20 



PHYSIOLOGY OF THE FUNGI 



another, it should be done in such a way that the same amount of the 

 essential element is present in all the media. If this is not done, the basis 

 upon which the replacement was made should be stated. If 25 g. of 

 glucose, C6H12OC is replaced by 25 g. of sucrose, C12H22O11, it should be 

 realized that the carbon contents of the two media are different. It is 

 frequently difficult or impossible to find out from some papers in the 

 literature how substitutions in the media were made. 



Table 3. A Compakison of Two Synthetic Media upon the Basis of Amounts 



OF Essential Elements and Compounds Present in One Liter 



Both media were made with double-distilled water. 



* Medium 5, Suggested Laboratory Exercises, 

 t Steinberg, 1941. 



Finally, it should be noted that the common practice of using one com- 

 pound as the source of two essential elements does not permit perfect 

 freedom in adjusting the composition of a medium. If magnesium sulfate 

 heptahydrate is used to supply both magnesium and sulfur, it is obvious 

 that the ratio Mg/S is fixed. If it is desired to vary the amounts of 

 magnesium and sulfur independently, it is necessary to use different com- 

 pounds of magnesium and sulfur; e.g., magnesium chloride and sodium 

 sulfate. This practice introduces other elements into the medium. 



