GROWTH 



27 



WAYS OF MEASURING GROWTH 



The discussion of phases of growth presupposes methods of measuring 

 growth. In choosing a method of measuring growth, or any other physio- 

 logical process, the accuracy and type of information desired must be kept 

 in mind. For some purposes the simplest methods are satisfactory; for 

 others the most accurate methods should be chosen. 



Visual inspection. The simplest way to measure growth is by inspec- 

 tion and comparison. The value of this method lies in the speed with 

 which growth measurements are made. Elaborate equipment is not 



400 



300 



• 200 



T5 



'o 



S 



too 



16 



18 



20 



4 6 8 10 12 14 



Doys of incubation 



Fig. 2. Growth of four fungi under the same conditions, in 25 ml. of liquid glucose- 

 casein hydrolysate medium at 25°C. 



needed, as test tubes and Petri dishes are satisfactory culture vessels. 

 This method has the further advantage that the same cultures may be 

 kept under observation. It is frequently the method of choice for pre- 

 liminary experiments, for the very appearance of the mycelium is a clue 

 to the amount of growth. Growth under varying conditions may be 

 compared if some condition is used as a standard for comparison (see 

 Suggested Laboratory Exercises). It is obvious that a great deal of sub- 

 jective judgment enters into this method of estimating growth, but it is 

 veiy useful where fine distinctions are not required. 



Linear growth. A second widely used method of measuring growth 

 consists in growing fungi in Petri dishes and measuring either the diameter 

 or the area of the colony. This is a useful method in some instances but 



