78 



PHYTOHORMONES 



of growth rates of internodes over the whole stem, is similar 

 to that in Avena. Went has explained this growth distribu- 

 tion, in Avena, by the interaction of two factors: 1, auxin, 

 coming from the tip, and 2, a food factor or complex of fac- 

 tors, coming from the base (Went, 1928, 1935). The auxin 

 content would thus decrease with distance from the tip, 



40- 



30- -0.2 



20- 



25 mm. 



Fig. 34. Distribution of growth rate (crosses) and auxin concentration 

 (circles) over the length of a coleoptile of Avena saliva. Abscissa, distance 

 from tip of coleoptile in mm.; ordinates (outer figures), increase in length in 

 per cent of original length per 8 hours, (inner figures), auxin present (by extrac- 

 tion) in plant units per mm. length. The auxin curve continues upward at the 

 left to reach a very high point for the region close to the tip. Thin broken 

 curve to right represents distribution of hypothetical food factor. (After 

 Thimann, 1934.) 



and the food factor with distance from the base, because 

 both are used up during the actual process of growth. 



That the auxin concentration within the coleoptile does 

 actually fall off with increasing distance from the tip, and 

 that there is no maximum in the middle to correspond to 

 the maximum in growth rate, was shown by Thimann (1934) 

 with his chloroform extraction method. Figure 34 combines 

 the curve for distribution of growth rate with part of the 



