204 GROWTH HORMONES IN PLANTS 



about 36 per cent. Then Nielsen (1930a) showed that rhizopin 

 (3-indole acetic acid) can completely inhibit root growth in Lup- 

 inus albus and ViciaFaha without permanently injuring the roots. 

 Other work by Cholodny (1926) supplied the information that 

 although decapitation has a retarding effect upon the rate of growth 

 of the coleoptile, it has a promoting effect upon the growth of the 

 root. The increase amounted to only about 12 per cent. Accord- 

 ing to Keeble, Nelson, and Snow (1930), the growth-retarding sub- 

 stance accumulates in the wound when the root is decapitated. 

 When the substance is removed by washing, growth of the decapi- 

 tated root again takes place. If the excised tip is replaced, the 

 rate of growth is decreased again. All these observations conform 

 with the theory of positively geotropic curvature in the root as pro- 

 pounded by Cholodny (1927). According to Cholodny's theory, 

 a growth substance, which is identical with that formed by the 

 coleoptile tip, is secreted by the root tip. This growth substance 

 has a retarding effect upon the rate of growth of the root. The 

 movement of it is influenced by gravity in such a way that it 

 accumulates upon the lower side of horizontally placed roots. As 

 a result, the rate of growth of the root is retarded on the under 

 side, and a geotropic curvature results. 



In 1932, Hawker published some experimental data which 

 pointed to the presence of growth substance in the root of Vicia 

 Faha. Geotropically stimulated root tips were cut longitudinally 

 into an upper and lower half. Four such pieces (all either upper 

 or lower) were placed on each of a series of gelatin blocks. After 

 one hour the half tips were removed, and the gelatin blocks were 

 placed unilaterally upon decapitated, vertically suspended Vicia 

 Faha roots. Curvatures appeared, due to bending toward the 

 side with the gelatin block. The curvatures were greatest in 

 those roots with gelatin blocks which had obtained something 

 from the lower halves of the original root tips. 



In a study of the influence of temperature upon geotropism in 

 seedlings of Vicia Faha, Hawker (1933) demonstrated conduction 

 of the geotropic stimulus into the root from excised and replaced 

 root tips. Excised tips from plants grown at 20°C. were found 

 capable of causing greater geotropic bending than tips taken from 

 plants which had grown at the same temperature but had been 

 kept at 5°C. for 24 hours immediately preceding the experiment. 

 A period of 24 hours in the cold apparently decreased the produc- 



