GENERAL CONCLUSIONS 231 



A. Quantitative Relations between Auxin and 



Growth 



The development of our knowledge of auxins has brought 

 out the fact that their activity is quantitative as well as 

 qualitative. The extent of growth, of root formation, etc., 

 is directly dependent on the amount or concentration of 

 auxin present. Since these quantitative relations have been 

 proven for the simplified cases of direct experiment it is 

 interesting to consider their bearing in general on growth 

 and correlation. 



Thus absolute size, among individual plants of a species 

 (within normal environmental conditions), remains fairly 

 constant and is even used as a taxonomic character. The 

 experiments described in Chapter VI allow us to ascribe 

 this to genetically determined rates of auxin production. 



Ajnong relative size relationships, one of the most stressed 

 is that between the above-ground and below-ground parts 

 of the plant, the "shoot-root ratio." Such constancy as 

 this ratio shows is probably due to a reciprocal control sys- 

 tem in the plant. On the one hand the number and size of 

 buds and leaves determines the rate of auxin production, 

 and this auxin, reaching the roots, influences the develop- 

 ment of the root system (c/. XI F) . On the other hand, 

 the root system probably forms a factor or factors necessary 

 for shoot growth (c/. XII C) and hence the increased root 

 system increases the shoot. 



Another interesting relation is that between the place 

 of auxin production and the length of the growing zone in a 

 stem. In many stems {e.g. most seedlings and annuals — 

 Pisum, Vicia, Nicotiana), the growing region is restricted to a 

 few cm. below the terminal bud, and in these stems auxin 

 production is mainly in the terminal bud and youngest 

 leaves. Other plants exhibit growth over a longer region, 

 and in these auxin production is probably vigorous in axillary 

 buds or lower leaves as well as in the terminal bud. Exam- 

 ples are Asparagus (Oosterhuis, 1931), Polygonum (Fig- 



