Chemical Nature of Growth Regulators 167 



are schematically portrayed in figure 75. Assuming that both points 

 of attachment of auxins are on the same receptor substance, a true 

 auxin can satisfactorily fill both (figure 75 A). If the ring attachment 

 is obstructed, incomplete attachment results (B). If the appropriate 

 acid group on the side-chain is lacking, incomplete attachment again 

 occurs, as shown in C. If both ring and acid groups are satisfactory 

 but a steric hindrance exists for the two-point attachment, attach- 



A. Complete 2- Point Attachment 



B. Limitation in Ring 



Limitotion in Acid Group 



D. Limitation in Configuration 



Fig. 75. Diagrammatic representation of the two-point attachment scheme of 

 auxin and anti-auxin action. A represents a complete auxin which completes both 

 attachments. B, C and D represent anti-auxins which complete only one point of 

 attachment because of structural limitations (redrawn from McRae and Bonner, 

 1953) . 



