148 PHYTOHORMONES 



the leaf; on removal of the leaf the roots resumed their 

 normal growth. Kogl, Haagen Smit, and Erxleben (1934o) 

 in showing that pure auxins inhibit growth of Avena roots, 

 mention that the short roots were abnormally thick. Sub- 

 sequently numerous workers have noticed the thickenings 

 which are almost always produced by the action of auxin, 

 particularly with high concentrations. In the shortened and 

 thickened zone, there is radial elongation of the cells in the 

 cortex, which are actually shortened in the longitudinal 

 dimension (u). The thickening need not necessarily accom- 

 pany inhibition of elongation, since Lane (1936) has shown 

 that Avena roots may be almost completely inhibited in 

 length without any appreciable thickening, by soaking the 

 seeds in auxin solution. 



A special case of root thickening and lateral root forma- 

 tion is provided by the bacterial nodules on the roots of 

 legumes. Thimann (1936) has shown that young growing 

 nodules produce considerable amounts of auxin, which 

 diffuses both from the apical and from the basal portion. 

 He also showed that local application of auxin to a lateral 

 root at its first appearance produced its complete inhibition, 

 and thus deduced that nodules are due to the auxin formed 

 by the bacteria in the tissues; this auxin induces a lateral 

 root at the point of infection, but then inhibits its growth 

 and causes it instead to swell. 



D. The Role of Auxin in the Growth of Roots 



The formation of and response to auxin in the root is so 

 different from that of shoots, that the two-factor scheme, 

 suggested to explain the distribution of growth in shoots, 

 cannot be applied to roots. How then do roots grow? Ac- 

 cording to Czaja (1935) auxin in roots has fundamentally 

 the same function as in shoots, that is, it causes elongation. 

 He assumes two opposite streams of auxin, one coming 

 from the stem and the other from the root tip, each of which 

 by itself promotes growth, while the two in combination 

 cause a growth retardation. This view, based on a few ex- 



