144 PHYTOHORMONES 



then practically nil, and 2.10"^ mol. indole-acetic acid (0.1 7 

 per liter) caused about 30 per cent acceleration in growth, 

 while higher concentrations only inhibited. Thimann (1936a) 

 obtained a slight acceleration of the growth of Pisum roots 

 when auxin was applied to the base of the stem. He ascribed 

 this acceleration to the very low concentrations of auxin 

 which reach the root under these conditions, together with 

 the relatively small amount of auxin present in these roots. 

 Further, if Avena roots are inhibited in auxin solutions and 

 then transferred to water, their growth is accelerated beyond 

 that of water-treated controls; this, presumably, is due to 

 the rapid disappearance of auxin from the root, so that its 

 concentration eventually reaches the accelerating level (u). 



C. Auxin Production in the Root 



In order to complete the picture it is necessary to have 

 proof that root tips really do produce auxin, and this has 

 been given by a number of workers. First, Cholodny (1928) 

 showed that placing a root tip of Zea Mays on decapitated 

 Avena coleoptiles would increase their photo- and geotropic 

 responses. Then Hawker (1932) in experiments primarily 

 directed to the study of geotropism, placed root tips of 

 Vicia Faba on gelatin blocks and showed that if subsequently 

 applied to one side of decapitated root stumps the blocks 

 caused the stump to curve towards them, i.e. the stump was 

 inhibited on the side to which the gelatin was applied. 

 However, Boysen Jensen (1933a), who similarly placed root 

 tips on agar and then tested this agar on Avena coleoptiles, 

 could obtain no auxin in this way from Zea Mays root tips. 

 He found, however, that the tips of very young Zea Mays 

 roots (not longer than 5 mm.) would cause curvature of 

 Avena coleoptiles if placed one-sidedly upon them, but the 

 tips of older roots would not do so. Believing that the reason 

 for this failure was the need for nutrient, he used agar con- 

 taining 10 per cent dextrose, and this brought immediate 

 success. Root tips of Vicia Faba gave a small amount of 

 auxin to plain agar (as Hawker had found), but five times 



