76 CARBON NUTRITION 



Most investigators have used "soluble starch," a partially depolymer- 

 ized derivative prepared by acid treatment, rather than native starch. 

 Raw starch, unless the granule structure is destroyed by autoclaving, 

 will not support the growth of Phymatotrichum omnivorum (18) and, 

 indeed, is not hydrolyzed by purified amylase. This raises, of course, 

 the question of the utilization of plant starches in nature by plant 

 pathogens, and by saprophytes acting on unmodified plant materials. 



Starch is often a better substrate for growth than glucose; although 

 no such case has been studied in detail, this effect presumably results 

 either from contamination of the starch with growth factors or from 

 the fact that utilization of a slowly hydrolyzable compound is accom- 

 panied by less accumulation of acids than utilization of glucose. 



Dextrins, which are chemically or enzymatically modified starches 

 of uncertain structure, are generally satisfactory sources of carbon for 

 fungi. Quantitative differences between starch and dextrins as carbon 

 sources are frequently reported but cannot easily be interpreted until 

 the chemistry of these polysaccharides is better known. Chalara 

 quercina, unable to utilize starch, grows very well on dextrin (12). 



Inulin, a polymer of D-fructose possibly containing a small amount 

 of D-glucose, is found in several of the Compositae, e.g., Dahlia. It is 

 not hydrolyzed by enzymes which act on starch or on sucrose. Limited 

 data suggest that it is a good carbon source for many but not all fungi. 

 It should, however, be noted, that inulin, like all fructans, is susceptible 

 to hydrolysis by heat, and in careful work it is necessary to use filter 

 sterilization (18). 



Unidentified bacterial fructans are probably utilized by Streptornyces 

 coelicolor and related species (127). 



Glycogens, the reserve polysaccharides of animals and some thallo- 

 phytes, are glucose polysaccharides quite similar to starch, or, more 

 precisely, to the amylopectin fraction of starch. Glycogens have not 

 been tested extensively in the nutrition of fungi, but the data suggest 

 that they are quite generally available to fungi of widely different eco- 

 logical groups (17, 45, 82, 201). Those fungi unable to use starch 

 can be expected not to use the glycogens, since, so far as is known, the 

 same enzymes attack both polysaccharides. 



Hemicelluloses. The hemicelluloses comprise a group of insoluble 

 polysaccharides of plant cell walls. The group as a whole is not well 

 defined, and there is much confusion in the literature. The hemicel- 

 lulose fraction as usually isolated includes pentosans, hexosans, and 

 polyuronides; the main constituent is probably a pentosan or 

 mannan (235). Fungi are active in the breakdown of hemicelluloses 



