286 Auxins in Agriculture 



1951). The tetra- and penta-chloro derivatives are apparently not 

 auxins, but they are effective herbicidal materials. They bring about 

 a stasis of growth without much leaf distortion (Miller, 1952). One of 

 the most outstanding characteristics of the benzoic acid herbicides is 

 their extensive persistence, both in plants and in soils. Whereas 2,4-D 

 persists in bean plants only long enough to cause extreme distortion 

 to 3 or 4 leaves, the substituted benzoic acids persist long enough to 

 distort up to 11 leaves (Minarik et al, 1951). Field observations 

 similarly indicate much longer persistence in soils. 



In 1953 it was reported that halogenated phenyl alkyl ethers pro- 

 duce strong distortion effects on plants after a delay in time, and 

 these compounds show some promise as herbicides (McLane et al, 

 1953). Structurally these materials are comparable to 2,4-D except that 

 they have long straight side-chains with no acid group at the end. 

 Only even-numbered carbon side-chains are active, and the side-chain 

 must be from eight to twelve carbon units in length for herbicidal 

 activity. This strongly suggests that the alkyl ethers are metabolized 

 down to 2,4-D, for biological oroxidation of straight-chain hydro- 

 carbons to produce a terminal carboxyl is known to be limited to those 

 of eight to twelve carbon lengths, and only the even numbered side- 

 chains would result in an acetic acid final derivative, because fatty 

 acids are broken down in two-carbon fragments. 



Solvents 



For approximation of herbicidal amounts and concentrations it 

 is convenient to know that when a solution is applied at the rate of 

 100 gallons per acre ("to runoff"), a concentration of 1000 mg./l. will 

 furnish an application of almost exactly one pound per acre. When 

 concentrate sprays are applied at the rate of five gallons per acre, a 

 concentration of 20,000 mg./l. (2%) will result in an application of 

 one pound per acre. 



The use of various solvent carriers has been discussed in chapter 

 IX. The use of polyalkylene glycols (Carbowax) to increase solubility 

 and effectiveness of 2,4-D originated with some of the early herbicidal 

 auxin studies (Mitchell and Hamner, 1944). Detergents in general 

 similarly increase herbicidal effectiveness (Staniforth and Loomis, 

 1948), undoubtedly by facilitating entry of auxin into the plant in 

 the manner described in chapter VI. The use of oil carriers for esters 

 is common practice and light oils have been consistently more 

 effective than heavy oils (Kraus and Mitchell, 1947). The use of oil 

 or diesel fuel carriers permits the use of very high concentrations of 

 auxin such as the 44% 2,4-D commonly used in airplane applications. 



