TEMPERATURE AND GROWTH 13 



We may conclude that the agar method is adequate for studies of 

 some environmental factors, e.g., temperature and (probably) com- 

 position of the atmosphere, although even in these experiments the 

 results are not always correlated with dry weight (94, 134, 145, 250). It 

 is decidedly not adequate for nutritional studies. Whether it should 

 be used for essentially chemical problems like the effct of pH or toxic 

 agents is still uncertain. The burden of proof that spread on agar is 

 a fair index of growth rests on the investigator who chooses to use the 

 method. Its value cannot be assumed. 



6. TEMPERATURE AND GROWTH 



Temperature affects growth, spore germination, reproduction, and 

 indeed all activities of the organism. Typical curves of growth as a 

 function of temperature are shown in Figure 5. There is characteristi- 

 cally a linear portion in which growth increases directly with tem- 

 perature, an optimum range which may be narrow or rather broad, 

 and a descending limb as the temperature becomes too high for growth. 

 The curve as a whole is usually skewed to the right; an empirical 

 method for converting this skewed curve to a straight line is described 

 by Cohen and Yarwood (58). 



The temperature-growth curve tends to become more nearly sym- 

 metrical as the optimum becomes lower: fungi with optima of 22-24°C 

 or less often have a much less skewed curve (53, 98, 143, 171, 311). Not 

 all low-temperature fungi, however, follow this pattern (113, 164). 



Fungi with high temperature optima generally have the typical 

 skewed curve even more pronouncedly; in a few cases, the high opti- 

 mum is associated with a broader range of good growth, but this is not 

 the rule. 



It should be noted that the concept of an optimum, usually taken 

 for granted, is not in fact very clear. The response of Phycomyces 

 blakesleeanus to temperature (247) illustrates the principal ambiguities 

 in the concept of optimum temperature. First, growth is most rapid 

 at 20-25 °C, but the maximum total amount of growth at limiting 

 levels of thiamine is greatest at 10°. Second, the effect of temperature 

 on final dry weight can be almost wiped out if the nitrogen supply is 

 made limiting. 



Another example of the dependence of temperature characteristics 

 on other factors has been elucidated by L. Fries (103). Coprinas 

 fimetarius grows poorly at 44°C because of the failure of methionine 

 biosynthesis to keep pace with other processes; if exogenous methionine 

 is supplied, growth at the elevated temperature is normal. The ap- 



