64 Fundamentals of Auxin Action 



teristically exist in seeds or structures which are part of seeds, in stem 

 tips, and in young expanding leaves. 



CORRELATION WITH GROWTH 



Many examples of a strong correlation between auxin content 

 and growth have been demonstrated. A particularly graphic correla- 

 tion of this sort was shown by Went and Thimann (1937, p. 59). They 

 found that a similar rise in diffusible auxin available from the coleop- 

 tile tip was associated with the rise in growth rate of Avena coleoptiles 

 (figure 31). As the cells of the coleoptile approach maturation and 





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AGE OF COLEOPTILE 



5 DAYS 



Fig. 31. The proportionality between auxin production and growth rate in Avena 

 coleoptiles of various ages (Went and Thimann, 1937, p. 59). 



are no longer capable of responding to growth stimuli, the growth rate 

 of the coleoptile falls off rapidly to zero. At the same time the auxin 

 production does not decrease, but an additional supply is produced 

 over a short subsequent j^eriod. The close correlation between growth 

 rate and auxin production is thus obtained as long as the capacity for 

 growth persists, and after that time the correlation is not to be ex- 

 pected. 



In several instances the commencement of growth has been cor- 

 related with the appearance of free auxin. Almost universally there 



