FORMATION AND OCCURRENCE OF AUXINS 67 



tant to note that for the growth of seedlings, which have to 

 begin their Hves in darkness, not only carbohydrates and 

 other nutrients, but also auxin, is thus stored in a readily 

 available form. 



It should be added that auxin production is not in general 

 due to meristematic activity or to the presence of embryonic 

 cells, as is proven by its production in coleoptile tips and 

 in leaves. (A very clear case is that of the saprophytic 

 tropical orchids Gastrodia and Didymoplexis, whose ovaries 

 strongly promote the elongation of their pedicels, but only 

 after the growth of the ovary and embryos is complete 

 {u\.) The exact significance of its production in growing 

 regions is discussed in IV C. Goodwin's finding (1937) 

 that, weight for weight, auxin production is the greatest 

 in the very youngest leaves of the bud is of interest in this 

 connection. 



On the other hand, there is a distinct correlation between 

 the position of an organ and its auxin production; only the 

 most apical regions form auxin, and the more terminal 

 their position the greater their production. 



C. Distribution of Auxin in the Plant 



Besides its presence in the producing zones, auxin is also 

 distributed throughout other parts of the plant. Soding 

 showed (1929) by straight growth measurements that auxin 

 was present in small amounts in sections of green Avena 

 coleoptiles immediately below the tip, but that further 

 down than 2.5-5 mm. from the tip no growth-promoting 

 effect was detectable. 



Determinations of the auxin content of different zones 

 of the etiolated coleoptile and roots of Avena, using the 

 method of extraction with chloroform, were made by Thi- 

 mann (1934). The results, summarized in Figure 30, show that 

 auxin is present everywhere in the seedling, but that its 

 concentration falls off rapidly with increasing distance from 

 coleoptile tip and root tip. His finding auxin throughout 

 the plant, where earlier workers found none (Stark, 1921; 



