154 Fundamentals of Auxin Action 



(IV), or as large as the anthracene ring (V) which contains 14 carbon 

 atoms. 



CHzCOOH 

 CH,COOH - - - 



IV V 



The ring must be unsaturated; that is it must have at least one 

 double bond. Went (1949) stated that one double bond must be lo- 

 cated adjacent to the side-chain. For example, the saturated cyclo- 

 hexane acetic acid (VI) is inactive, whereas 1-cyclohexene-l -acetic 

 acid (VII) possesses auxin activity, and 2-cyclohexene-l-acetic acid 

 (VIII) is inactive. 



CHjCOOH [^ \cH2COOH 



VII VIII 



Substitution of various groups into the unsaturated ring may pro- 

 foundly affect auxin activity. The nature of the substituent and its 

 location on the ring will both influence activity. A graphic example 

 of this is the substitution of a single chlorine atom on each of the 

 possible positions on the ring of phenoxyacetic acid. The striking dif- 

 ferences in activity obtained in the ortho, meta and para positions 

 using this same single substituent are shown in figure 69. Curiously 

 enough, the ortho chlorine increases growth activity but slightly, while 

 the meta and para chlorines each furnish very large increases in ac- 

 tivity. Again, substitutions in more than one position on the ring will 

 have strong effects on activity both in regard to what groups are 

 substituted and the position at which they are substituted. A striking 

 case in point is the substitution of more than one chlorine atom into 

 phenoxyacetic acid as shown in figure 69. It can be seen that chlorines 

 at both the 2 and 4 positions (2,4-D) impart immensely more growth 

 activity than did either one of the chlorines alone. The further addi- 

 tion of a chlorine in the 5 position (2,4,5-T) alters the growth re- 

 sponse very little, whereas the addition in the 6 position essentially 

 eliminates activity (figure 69). If other groups such as bromine, iodine, 

 methyl or nitro groups are used for substitutions, somewhat smaller 

 effects are generally obtained (Koepfli et al, 1938; Thimann, 1952). 



An interesting and productive finding concerning substitutions in 

 the ring of the phenoxyacetic acid series has been made by Muir et al 



