442 



PLANT GROWTH SUBSTANCES 



the detailed mechanism of such effects, an explanation which undoubt- 

 edly holds in many instances may be summarized as follows. 



It may be supposed that a bacterium requires a vitamin to carry out 

 a number of catalytic functions necessary for growth. For example, 

 the vitamin may be necessary for synthesis of a number of different 

 essential components (Pi, P2, P3, etc.) of protoplasm, from their respec- 

 tive precursors (pi, p2, ps) as indicated in the accompanying diagram. 



pi_- 

 P2- 



P3^- 



Pl 

 P2 

 P3 



It is logical to assume that if a medium supplies one or more of the 

 products (Pi, P2, etc.) preformed, the metabolic requirement for the 

 vitamin involved in synthesis of these products might be greatly reduced 

 as compared with the requirement in a medium which did not supply 

 these products. 



Such relationships apparently explain the data of Figure 5. It will be 



L06 



O tl +2 



10 MILLIMICR06RAMS FOLIC ACID PER 6 ML. 



Figure 5. The effects of serine, and of purine bases and thymine on the 

 requirement of Streptococcus faecalis for folic acid. Curve i, serine, thymine, 

 and purine bases present. Curve 2, purine bases and serine present, thymine 

 omitted. Curve 3, purine bases present, thymine and serine omitted (6). 



