126 PHYSIOLOGY OF THE FUNGI 



utilize galactose to better advantage in the presence of glucose. The 

 experiments of Steinberg (1939) on the effect of two poor carbon sources 

 on the growth of A. niger were made at 35°C. Some combinations of 

 poor carbon sources supported more growth than when these sources 

 were used singly. Thus, the calculated weight of mycelium for the 

 combination, D-mannitol-lactose was 21.4; the actual yield was 233.6 mg. 

 Some combinations of poor carbon sources resulted in a decrease in 

 amount of mycelium formed (glycerol-D-galactose : calculated yield, 

 243.7 mg.; actual yield, 154.7 mg.). The effect of mixed carbon sources 

 in the amount of growth of Phy corny ces blakesleeanus and Pythiomorpha 

 gonapodyoides appeared to be purely additive (Margolin, 1942). 



Table 26. The Effect of Galactose and Glucose, Singly and in Combination, 

 UPON the Amount of Growth of Aspergillus niger 

 Cultures incubated 7 days at 20°C. (Horr, Plant Physiol. 11, 1936.) 

 Grams of Sugars Used per Liter Yield, Mg. per Culture 



10 galactose 45 . 1 



18 galactose 42.4 



20 galactose 44.3 



2 glucose 145 . 6 



10 glucose 411 .0 



18 galactose + 2 glucose 577.4 



10 galactose + 10 glucose 1,151.6 



All these results indicate that the effect of mixed carbon sources is 

 highly specific. A mixture of poor carbon sources may or may not 

 result in increased growth, depending on the carbon sources involved as 

 well as the fungus concerned. 



The favorable effects of mixtures of poor carbon sources on the rate 

 and amount of growth have been ascribed to the ease with which a fungus 

 is able to synthesize certain key intermediates. If the synthesis of 

 intermediate X from carbon source A is slow and difficult, and the syn- 

 thesis of X is rapid from carbon source B, it is probable that growth will 

 be more rapid on media which contain both carbon sources. 



ORGANIC ACIDS 



An organic acid is characterized by having one or more carboxyl 

 ( — COOH) groups. Some organic acids are utilized as sources of carbon 

 and in other ways. Two series of organic acids are especially interesting 

 from the standpoint of physiology. The fatty acids are monocarboxylic 

 acids; the higher members, when esterified with glycerol, form fats. 

 The dicarboxylic acids, especially those which contain four carbon atoms, 

 enter into the metabolic pathways of the fungi in various ways; e.g., 

 utilization of ammonium nitrogen (Chap. 6). 



I 



