18 PHYSIOLOGY OF THE FUNGI 



avoid the use of percentages, but for routine work, where the composition 

 of media is of less importance, the use of percentages may be allowed. 

 Before the same medium can be prepared repeatedly, it is necessary to 

 know what constituents are used and the amount of each. 



Two general methods are used for reporting the composition of media. 

 Either the weights of the constituents and the volume of water used are 

 given, or the weights of the constituents are given and the medium made 

 up to a definite volume. The first method is in common use; its sim- 

 plicity conceals its disadvantages. The volume of a medium prepared by 

 this method is never the same as the volume of water used. It is neces- 

 sary to measure the volume of the medium after preparation in order to 

 calculate the amount of any constituent in an alicjuot. 



The method of choice in accurate work is to weigh the constituents and 

 make the medium up to a given volume. The amovmt of any constituent 

 in any volume of medium may then be calculated. If a liter of medium 

 contains 25 g. of sucrose, and 25-ml lots are dispensed, each lot contains 

 ^/iooo X 25, or 0.625 g. of sucrose. 



Direct units. The units of volume most used are the liter (1.) and the 

 milliliter (ml.). A cubic centimeter (cc.) is nearly, but not exactly, 

 equivalent to a milliliter. Its use should be discouraged. The formulas 

 for media are usually given on the basis of a liter. This practice is to be 

 encouraged, as the liter is a convenient volume in preparing media. The 

 weights of solid constituents should be reported as grams (g.) or decimal 

 divisions thereof. The most commonly used decimal fractions of the 

 gram are the milligram (mg.), the microgram (jug), and the millimicrogram 

 (m/ig), each of which is one-thousandth of the preceding weight. Since 

 it is easy to make mistakes in reading small decimals, it is recommended 

 that no decimals smaller than 0.1 be used. The use of 12 mg. is preferable 

 to 0.012 g., although both mean exactly the same. It is easier to read 

 5 jug than 0.000005 g. One milligram of a substance in a Uter of solution 

 equals one part per million (p. p.m.). Each milliliter of such a solution 

 wiU contain 1 fxg of the substance. Similarly, a microgram of a substance 

 in a liter of solution is present as one part per billion. The microgram 

 has also been called the gamma (7), but this usage should be abandoned 

 inasmuch as gamma is not a regular prefix used in the metric system. 

 The necessity of using such small units of weight arises from the physio- 

 logical activity of certain compounds and elements. For example, a 

 concentration of 1 mg. of biotin in a liter of medium is a relatively enor- 

 mous concentration. 



Derived units. Derived units must be used in comparing the effect of 

 compounds which have different molecular weights. Among these 

 derived units the mole is the most useful. A mole is the molecular weight 

 of a chemical compoimd expressed in grams. A mole of glucose is 180 g., 



