ROOT FOLIATION 



195 



been purified by ether extraction (c/. above) gives longer 

 roots than if unpurified {u). Green plant parts, producing 

 or containing ample carbohydrate, are of course not de- 

 pendent on added sugar. 



Interrelations between auxins. It was mentioned above 

 that the activity of various extracts was not always parallel 

 to their auxin content. It has since been found {u) that 

 the activity of auxin is increased by a number of other 

 factors. One of the most interesting of these relationships 

 is the effect of auxin a on the action of indole-acetic acid, 

 of which Table XV {u) gives an example. In this experiment 

 the cuttings were treated either with one auxin alone, or 

 with indole-acetic acid in the highest concentration together 

 with auxin a in varying concentrations. It will be seen that 

 auxin a, even down to its lowest concentration (0.002 per 

 cent = 2.10"^ mg. per cc), increases the number of roots 

 formed in presence of excess indole-acetic acid by about one 

 fourth. Urine, in concentrations which are not toxic, has a 

 similar effect, which is undoubtedly due to its auxin a con- 

 tent, as is shown by the fact that its ether extract is also 

 active in the same way {u). On the other hand, the reverse 

 procedure, namely the addition of small amounts of indole- 

 acetic acid when auxin a is in excess, has no such effect. 



TABLE XV 



Effect of Auxin a on the Root Form.\tion of Pea Seedlings by Indole- 

 3-AcETic Acid. Each Figure Represents the Number of Roots per 10 

 Pl.\nts and Is a Mean of 30-100 Plants 



