CHEMISTRY OF THE GROWTH SUBSTANCES 



765 



analysis, that the antagonism is competitive {cf. section Vllb). In other cases, e.g. 

 cyclohexaneacetic acid, XXI, or 2,3,5-triiodobenzoic acid, XXII, they act as 



CH, 



-C— COOH 



HjC 



COOH 

 I 

 .CH 



CH; 

 I 



CH, 



Ha 



2,4,6-Trichlorophenoxy 



acetic acid 



XIX 



a-Indoleisobutyric 

 acid 

 XX 



Cyclohexaneacetic 

 acid 

 XXI 



2,3,5-Triiodo- 



benzoic acid 



XXII 



synergists in the slit pea stem test, greatly increasing the apparent activity of 

 added auxin. 



High auxin concentrations, as mentioned above, inhibit growth and very high 

 levels are toxic. Hence a curve plotting growth against auxin concentration 

 necessarily passes through an optimum (see Fig. 7, p. 776). This phenomenon is dis- 

 cussed in detail in section VIb (p. 776). 



The gibberellins are quite unrelated to the compounds described above, 

 although in their physiological effects they have some properties in common (see 

 section VId, p. 780). GibberelHc acid (also termed gibberelhn A3 and gibberelHn X) 

 is a lactone-acid derived from 1,7-dimethylfiuorene and has the tentative formula, 



XXIII, Ci.H^P^: 



CH3 



Gibberellic acid 

 XXIII 



(Curtis and Cross, 1954; Stodola, 1956). GibberelHn Aj of Sumiki (Takahashi et 

 aL, 1955) has been termed gibberelhn A in the USA and is probably CJ9H24O6, 

 while gibberelHn A^ of Sumiki and co-workers is Cj^Hj^Og. The crystalline 

 product obtained from cultures of Fusarium moniliforme in the USA is a mixture 

 of Aj and A3 (Stodola et aL, 1955). The original "gibberelHn A", first isolated 

 by Yabuta and Hayashi (1939) may have contained ah three. So far as their 

 potencies have been compared all three seem to be about equally active, but 

 as yet the comparisons are very incomplete. On the other hand the breakdown 

 product ahogibberic acid, (gibberelHn B of Sumiki) XXIV, CjgHjoOj, may have 

 lower activity, while its rearrangement product formed in hot dilute acid, gibberic 

 acid, XXV, is apparently inactive. A3 has also been isolated from higher plants. 



CH3 H COOH 



Allogibberic acid 

 XXIV 



- X CH2- 



*-^3 H COOH 



Gibberic acid 

 XXV 



Literature p. 8i6 



