Occurrence and Role of Growth Hormones 



61 



that of the etiolated Avena seedling carried out by Thimann (1934). 

 Using a solvent extraction technique, he demonstrated an auxin gra- 

 dient through the plant with by far the highest concentration occur- 

 ring at the coleoptile tip and a lesser concentration at the root tip. A 

 sagging gradient was found to lie between these two peaks (figure 28). 

 It is evident that the coleoptile apex and the root apex are the two 

 primary loci of auxin formation in the etiolated oat seedling and that 

 the root apex is much less active in this respect than the coleoptile. 



2 5 8 13 24 



59 MM 



DISTANCE FROM APEX 



Fig. 28. Distribution of auxin in an etiolated Avena seedling (Thimann, 1934). 



Another survey of auxin distribution through a plant was an 

 examination of Vicia faba seedlings grown in normal lighted condi- 

 tions (Thimann and Skoog, 1934). In this instance, the lateral buds 

 and leaves and the apical bud were analyzed. Small amounts of auxin 

 were found in all lateral buds and leaves, with the young expanding 

 leaves containing most of the substance. The apical bud, however, 

 yielded almost six times as much auxin as the most productive leaves 

 (figure 29). These data provide a revealing instance of the relative 

 productivity of the growing apex as an auxin source. 



As pointed out in the preceding chapter, auxins exist in plants in 

 more than one form. We speak of "free" auxin as being that readily 

 available by diffusion or rapid extraction and "bound" auxin as that 



