:36 



PHYSIOLOGY OF THE FUNGI 



hot water. Frequently the mat can be removed from the melted agar 

 with a pair of forceps instead of by filtering. Autoclaving removes some 

 soluble constituents from the mycelium, but if a uniform procedure is 

 adopted, the results are comparable. 



Measuring yeast growth. The growth of yeasts may be measured by 

 four methods. (1) Yeast cells may be counted in an aliquot of the 

 medium by the use of a hemocytometer or other counting chamber. The 

 method is tedious. (2) The volume of yeast cells in a given volume of 

 medium may be measured in special graduated centrifuge tubes. Yeast 



80r 



3.1 



6.2 



0.8 1 .6 



iig. fhiomine per culture 



Fig. 4. Direct comparison between diameters and dry weights of the same 10-day- 

 old cultures of Ceratostomella fimbriata in the presence of varying amounts of thiamine. 

 Cultures were grown in Petri dishes on 25 ml. of glucose-casein hydro lysate agar 

 at 25°C. 



cells are large and easily separated from the medium by centrifuging. 

 This method is less tedious than counting. (3) Turbidity may be used to 

 measure the amount of yeast growth. Accurate determinations by this 

 method require the use of a photoelectric photometer. This method is 

 rapid and sufficiently accurate for many purposes. Lindegren and Raut 

 (1947) have cultivated yeasts in colorimeter tubes and have followed the 

 rate and amount of growth for as long as desired. (4) Yeast cells may be 

 filtered under vacuum, washed, dried, and weighed. Selas porcelain 

 crucibles with fritted bottoms are suitable. This method is accurate but 

 somewhat time-consuming. 



Comparison of methods. It should be clearly recognized that one 

 method of measuring growth may not agree with another. This is illus- 



