136 PHYSIOLOGY OF THE FUNGI 



Saproliginales to utilize starch and its hydrolytic products (dextrin, 

 maltose, and glucose). Thirteen other carbon sources, including fructose, 

 were not utilized. Margolin (1942) found that 19 out of 21 fungi which 

 utilized maltose also utilized dextrin. 



The nonutilization of starch by Sclerotinia libertiana has been suggested 

 as the basis of a method of preparing potato starch (Kakeura, 1946). 

 Few yeasts utilize starch, although maltose and glucose are readily 

 utilized. 



All the fungi listed in Table 22 except Pythiwn ascophallon and Phy- 

 tophthora jagopijri utilized dextrin. A comparison of the ability of fungi 

 to utilize glycogen and starch has not been investigated thoroughly. 

 Tamiya (1932) found the yield of mycelium of Aspergillus oryzae to be 

 greater on glycogen than on dextrin. Dextrin was a better carbon source 

 than starch. 



The role of the pectin-destroying enzymes in parasitism and the rotting 

 of fruits and vegetables is discussed in Chap. 17. Presumably these fungi 

 utilize some or all of the hydrolytic products of pectin (n-galacturonic acid 

 and methyl alcohol). None of the fungi, in so far as is known, utilize agar 

 as a source of carbon. A . niger utilizes the arabo-galactan from western 

 larch as a source of carbon (Ratajak and Owens, 1942). 



HETEROTROPHIC UTILIZATION OF CARBON DIOXIDE 



The assimilation of carbon dioxide is not restricted to green plants. 

 Carbon dioxide fixation has been demonstrated in bacteria, fungi, 

 protozoa, liver slices, barley roots, and intact green plants in the absence 

 of light. The basis for classifying organisms according to the way they 

 utilize carbon dioxide is discussed by Werkman and Wood (1942), By 

 the use of carbon isotopes an elegant method is available for demonstrating 

 carbon dioxide assimilation. In addition, the mechanism of fixation can 

 be studied. This involves isolation and degradation studies of the com- 

 pounds synthesized Avhile the organisms were exposed to isotopic carbon 

 dioxide. Either stable or radioactive carbon isotopes may be used. The 

 finding of isotopic carbon in compounds synthesized is proof of assimilation. 



Aspergillus niger and Rhizopus nigricans were shown to assimilate car- 

 bon dioxide (Foster et al., 1941). Radioactive carbon dioxide (C^i02) 

 was used in these experiments. Mycelium of R. nigricans was suspended 

 in 5 per cent glucose solution and agitated in a closed system containing 

 isotopic carbon dioxide. At the end of the experiment the mycelium and 

 the medium were analyzed for radioactivity. More than one-third of 

 the carbon dioxide assimilated was incorporated into cell constituents 

 which were not decomposed by boiling for 1 hr. with 2M hydrochloric 

 acid. Carbon dioxide was assimilated under aerobic and anaerobic 

 conditions. The data of such an experiment are given in Table 28. 



