THE GROWTH OF ROOTS 149 



periments only, and theoretically impossible (see VI D), has 

 been disproven both by Faber (1936) and by Thimann 

 (1936). Czaja's experimentum crucis: negative geotropic 

 curvatures in decapitated roots, which should have only one 

 auxin "stream," could not be repeated. Further, auxin, 

 when applied to the epicotyl stump of decapitated Pisum 

 seedlings does not inhibit root growth, but even slightly 

 accelerates it (Thimann, 1936), and the presence or absence 

 of the root tip (i.e. the second auxin ''stream") does not 

 affect the result. In Avena, in which the same treatment 

 produces a slight inhibition, the presence or absence of the 

 root tip is also without effect. 



One obvious suggestion as to how it is that auxin inhibits 

 root elongation is that its action is exerted on the transverse 

 walls and thus causes thickening instead of elongation (c/. 

 e.g. F. A. F. C. Went, 1935). However, since inhibition 

 appears to be independent of thickening, there must be 

 another explanation. 



Another suggestion comes from Boysen Jensen (1936a). 

 According to this, roots are much more sensitive to auxin 

 than shoots; very low concentrations of auxin promote 

 their growth, but the ordinary concentrations worked with 

 are high enough to be above the optimum and thus cause 

 inhibition. This is supported by the experiments of Amlong 

 (1936a) and Fiedler (1936) on roots made very poor in 

 auxin, whose growth is then slightly promoted by the lowest 

 auxin concentrations (see IX B). A comparable inhibition 

 of elongation is produced in stems, e.g. of pea and Helianthus, 

 by very high concentrations of auxin. In stems, therefore, 

 inhibition is caused only by the highest concentrations, 

 while in roots it is caused by all but the very lowest. This 

 raises the question as to why different roots react differently 

 to auxin. Thus in Thimann's experiments (1936) Avena and 

 Pisum behave in the opposite way to auxin applied basally; 

 in Faber 's experiments (1936) Avena and Vicia also react 

 oppositely. Both have suggested that the difference is due 

 to different concentrations of auxin already in the root, 



