168 Fundamentals of Auxin Action 



ment is incomplete as shown in D. A few examples of each of these 

 types of anti-auxins are given in table 9. 



TABLE 9 



Examples of Auxins and Anti-auxins Belonging to the Classes in Figure 75 

 (Adapted from McRae and Bonner, 1953) 



A compound which lacks both a reactive ring position and the 

 proper acid group will not be an anti-auxin. If it does not react at 

 either of the points of attachment, it will obviously not interfere with 

 auxin molecules doing so. An excellent example of this has been 

 cited by McRae and Bonner (1953) in the case of 2,4,6-trichloroani- 

 sole (XXVI), in which the ortho positions of the ring are both occu- 

 pied and there is no acid group. 



A fourth type of anti-auxin recognized by McRae and Bonner 

 (1953) is the type which can apparently fill both of the points of 

 attachment satisfactorily but whose activity in that position is weak. 

 Perhaps the most clearcut examples of this type are y-phenylbutyric 

 acid (XXIV) (Skoog et al, 1942) and phenoxyacetic acid (Ingestad, 

 1953). It is believed that these compounds have strong affinities for 

 one or both of the two points of auxin attachment, but that they 

 show only weak activity in the final auxin function. Many of the anti- 

 auxins show weak auxin activity, for example the two compounds 

 cited as belonging to this class, and 2,6 and 2,4,6 chlorophenoxyacetic 

 acids and most of the a-phenoxypropionic acids (Thimann, 1952; 

 Smith et al, 1952). The very fact that they are such weak auxins is 

 undoubtedly partly responsible for their anti-auxin properties. They 

 are excellent examples of compounds which probably have good 

 affinity for the receptor but poor activity after being attached. 



