AUXIN AND GROWTH 79 



curve for auxin content and shows that three zones are dis- 

 tinguishable : one, the most apical, in which growth is inde- 

 pendent of the auxin concentration; a second in which 

 growth is proportional to the auxin ; and a third, most basal, 

 in which the growth is shghtly less than would be expected 

 from the auxin content, due to aging. 



B. The Role of the Food Factor 



In the most apical zone where auxin does not limit growth, 

 there is good evidence that growth is limited by a food 

 factor or factors. To avoid misunderstanding it should 

 be pointed out that the term "food factor" is used in its 

 broadest sense. There is evidence that, like auxin, it is more 

 of the nature of a hormone than of a nutrient. 



The limiting influence of auxin and the effect of aging 

 can both be removed by applying very high concentrations 

 of auxin; if this is done, then the limitation of growth 

 exerted by the food factor should become apparent. In the 

 experiments of Went (1935), high concentrations of auxin, 

 in paste form, were applied to the tips of intact coleoptiles 

 at five different stages of growth, i.e. at five different physio- 

 logical ages. The final lengths of the upper 15 mm. of the 

 treated coleoptiles after 22 hours were 248, 246, 253, 251, 

 and 244 per cent, and of the controls 192 and 198 per cent, 

 of the original lengths. Hence in these zones the growth 

 increase was the same irrespective of the age at which the 

 excess auxin was applied, and hence their growth must be 

 limited by some factor other than auxin or aging. If, on 

 the other hand, the plants are de-seeded some time before 

 auxin is applied, this would reduce the amount of the food 

 factor present, and the excess auxin could then not bring 

 about so much increased growth. To test this view, normal 

 and de-seeded plants were compared. To half of each group 

 excess auxin was applied. After 22 hours the de-seeded 

 controls had increased 21.0 per cent and those treated with 

 auxin 30.4 per cent; on the other hand, the intact controls 

 had increased 37.7 per cent and the intact plants treated 



