VII GROWTH OF SPECIFIC ORGANS 803 



around lo '^M. The parallel with cell division is again noticeable though not quite 

 so directly proportional. Compounds 3 and 10, phenoxyacetic acid and its 3,5- 

 dichloro derivative, behave similarly; these two also have virtually no growth- 

 promoting activity on shoots. Thus it appears that every auxin, as defined by 

 growth promotion of shoots, inhibits growth of roots, while the compounds which 

 promote growth of roots are those with little or no auxin activity on shoots. Fur- 

 thermore, the most active growth-promoting compounds on shoots are in general 

 the most active inhibitors on roots. Note that compound No. 2, phenylacetic acid, 

 is certainly less active as an auxin on shoots than the other compounds (except 

 Nos. 3 and 10) in that figure. 



The special case of indoleacetonitrile (III p. 761) may be mentioned. As shown 

 in section IV the activity of this compound on shoots can be explained by its conver- 

 sion to lAA, the extent of which diflfers in different plants. In tomato roots, how- 

 ever, its action differs from that of lAA in that it promotes formation of laterals 

 and it does not decrease the survival of the meristem as does lAA (Street, 1954; 

 Street et al., 1954) The conclusion that its mode of action is quite different from 

 that of lAA is, however, unjustified. The concentrations of nitrile needed for 50% 

 growth inhibition are 5000-10,000 times those of lAA which means that activity 

 could be due to a conversion of < 0.1%. The large excess of unconverted nitrile 

 would not be wholly inactive but probably would weakly antagonize one or the 

 other activity of lAA, just as it weakly synergizes with lAA on pea stems. 



The exact activity of indoleacetic acid on roots is hard to estimate because this 

 substance is rapidly oxidized in root tissue. Furthermore, as mentioned in VIg (p. 

 787), peroxidase increases in lentil roots exposed to lAA, and since peroxidase can 

 oxidize lAA it follows that the ability of root tissue to destroy lAA would increase 

 with time of immersion. Roots of flax and cress do not show this effect, while 

 roots of oats (Bonner and Koepfli, 1939) and of wheat (Aberg and Jonsson, 1955) 

 destroy lAA very actively. A complication is that at subinhibitory concentrations, 

 lAA reduces the survival of the main axis meristem (Street, 1954). 



At concentrations lower than the inhibiting level some true auxins cause a pro- 

 motion of root growth. The effect is seldom more than 20% and is not obtained 

 with all roots. Flax roots, studied by Aberg, for instance, do not show it at all, 

 while wheat, oats and corn roots show clear promotions, 4-methyl-phenoxyacetic 

 acid giving a 35% increase of the elongation of wheat roots (Aberg, 1956). In the 

 older literature many workers have found lAA to cause increased elongation. 

 Most of these results were obtained after one or more days, when presumably the 

 lAA had been partially destroyed (see Table i in Larsen, 1956, and Fig. 2 of 

 Geiger-Huber and Burlet, 1936). 



When elongation is being inhibited by an auxin, the inhibition can be relieved 

 by one of the substances on the right-hand side of Fig. 1 1 which promote elonga- 

 tion. Such substances, which remove the inhibiting effect of auxins, are considered 

 to be auxin antagonists. As a rule they inhibit the elongation of Avena coleoptile 

 sections in presence of an auxin, which is of course another criterion of antagonistic 

 activity. While in general they promote the growth of intact roots, especially wheat 

 roots, the correspondence is not absolute, and in isolated roots the growth pro- 

 motion depends upon the sucrose content of the medium (Street, 1955). For these 



Literature p. 816 



