266 Teubner, Wittwer, and Shen 



because of solubility difficulties, loss of activity of solutions on stand- 

 ing, and the possible hydrolysis of the amide liberating the aromatic 

 amine and phthalic acid (3). The measures taken to avoid these diffi- 

 culties are given under Methods. 



The results presented in Table 3 are those of a single test, but 

 have been confirmed for the chloro-substituted derivatives in at least 

 two other tests. The per cent elongation relative to controls shows 

 essentially the same structure-activity relationships that Avere apparent 

 in both flower formation and fruit parthenocarpy. Mono-para and di- 

 ortlio substitutions either abolish or have no effect on the activity of 

 N-phenylphthalamic acid. The most active compounds were the N- 

 2,3-dichloro-, A''-2,3,5-trichloro-, and N-3,5-dichlorophenylphthalamic 

 acids. The high activity of the latter was surprising in view of its 

 lower activity in tomato flower formation and ovary development. 

 In addition to the derivatives listed in Table 3, the three (ortho, meta, 

 and para) mono-hydroxy and mono-nitro derivatives were tested and 

 were inactive. The closely related N-1-naphthylphthalamic acid was 

 almost as active as A^-2-chlorophenylphthalamic acid at 10"^ and 10"^ 

 M, giving 115 and 117 per cent elongation, respectively. xV-1-naphthyl- 

 ph thalamic acid did not, however, stimulate the growth of Avena sec- 

 tions at concentrations lower than 10-^Af and retardation (88 per 

 cent of controls) occurred at lO-^M. On the other hand, N^-2-naphthyl- 

 phthalamic acid was one of the more active derivatives tested with 

 133, 125, and 120 per cent elongation at 10 3, lO*, and 10-^M concen- 

 tration, respectively. It should be noted (Table 3) that the maximum 

 elongation obtained with any of these derivatives was only 129 per 

 cent, while comparable results with indole-3-acetic acid at \Q''M were 

 147 per cent of the final length of control sections. 



DISCUSSION 



The results clearly indicate that modifications of structure, through 

 substitution of chloro- and methyl- groups on the phenyl ring, have 

 precisely the same effects on auxin activity (Tables 2 and 3) as on ac- 

 tivity in tomato flower formation (Table 1). It is not possible, at 

 present, to resolve the question of whether these chemicals function 

 as auxins, or in some other manner, in the control of tomato flower 

 formation. However, it is evident that only those derivatives which 

 have activity in stimulating Avena straight growth and tomato par- 

 thenocarpy are able to increase flower numbers in the tomato. 



Consideration of the structural relationships of the substituted N- 

 phenylphthalamic acids on the basis of their activity in Avena straight 

 growth (Table 3) indicates requirements for activity distinctly dif- 



