294 



Freed, Reithel, and Remmert 



sorption and their activity. Accordingly, the ultraviolet absorption 

 spectra of a number of these compounds were determined with a Gary 

 recording spectrophotometer and the molar extinction coefficient in 

 the neighborhood of 280 fx calculated. The results of these determina- 

 tions are given in Table 3. The results in Table 3 are not as unequivo- 

 cal as was the case with the infrared absorption. It will be noted in 

 the monosubstituted compounds that the molar extinction coefficient 

 is actually less for the compound of greatest activity. However, the 

 relationship between absorption spectra and biological activity holds 

 very well for the di- and tri- substituted isomers studied. However, 

 when these ultraviolet absorption data are studied in relation to the 

 solubility of the compounds, a slightly different picture emerges. Thus, 

 the molar solubility of the 2- and 3-chlorophenoxyacetic acids is ap- 

 proximately twice and three times that of the p-chlorophenoxyacetic 

 acid, being 7.2 X 10-=^Af, 11.6 X lO-^M, and 4.3 X lO-^M, respectively, 

 for these compounds. In light of the relationship of water solubility to 

 biological activity, it is reasonable to suppose that a higher molar 

 dosage of the more soluble compounds would be required to achieve 

 the same degree of biological activity, all other factors being equal 

 (12) . Table 4 represents an attempt to summarize the relation of the 

 physical properties to biological activity. It will be noted that a 

 negative value for any of the properties is associated with a compound 

 of low activity. Thus, the importance of these properties to activity is 

 clearly demonstrated. 



ACTION OF 2,4-D ON ROOTS AND MITOCHONDRIA 



If, as previously suggested, the primary event in ihc action of 

 these synthetic growth regulators is adsorption, it should be possible 

 to demonstrate a ready reversibility of the action of these substances 

 (9). Tims, it would seem that removal of a plant or an organism from 

 a short exposure to the chemical should result in a recovery from 



