VITAMINS 



177 



from its substrate. Figure 33 emphasizes two important features which 

 must be considered in vitamin studies: (1) the effects of different amounts 

 of the vitamin in the medium, and (2) the response of the fungus over a 

 period of time sufficiently long to allow maximum growth. The three- 

 dimensional graph permits one to plot dry weight against both variables. 

 The failure of a fungus to make an appreciable amount of growth even 

 after an extended period of incubation on a medium essentially free of a 



Asparagine 0.1 % 



r, 9 ^ 



Mg. 

 ^90 



Fig. 33. Three-dimensional graph showing growth of Phycomyces blakesleeanus on a, 

 synthetic medium as a function of thiamine concentration and time. (Courtesy of 

 Schopfer, Protoplasma 28: 383, 1937; also from the book "Plants and Vitamins," 

 p. 102, 1943. PubUshed by permission of Chronica Botanica Co.) 



particular vitamin, like the case illustrated by P. hlakesleeanus and 

 thiamine, indicates that the deficiency is total; i.e., the synthesis of that 

 vitamin is zero. Vitamin deficiencies of many fungi are only partial, as 

 shown by a slower rate of growth in a vitamin-free medium than in the 

 presence of added vitamins. The degree of partial deficiency may vary 

 M^dely, from slight to nearly total. Partial deficiencies may be easily 

 overlooked by terminating an experiment too soon. An incubation 

 period of 1 or 2 months is often required to distinguish between partial 

 and total deficiencies of some fungi. 



An example of partial thiamine deficiency is illustrated by Lenzites 

 trabea (Fig. 34). In a medium containing thiamine, maximum Aveight 



