KENNETH V. THIMANN 23 



good curvatures, though with only about i per cent of the activity of 

 indoleacetic acid, but the curvatures are hmited to a very short apical 

 zone. In growth tests in which the plant parts are immersed in the solu- 

 tion, the activity is much higher relative to that of indoleacetic acid 

 than in the Avena test, reaching 20 per cent in the curvature of slit 

 pea stems. Further, in the formation of roots on pea stems III has low 

 activity when applied to the apex of the stem but is highly active when 



CH^COOH 



I I 



CHjCOOH ^<^ ^0' 



applied to the base; the difference is of the order of 100 times. (In later 

 studies we have found this compound excellent for the rooting of woody 

 cuttings, when appHed to their bases.) These observations show that the 

 activity of III is being hmited by the ability of the plant to transport it. 

 A similar analysis of the activity of benzofurane-2-acetic acid (IV) gave 

 comparable though more extreme results. It is completely inactive in 

 the Avena curvature test but active on immersed sections; and in root 

 formation it is inactive when appHed to the tip but active at the base. 

 Benzofurane-3-acetic acid (V), the spatial analogue of II, is about twice 

 as active as IV but is also limited by transport. 



Activity in inducing growth thus depends not only on activity of the 

 compound per se, but on its abiUty to be transported in the plant. It is 

 obvious that there may be many other such secondary characteristics 

 which influence the apparent activity of a compound. The following may 

 be suggested: i) ability to enter the cell (solubility in the plasma 

 membrane); 2) resistance to inactivation by plant enzymes; 3) stability 

 to hght (in certain tests), or to the combination of light and photo- 

 dynamic substances hke eosin, riboflavin, or carotenoids; 4) dissociation 

 constant of the acid group, since the anion is in general less active than 

 the undissociated acid. 



The influence of many such factors can be minimized by using a short- 

 time test with a single standard tissue immersed or floating in a solution 

 of known pW in darkness. The curvature of slit halves of etiolated pea 

 stems has been most widely used and most of the conclusions which 

 follow are based on this test. To avoid curvatures due to release of 

 residual auxin in the tissues, the sht halves are placed in water for two 

 hours before exposure to the test solution (16). 



