98 



PLANT GROWTH SUBSTANCES 



Malate, fumarate, succinate, isocitrate, and pyruvate are all effective 

 in this way (Fig. 2). Concentrations of about M/iooo are needed. With 

 these acids added to the auxin and sucrose solution the coleoptile sections 

 grow at least as much as though the iodoacetate were not present, and in 

 general somewhat more. Even malonate, which at higher concentrations 

 is itself a growth inhibitor, protects against iodoacetate. 

 160 



2 3 4 5 6 7 



CONCN. OF IODOACETATE X lO' 



10 



Figure i. Effect of iodoacetate on the growth of A vena coleoptile sections 

 after 48 hours at 25°C. in darkness in indoleacetic acid i mg. per liter plus 

 sucrose i per cent. Sections cut from plants aged: curve A, 74 hours; B, 64-66 

 hours; C, 54-56 hours; D, 96 hours. The growth of the uninhibited controls 

 is placed at 100 per cent for each age. (10). 



It follows from these observations that the amount of the dicarboxylic 

 acids normally present in the coleoptile must exert considerable influence 

 upon the response of the plant to iodoacetate. If the content of these 

 acids were high enough, presumably iodoacetate would not inhibit 

 growth at all, and if it were to decrease with increasing age, the decrease 

 in resistance to iodoacetate would be explained. Accordingly analyses 

 were made for total organic acids, malic and citric acids, in coleoptiles 

 of different ages. The methods of Pucher and Vickery (8) were used, 

 the same 9 mm. zone of the coleoptiles being analyzed as was used for 



