Herbicides 



283 



OCHoCOOH 



MCPA 



The relative effectiveness of different derivatives of auxins is 

 altered by the age of the plant. For example, it has been observed that 

 esters and salts are essentially equally effective at the seedling stage, 

 but as the plants become older the esters become more and more 

 effective in comparison with the salts (Lee and Bewick, 1947). This is 

 consonant with the generalization of Crafts (1948) that lipoidal or 

 non-polar materials enter the plant more readily when the cuticular 

 layers of the epidermis haye matured, for these cuticular layers them- 

 selves are primarily made up of lipoidal, non-polar materials. 



Comparison of different salts of auxins indicates that salts of low 

 solubility are relatively less effective when applied to plants and con- 

 versely that those of high solubility are more effective. Some compari- 

 sons are shown in figure 116. Uniform quantities of the various salts 



Control 



AUXIN DERIVATIVE 



Fig. 116. Relative effectiveness of various salts of 2,4-dichlorophenoxyacetic 

 acid and 2,4,5-trichlorophenoxyacetic acid in inhibiting bean shoot growth. All 

 applications were of 6 micrograms of acid equivalent per plant (Mullison, 1951). 



