8o4 PLANT GROWTH lO 



reasons Hansen (1954) has suggested that root growth promoting compounds 

 be called "root auxins," and only those compounds which relieve the inhibi- 

 tion due to externally applied auxin be considered true antagonists. The case 

 is particularly complex with indoleisobutyric acid (XX, p. 765) which promotes 

 root cell elongation by about 200 my. whether or no the root is being simultane- 

 ously inhibited by lAA (Burstrom and Hansen, 1956). The liBA is thus not really 

 acting as an antagonist here, but as a simple root growth promoter, and the logical 

 conclusion is that the two substances "act in two physiologically different systems, 

 and that they do not simply compete in one single reaction mediating the cell 

 elongation". (It may be noted that XX also acts as a true auxin on shoots). 



Synthetic auxins with an aryl group in the a position of a propionic or butyric 

 acid side-chain exist in d- and L-forms. The D-forms are invariably the ones with 

 auxin activity, the L-forms having always less activity and sometimes none at all. 

 Most of these L-forms act as auxin antagonists, L-a-(2-naphthoxy)-;2-butyric acid, 

 XXXn, being particularly effective (Matell, 1953; Aberg, 1953). Some, however, 

 are inactive even as antagonists. 



O — C — GOGH 

 H 



a-2-naphthoxy-«-butyric acid 

 XXXII 



All these data have given rise to intensive theorizing. Many of the antagonisms 

 are apparently competitive, i.e. the effects depend on the concentrations of both 

 auxin and antagonist, and in a manner predictable from theory (Skoog et al., 

 1942; McRae^ia/., 1953; McRae and Bonner, 1953; Linser, 1954; Kaindl, 1956). 

 Similar competitive antagonism can often be shown in coleoptile sections. There 

 are also compounds of an intermediate type, which act as auxins where the auxin 

 concentration is low, but as antiauxins where it is high or when effective auxins 

 are added (see Aberg, 1956). These compounds cause peculiar effects on shoot 

 tissues such as coleoptiles, inhibiting growth at low concentrations and promoting 

 it (or inhibiting it less) at higher ones. Evidently the relations are a little too com- 

 plex to be expressed as yet in any simple theory (see Torrey, 1956b, for review). 

 All the above considerations exemplify the opposite way in which roots react 

 from shoots where auxin is concerned. The tropisms offer another example. 

 Shoots curve upwards away from the earth (negative geotropism), roots down- 

 ward (positive geotropism). Shoots curve towards high light intensities (the reac- 

 tion to low light dosages being complex) while roots frequently curve away. In 

 shoots the curvatures are well known to be due to differences in the concentration 

 of auxin on the two sides, though the method by which this is achieved is still 

 under discussion (see Went and Thimann, 1937, and Brauner, 1954, for reviews; 

 Rufelt, 1954; Larsen, 1956; and Curry et al., 1956, for recent literature). In roots 

 the proof of such a distribution is really not quite complete. For geotropism, an 

 increase in auxin on the lower side of a horizontally-placed root has been shown by 

 several workers in the past, and this would cause inhibition of the extension on the 



