THE GROWTH OF ROOTS 



143 



the lowest active concentration is about that which gives 

 5° curvature on the Avena coleoptile (Lane, 1936; Meesters, 

 1936). The inhibition is not due to pH, and is not com- 

 parable with that produced by toxic substances. Since 

 indene-3-acetic acid exerts the same inhibiting effect (Thi- 

 mann, 19356), it is presumably a general property of growth 

 promoting substances to inhibit root elongation. Root in- 

 hibition can even be used as a method for the assay of auxin, 



o 

 < I- 



H O 



5° 



o " 



Ul 



100 



80 



60 



40 



20 



0050 



Hz^ 



Q020 0.004 0.0008 0.00016 .00003 2 



CONCENTRATION IN MG. PER CC. 

 I. IND0LE-3-ACETIC ACID 2. I ND0LE-3-PR0 PION IC ACID 



Fig. 46. Relation between auxin concentration and the inhibition of root 

 elongation in Avena. The seeds were soaked 24 hours in water and then 24 

 hours in test solution. The concentration of indole-propionic acid necessary 

 to cause a given inhibition is 25 times that of indole-acetic acid. (From Lane, 

 Am. J. Bot. 23: 532-535, 1936.) 



for which it is both simple and sensitive, the inhibiting 

 concentrations of auxin being much lower than those of 

 ordinary toxic substances such as mercury and silver ions 

 (Lane, 1936). The ratio of activities of indole-propionic and 

 indole-acetic acids, which in Avena coleoptile curvature is 

 0.2 per cent, in Avena root inhibition is about 4 per cent, 

 (see Figure 46). 



Extremely low concentrations of auxin may produce 

 acceleration of root growth. Thus Amlong (1936) finds that 

 if Vicia Faba roots are decapitated and 3 hours later, when 

 their auxin content will have somewhat decreased, are 

 treated with auxin solutions, then 10"^ mol. indole-acetic 

 acid causes slight but definite growth promotion. Higher 

 concentrations still inhibited growth. Fiedler (1936) ob- 

 tained a similar result with isolated Zea Mays roots in cul- 

 ture medium without yeast extract ; their auxin content was 



