388 Morpho genetic Factors 



multiflorus, on the other hand, is much branched and bushy. Delisle 

 ( 1937 ) found that in the former species the concentration of auxin at the 

 tip was high (Fig. 18-12). In the latter it was much lower, suggesting 

 that the copious branching resulted from weak inhibition. After re- 

 moval of the apex in the young plant, A. novaeangliae became much 

 branched, and application of auxin to the tip of A. multiflorus resulted 

 in the growth of a main stem with relatively small branches. Thus an 

 important taxonomic character is related to the amount of auxin present 

 in the plant. 



Many of the effects of growth substance in correlation are stimulatory 

 rather than inhibitory. There is a close relation, for example, between the 

 presence of growing seeds and the development of a fruit, the seeds pro- 

 ducing the auxin necessary for fruit growth. In some cases, as has been 





20 



5 15 



8 





» 



20 



30 40 SO 



LENGTH Or TIP IN MM 



60 



SO 



Fig. 18-12. Effect of auxin on inhibition of branching in Aster. Auxin concentrations 

 at successive distances from apex in A. novaeangliae, an unbranched species. The 

 amount of auxin in A. multiflorus, which is more branched., is considerably less. ( From 

 Delisle. ) 



mentioned, synthetic growth substances may be substituted for the 

 natural auxin and seedless fruits produced. Growth of the receptacle 

 may also be related to seed production. When Nitsch (1950) removed 

 the growing achenes from a strawberry, the fleshy portion stopped grow- 

 ing, but if synthetic growth substance replaced the achenes, the normal 

 development of the strawberry was resumed (Figs. 18-13, 18-14). If 

 only some of the achenes were removed, the weight of the mature ripe 

 fruit was proportional to the number of achenes remaining. Nitsch found 

 that the achenes contained a large amount of auxin but that this was 

 absent from the receptacle. 



Galston ( 1948 ) has described an example of competitive correlation 

 in asparagus. Root primordia, stimulated by auxin, are formed during 



