All the mono- and di-chlorophenylacetic acids, together with certain analogous trichloro-derivatives, have recently been prepared in this laboratory and their growth-regulating activity has been assessed in the wheat cylinder, pea segment and pea curvature tests3. During this investigation, the activity in the above three tests of all the corresponding benzoic and phenoxyacetic acids was re-examined for comparative purposes. With phenoxyacetic acids, high activity occurs only when a substituent group such as chlorine is present in the)ara-position of the ring ; thus, for example, the 4-chloro-, 2 : 4- and 3 : 4-dichloro-, 2:3:4- and 2:4: 5-trichloro-acids are all highly active4. In the benzoic series, however, compounds substituted in the para-position possess only weak activity ; when this position is free, high activity is found to be associated with ortho-substitution. Thus the 2:3-, 2:5- and 2 : 6-dichloro-and 2:3: 6-trichloro-benzoic acids are all active compounds5. With the phenylacetic acids, we have found that all the mono- and di-chloro-derivatives exhibit activity3, the 2 : 3- and 2 : 6-compounds being extremely active. The high activity of 2 : 3- and 2:6-dichlorophenylacetic acids led logically to an examination of 2 : 3 : 6-trichlorophenylacetic acid and, as expected, this compound proved extremely active. The performance of these three acids in our standard laboratory tests, together with those of the corresponding benzoic and phenoxyacetic derivatives, are presented in Table 1, where comparable data for the activity of 2 : 4-dichlorophenoxyacetic acid are also given. The results clearly reveal the high growth-promoting properties of 2:3- and 2 : 6-di-chloro- and 2:3: 6-trichlorophenylacetic acids. It will be noted that the activity of the 2:3- and 2:3:6-derivatives in these tests was at least equal to that of 2 : 4D and far superior to that shown by the corresponding 2 : 3-dichloro- and 2:3: 6-trichloro-benzoic and phenoxyacetic acids.Table 1. GROWTH-PROMOTING ACTIVITY IN THE WHEAT CYLINDER, PEA SEGMENT AND PEA CURVATURE TestOF 2:3- AND 2:6-DICHLORO-AND 2:3:6- TRICHLORO-DERIVATIVES OF BENZOIC, PHENLACETIC AND PHENOXYACETIC ACIDS . THE ACTIVITY OF 2:4-DICHLOROPHENOXYACETIC ACID IN THESE TESTS IS ALSO GIVEN Acid Concentrations tested (molar)Wheat cylinder test: Pea seagment test: Pea curvatu re test: Xl0-7 x l0-6 x l0-5 x l0-4 x l0-3 Xl0-7 xl0-6 xl0-5 xl0-4 xl0-3 Xl0-7 xl0-6 xl0-5 xl0-4 Xl0-32 : 3-Dichlorobenzoic 2 : 6-Dichlorobenzoic 2:3: 6-Trichlorobenzoic 2 : 3-Dichlorophenylacetic 2:6-Dichlorophenylacetic 2:3:6-trichlorophenylacetic 2:3-Dichlorophenoxyacetic 2:6-Dichlorophenoxyacetic 2:3:6-Trichlorophenoxyacetic 2 : 4-Dichlorophenoxyacetic j 101 99 103 138 102 110 99 101 100 114 1029910915210914499100100135 10910013015414115110010099150 125106144153148150105105102154 121141141126127142113114111128 100 102 104 103 100 119 101 103 100 112 10299115114108129100102102122 10510212512512013098107107127 117106129130130126108119117125 120116125114121116118117113115 0001040004 0024360005 1046660216 2166663436 3262224224The responses obtained in all tests were assessed after a 24-hr, treatment period. In the wheat cylinder and pea segment tests, activity is expressed as a percentage of the final length of control segments. Figures in italics are significantly different from controls at the 0 .1 per cent level. In the pea curvature test, activity was assessed using a scale from 0 (inactive) to 6 (highly active). All compounds were examined at the stated concentration of the acid in the form of its sodium salt.The marked activity of these three phenylacetic acids in our laboratory tests prompted us to investigate their effect on weed and crop plants. The results of experiments carried out to date indicate that all show selective herbicidal activity, and that the 2:3- and 2:3: 6-derivatives are as effective as 2 : 4D in controlling many weed species. Further investigations on the herbicidal properties of these compounds and their effect on crop plants would appear to be warranted.