100 GROWTH HORMONES IN PLANTS 



were not permanently injured by the rhizopin, for if the rhizopin 

 solution was replaced later by water, the roots immediately 

 resumed their growth. 



In contrast with these results, Gorter (1932) concluded that 

 growth substance had no influence upon the growth of roots of 

 Pisum and maize. The roots were decapitated, and agar blocks 

 containing growth substance from the coleoptile tips of maize 

 were placed upon the wound surface. The experiment included 

 six Pisum roots (three with agar and growth substance, three 

 with agar without growth substance) and three maize roots. 

 When growth substance was added, the rate of growth in Pisum 

 was greater, in two cases, than when it was not present. Growth 

 was greatest in maize without added growth substance. The 

 author concluded from her experiments that growth substance has 

 no influence upon the growth of the root. Her data were too few 

 to permit a final conclusion on this matter. To clear up the 

 existing disagreement in reports of the different workers, Boysen 

 Jensen (1933c) carried out further experiments on the influence 

 of growth substance upon the rate of growth in roots of Vicia 

 Faba. In the first of these studies the methods of Nielsen were 

 used. The rate of growth was determined for root tips, some of 

 which were immersed in pure water and others in a growth- 

 substance solution containing 2 WAE per 100 cc; in the latter 

 case, growth decreases by about half. The use of Gorter's 

 method also showed that growth substance influences the rate of 

 growth of the root, but with her procedure it was necessary to use 

 far higher concentrations of growth substance. In this method, 

 the roots were decapitated 1.25 mm. back of the tip, and agar 

 blocks were placed upon the wound surfaces, either without or 

 with growth substance in concentrations of 25 to 50 WAE per 100 

 cc. The rate of growth was reduced about one-half by this con- 

 centration of growth substance (see also Cholodny, 19336; Navez, 

 19336). 



Kogl, Haagen-Smit, and Erxleben (1934, Mitt. XII) have 

 shown that 3-indole acetic acid when added to the culture 

 solution in concentrations of 0.01 to 1 mg. per Uter inhibits 

 root growth; auxin a and 6 have similar effects. The work of 

 Meesters (1936) has shown further the inhibiting influence of 

 3-indole acetic acid on the growth of root hairs and roots of 

 Agrostemma. Growth in length of the root hairs was retarded 



