12 CULTIVATION AND GROWTH 



assembly are practical filtration devices. Gelatinous growth which 

 filters slowly may be separated by centrifugation. The use of solvents 

 (16) to hasten drying is practicable, of course, only if the extracted 

 lipids are negligible. Drying is usually effected by high temperature, 

 70-80°C. 



The dry weight of agar colonies, although of dubious significance in 

 most contexts, has been determined by scraping or peeling off of the 

 surface growth (91). Removal of agar with hot water results in a 

 significant loss in dry weight (67). 



Turbidimetric measurements of growth are suitable for those fungi 

 which grow as single cells or very short dispersed hyphae. Ustilago 

 xnolacea is of the first type, and turbidity is correlated with dry weight 

 (22). Methods have been evolved primarily for studies on bacteria 

 (305, 306); although most workers use a colorimeter, in principle the 

 nephelometer is better adapted to turbidity measurements. A turbidi- 

 metric method should, of course, always be calibrated against dry 

 weight or other standard measure; Figure 1 illustrates the correlation. 



Determination of growth by measurement of the total cellular 

 nitrogen has been used only rarely in work with fungi (222). Although 

 the chitin nitrogen complicates interpretation, this is the best method 

 for determination of growth defined as synthesis of protoplasm, and 

 deserves wider use. 



Linear growth on agar is the least laborious method of estimating 

 growth and has been correspondingly popular. The radial spread of a 

 colony in a petri dish or linear advance in a growth rate tube (93, 

 253) are most frequently determined. Growth is commonly expressed 

 as the constant rate established after the initial period of slow growth. 



Naturally, the impurities present in agar rule out this method for 

 certain problems. There are, however, more fundamental objections. 

 Chaudhuri (56) found no response of colony diameter to even a four- 

 fold dilution of the nutrient medium. Nor does the rate of advance 

 on agar always respond to necessary vitamins (106, 179) or essential ions 

 (238); strain differences in growth on agar are not always reflected in 

 dry weight measurements (133). 



Chaudhuri (56) found, on the other hand, excellent correlation of 

 radial growth on agar with dry weight and germ tube growth in tem- 

 perature studies on Verticillium albo-atrum; more recently, Domsch 

 (77) has confirmed the correlation in temperature response. Wood- 

 destroying fungi usually respond to temperature in agar culture about 

 as they do in wood blocks (317). One crucial detail is often over- 

 looked: the depth of the medium influences growth and must be 

 standardized (28, 36, 56). 



