Chemistry and Qrowth 



inactive in the Avena coleoptile test. The effects of other 

 substitutions were briefly discussed. 



The differing activities of the artificial growth-substances 

 has been referred to their different behaviour towards what 

 plant physiologists call "polar transport" in the plant. 



An inhibitory influence of the terminal bud of plants upon 

 the lateral ones has been known for a long time : it is the basis 

 for some operations of pruning. It has been shown by Thi- 

 mann and Skoog {1933) and others that the effect is due to 

 nothing else but the production of auxin by the terminal 

 bud : auxin is able to prevent lateral buds from developing. 



Van Overbeek (1938) has attempted to explain how this 

 inhibition might work. Went (1938) found that phenyl- 

 butyric acid has no activity in the Avena test, yet is able to 

 inhibit bud development to a considerable degree. Van Over- 

 beek made use of this property of phenyl-butyric acid in order 

 to be able to distinguish between the effects of hormone (auxin) 

 produced by the plant, and the effects of an artificial growth- 

 substance (phenyl-butyric acid) applied externally. Whereas 

 other workers have recorded amounts of auxin or the activities 

 of natural or synthetic growth substances in Avena units and 

 thus in terms of auxin itself, Van Overbeek employed the 

 novel unit of "gammas of indole-acetic acid equivalent per 

 1,000 gm. of water contained in the plant". This is a step 

 towards standardization in terms of a known and widely- 

 accessible pure substance. 



The reaction of dormant leafless cuttings of willow to 

 lanolin pastes or water solutions of indole-butyric acid has 

 been studied by Pearse (1938). The formation of roots was 

 greatly stimulated. When the portion of the base that had 

 been treated was removed, the effect of the treatment was 

 eliminated. A second treatment applied to a truncated 

 cutting shortly after the end of the first treatment (by a 24- 

 hour immersion in growth-substance solution) induced root- 

 formation. Pearse thought therefore that the indole-butyric 

 acid itself was the active agent that promoted root formation, 

 and was itself used up or chemically changed in the process. 

 He also thought that it may be more effective to apply 



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