GROWTH SUBSTANCES FOR NORMAL GROWTH 101 



by about 20 per cent in the presence of 0.5 mg. of 3-indole acetic 

 acid per liter; almost complete inhibition of root elongation 

 occurred with the same concentration of the hormone. Other 

 solutions of the same pH value, obtained by the addition of 

 acetic acid, did not show any inhibiting effects. 



It has been determined with certainty, therefore, that the rate 

 of growth of roots is retarded by the addition of growth substance, 

 and from this it might be concluded that growth substance is not 

 necessary for the growth of roots. In support of this interpreta- 

 tion is the fact that ageotropic roots, which can be obtained by 

 treating the seed with eosin or erythrosin (Boas and Merken- 

 schlager, 1925), often possess no demonstrable amount of growth 

 substance (Boysen Jensen, 1934), although the rate of growth is 

 not decreased. 



The evidence from normal distribution of growth substance in 

 roots makes another interpretation equally plausible. Boysen 

 Jensen (1933b) and Thimann (1934) both have shown that a 

 concentration gradient exists at the growing end of the root, the 

 tip possessing the most growth substance, and the concentration 

 falling off in a proximal direction. From this it might be con- 

 cluded that growth substance does take part in root growth and 

 that elongation of the root is taking place in the region of opti- 

 mum concentration. If this is the case, the optimum concen- 

 tration for root growth must be very low. Why the root and 

 shoot behave differently in the presence of a given concentration 

 of growth substance remains to be explained. [Czaja (19356) 

 discusses a possible explanation based upon the direction of 

 streaming of growth substance in roots.] 



Lower Plants. — Heteroauxin is produced by many lower 

 organisms, e.g., Aspergillus niger. This substance has a remark- 

 able effect upon the rate of growth of the Avena coleoptile, and it 

 is important to determine whether it has any demonstrable 

 physiological significance for the growth of Aspergillus itself. 



Boysen Jensen (1932) showed that if Aspergillus is cultivated 

 upon a glucose-nitrate solution, growth substance cannot be 

 demonstrated either in the fungus mycelium or in the culture 

 substratum. This would indicate that growth substance is not a 

 necessity for the growth of Aspergillus. Whether it has any 

 influence upon the growth of this organism was determined in the 

 following way: Aspergillus niger was grown upon a glucose- 



