174 CHEMICAL AGENTS AND GROWTH 



Table \TI. X-Ray, Light and L^A Versus Leaf Expansion 



not compelling, suggests that auxin is involved in leaf expansion and 

 that some of the mechanisms involved in oat coleoptile growth are 

 also operative in leaf expansion. Possibly such a system is also opera- 

 tive in seed germination, although suitable experiments have not yet 

 been devised to test this hypothesis. 



Since it is known that nucleic acids are affected by x-irradiation 

 (Scholes et ai, 1956) and since the experiments cited earlier indicate 

 that purines may be involved in leaf expansion, it became of interest 

 to study the abihty of some of the purines to reverse the x-ray-induced 

 inhibition of leaf expansion. Table VIII shows data from a typical 



" All concentrations behave alike. 

 * TPSA thiopurinesuccinic acid. 



experiment with purines in x-irradiated discs. Again x-ray reduced the 

 control growth by an appreciable amount. The purines overcome a 

 portion of this inhibition, but will not entirely replace the indoleacetic 

 acid destroyed by the x-irradiation. This evidence again suggests that 



