion, but it raises again the question of whether the 

 increased plasticity caused by auxin can be the direct cause 

 of growth. 



In view of the incompleteness of our knowledge as to the 

 mechanism of elongation, it is perhaps unreasonable to 

 expect any better understanding of the mechanism of in- 

 hibition. Doubtless further advances in the analysis of both 

 processes may be expected in the near future. 



