Natural auxins 



Clearly, both these zones arise from precursors which travel on the chro- 

 matograms nearer to the starting line. In tomato there is evidence of two 

 precursors which are interconvertible {Figure 5). It is not possible to say 

 from the evidence so far obtained whether these precursors are themselves 

 physiologically active or whether they are converted to an active substance 

 or substances during bio-assay. The first step to answering this question is 

 obviously chemical identification. 



Earlier workers have constructed theories (e.g. the Cholodny-Went 

 theory) to explain the growth of roots in terms of lAA. It is evident that the 

 hormone situation in roots is far more complex than had hitherto been 

 suspected. Obviously, more work needs to be done on the inter-relationships 

 between the water-soluble hormones and hormone precursors demonstrated 

 in the present work, and on their chemical identity, before it is possible to 

 construct satisfactory theories of hormone-controlled root growth. 



Yet another zone of activity nearer to the starting line than lAA has been 

 demonstrated in maize seeds, both in the aqueous fraction in the present 

 work, and in the ethereal fraction by earlier workers. It has been suggested 

 that activity in this region of the chromatogram is due to IPyA. Work on 

 synthetic IPyA has shown that it breaks down to form lAA, together with 

 other substances, tmder the conditions of chromatography used in these 

 studies. On chromatograms of maize extracts, FeCIg/HClO^ gives a yellow 

 colour at the position of I AA while other chromogenic reagents clearly show 

 that physiological activity at this position cannot be due to lAA. Therefore, 

 quite apart from the fact that IPyA does not survive under conditions of 

 ammoniacal chromatography, it cannot be responsible for the activity near 

 the starting line in the maize extracts, as there is no production of lAA. This 

 zone is therefore due to yet another unidentified auxin. 



We regard the presence of IPyA in plants as yet unproved, but even if it 

 does exist, it cannot be identical with the accelerator-a of Bennet-Clark and 

 Kefford (1953) as postulated by Stowe and Thimann (1953): the former 

 authors have shown that a promotes root growth, whereas the present work 

 shows that IPyA causes only inhibition. 



SUMMARY 



1 . A report in the literature indicates that an acidic precursor of a neutral 

 growth substance exists in cabbage. An alcoholic extract of mature cabbage 

 has been prepared, and from this an aqueous, ether-insoluble fraction has 

 been obtained. This fraction has been examined by chromatography in 

 ii^opropanol/ammonia and n-butanol/ammonia. It contains a substance 

 which is physiologically active on bio-assay with Avena coleoptiles. This 

 substance runs differently relative to 3-indolylacetic acid (lAA) controls in 

 the two solvent systems used, and gives a yellow colour with ferric chloride/ 

 perchloric acid and nitrous/nitric acid sprays. It gives rise to a further 

 substance by the action of mild alkali, including ammoniacal chromato- 

 graphy, and heat. This second substance is believed to be 3-indolylacetonitrile 

 (IAN). 



2. Aqueous fractions of seedling and excised tomato roots and of maize 

 roots gave a similar result, except that no evidence could be obtained that 

 the substance liberated from the precursor was IAN. The precursor could 



50 



