AUXIN AND GROWTH 75 



Beyer (1928a) was unable to confirm the fundamental 

 experiment of Dolk, finding instead that after two decapita- 

 tions growth still continued but at a lesser rate. He used, 

 however, very short plants, in which there is an excess of 

 auxin present (u); this means that a much longer period 

 without any regeneration would have been needed to de- 

 plete the plant of its auxin and to stop growth. 



Evidence similar to the above has been obtained from the 

 growth of immersed sections of Avena coleoptiles (Bonner, 

 1933; Thimann, 19356). These grow at a rather slow rate 

 and correspondingly their growth continues, at a gradually 

 decreasing rate, for a long time; they show, however, no 

 regeneration. If treated with acid their growth is tempo- 

 rarily accelerated by activation of the auxin present (see 

 VIII F), but subsequently it falls virtually to zero; on then 

 transferring them to auxin solution growth begins again 

 and may be quite considerable. 



It is satisfactory to note that on such a fundamental 

 point comparable evidence is available with another plant 

 and by another method. Cholodny (1926) showed that if 

 the central cylinder of Lupinus hypocotyls were bored out, 

 care being taken to remove all the phloem, the growth rate 

 falls to 30 per cent of the normal. If now the tips of Zea 

 Mays coleoptiles were inserted into the hollow, the growth 

 was greatly accelerated (see Figure 47). With one tip the 

 growth rate became 70 per cent, with 4 to 6 tips 90 to 100 

 per cent of the normal. ]\Ioissejewa (1928) varied the 

 experiment by inserting gelatin upon which Zea Mays 

 coleoptile tips had stood, with similar results. 



It is thus satisfactorily proven that cell elongation is 

 dependent upon, and controlled by, the presence of auxin. 



One of the complicating factors in the analysis of plant 

 growth in terms of auxin is the fact that the further we go 

 from the auxin-producing zone the less sensitive do the 

 cells become to applied auxin. This phenomenon is termed 

 "aging." It is produced, as has been shown by du Buy 

 (1933), by the continued decrease in auxin supply to those 



