10 PHYSIOLOGY OF THE FVSGI 



Semisynthetic media. A semisynthetic medium is one Avhich is com- 

 posed in part of natural materials of unknown composition. Such media 

 are made by adding compounds of known composition to one or more 

 natural materials. The widely used potato-glucose (dextrose) medium is 

 an example of this type. The addition of agar to an otherwise synthetic 

 medium introduces a natural material of unknown composition. Media 

 Avhich contain agar cannot be classed strictly as synthetic media. Semi- 

 synthetic media may be used for many types of physiological investigations. 



The composition of a given natural or semisynthetic medium is not 

 constant. Potato-glucose medium may vary greatly in composition 

 depending upon whether or not the potatoes were peeled and upon the 

 variety and age of the potatoes used. Neuberger and Sanger (1942) 

 found a twofold difference in the amide nitrogen (asparagine and gluta- 

 mine) among varieties. In this laboratory we have found that the 

 amount of potato pulp Avhich is allowed to enter this medium exerts a 

 marked influence upon growth and reproduction of certain fungi. These 

 differences, which may seem minor, are great enough to make comparisons 

 between work done in different laboratories difficult. 



Synthetic media. As the term is used in this book, a synthetic medium 

 is one of known composition and concentration. It does not mean that 

 every compound used is a product of the chemist's art. Some of the 

 constituents, such as the sugars, may be of natural origin. The important 

 condition is that the compounds used be pure, and this is difficult to 

 attain in practice. "Chemically pure" compounds are usually far from 

 being pure, as a glance at the labels will show. The ideal of using pure 

 compounds is seldom realized, but the closer it is approached, the more we 

 shall learn about the nutrition of the fungi. 



Natural media and most semisynthetic media are of limited usefulness 

 in studying nutrition of the fungi. The chief value of synthetic media is 

 for nutritional studies. However, growth and reproduction are fre- 

 quently poorer on a synthetic medium than on one containing some 

 natural material. For example, Aspergillus niger grew well in a synthetic 

 medium composed of sucrose, ammonium nitrate, magnesium sulfate, and 

 dipotassium hydrogen phosphate (Steinberg, 1939). In addition to 

 these major constituents, iron, zinc, copper, manganese, molybdenum, 

 and gallium salts were present. Extraordinary care was taken in prepar- 

 ing the medium. The concentration of every constituent was so balanced 

 that a decrease in concentration of any constituent resulted in diminished 

 growth. Growth and sporulation were excellent upon this medium. 

 When 20 mg. per liter of either peptone or yeast extract was added, the 

 rate of growth was greatly increased and the time required for sporulation 

 was decreased. The small amount of yeast extract or peptone used 

 could have added only an insignificant amount of material from which 



