Herbicides 285 



the data given in figure 117. It is evident that the inhibition of 

 germination of seeds when suspended over various esters of 2,4-D is 

 much the greatest when the alcoholic group is small as in the methyl 

 ester, indicating the greatest volatility for the small esters. Of course 

 high volatility is a treacherous characteristic for an herbicide because 

 of the danger of damaging plants away from the treated area. Al- 

 though the volatility decreases as the size of the alcoholic group is 

 increased, the toxicity is apparently unchanged (Robbins et al, 1952, 

 p. 149). Esters with intermediate-sized alcohols such as the isopropyl 

 derivative have achieved most widespread use in crops, and esters with 

 much larger alcohol groups — such as butoxy-ethanol esters — are used 

 generally for killing woody species. In the spraying of woody plants 

 along roadsides or railroad right-of-ways, it is very important to insure 

 against volatility which may result in damage to neighboring fields. 

 The use of esters results in a strong reduction of selectivity (Alex 

 et al, 1950) as we have already pointed out in another connection. 



Besides the derivatives of the phenoxyacetic acids, quite another 

 type of material has come into use as an herbicide, namely the phe- 

 noxyethyl acids. The commonest of these is 2,4-dichlorophenoxyethyl- 

 sulfate (King et al, 1950). This material is not an auxin, nor does 

 it have herbicidal activity. However, upon being applied to the soil 

 it is converted into 2,4-D either by microbial action (Audus, 1952) 

 or simply by acid hydrolysis (Vlitos, 1952). The 2,4-D thus formed can 

 then be taken up by plants and exert the herbicidal effect. A delay 

 in effect of five to fifteen days is caused by the necessity for con- 

 version into 2,4-D. Similar effects are obtained for 2,4-dichlorophenoxy- 

 ethylbenzoate. These materials are advantageous in that relatively 

 large amounts can be applied to fields without foliar damage to the 

 crop, and upon conversion into 2,4-D they can effectively control the 

 germinating weeds at the surface of the soil. 



JOCH2CH2SO^Na i^ |] OCH2CH2CH2CH2CH2CH2CH2CH3 



CI CiL Jci 



Sodium 2,4-dichlorophenyl 



2,4-dichlorophenoxyethyl octanoic ether 



sulfate 



Another group of herbicidal compounds which probably act 

 through auxin mechanisms are the chlorinated benzoic acids. Some 

 compounds in this group with three chlorine substituents are auxins, 

 as described in chapter VII. They produce leaf distortions and epinasty 

 in the manner common to most auxins (Zimmerman and Hitchcock, 



