60 CARBON NUTRITION 



and maintenance of preformed growth should be a factor in the inter- 

 pretation of results. 



12. Reasonable precautions should be taken against complications 

 arising from (a) contamination of the carbon source with impurities 

 which affect growth, or (b) the formation of toxic metabolites during 

 growth. Hydrogen ion is the most common toxic metabolite, and 

 consequently the most important precaution in this connection is the 

 periodic determination of the pH of the culture during the growth 

 period. 



The most frequent, and perhaps the least excusable, deficiency in 

 experiments on carbon nutrition is failure to meet the criterion of ob- 

 jective and quantitative measurement of growth. As a rule, it is wise 

 to disregard results which are reported in qualitative terms only, al- 

 though the complete absence of growth in such experiments may on 

 occasion be significant. This stricture applies with almost equal force 

 to the evaluation of growth on the basis of linear extension of a colony 

 on agar; this type of growth measurement has its uses (Chapter 1) but 

 is not sufficiently accurate to assess the relative utilizability of differ- 

 ent carbon sources. 



2. MONOSACCHARIDES AND THEIR DERIVATIVES 



Hexoses. Of the 6-carbon sugars, the hexoses, D-glucose is biologi- 

 cally the most important and is utilized for growth by virtually all 

 cultivable fungi. A few forms have been reported not to grow with 

 glucose; the only convincing evidence so far presented is the qualita- 

 tive observation of Schade (175) that Leptomitus lacteus uses none of 

 the common hexoses but grows with acetate or other fatty acids as the 

 sole source of carbon. This pattern resembles that of the "acetate 

 flagellates," e.g., Polytomella caeca, in which it has been suggested 

 (122) that enzymes necessary for the phosphorylation of glucose are 

 lacking. A similar deficiency in Pseudomonas putrefaciens has the 

 same effect (106). 



For the great majority of fungi and actinomycetes D-fructose and 

 D-mannose are equivalent to glucose for growth. Occasionally, as in 

 Stachybotrys atra (99), one of these may appear superior to glucose 

 for unknown reasons. Particularly among the lower fungi there ap- 

 pear to be numerous exceptions to the general rule that fructose and 

 mannose are equivalent to glucose. In conventional experiments of 

 limited duration it is found that either or both of these sugars fail to 

 support the growth of several members of the Chytridiales (2, 45) and 



