POLYSACCHARIDES 75 



Direct utilization may, of course, occur in the fungi as it does in bac- 

 teria (53), but the evidence is not yet at hand. 



The second general problem in oligosaccharide utilization is that 

 of adaptive growth. Operationally, nutrition experiments often show 

 that a given organism grows on a disaccharide only after a long in- 

 cubation period, but that once growth is initiated the final crop is as 

 heavy as that produced with glucose or other easily utilizable carbo- 

 hydrates. The study of Lilly and Barnett (116) contains many ex- 

 amples of this phenomenon, which, as discussed later, is probably to be 

 ascribed to the formation of an induced enzyme system. 



4. POLYSACCHARIDES 



Polysaccharides, as the name implies, are polymers of sugars or 

 sugar derivatives. Although the number of units may be as small 

 as 10, the natural polysaccharides have as a rule very high molecular 

 weights and contain up to several thousand saccharide units. They 

 can be divided into two broad classes: reserve or nutrient polysac- 

 charides (e.g., starch, glycogen, and inulin) and structural polysac- 

 charides (e.g., cellulose, pectic materials, and chitin). The structural 

 polysaccharides, found in cell walls and other extraprotoplasmic struc- 

 tures, are generally insoluble linear polymers; the reserve polysac- 

 charides contain a large percentage of branched chain structures and 

 are usually more soluble (235). Many polysaccharides are too poorly 

 known to be fitted into either category. 



Starches and Related Polysaccharides. Starch, the principal reserve 

 polysaccharide of plants (actually, the starches of different plants are 

 somewhat different) is an excellent carbon source for most fungi and 

 actinomycetes, even for rather fastidious forms. A few fungi are 

 known on acceptable evidence not to grow with starch as the sole 

 carbon source. They include representatives of widely different tax- 

 onomic and ecological groups: Ustilago violacea (180), Rhizophlyctis 

 rosea (160), Psalliota bispora (P. campestris) (216), Tricholoma im- 

 bricatum (142), Entomophthora (Empusa) spp. (242), Penicillium 

 digitatum (65), and Chalara quercina (12). The failure of Penicillium 

 digitatum to grow with starch finds its explanation in an inability to 

 form amylase (88). 



As would be expected from the fact that starch is hydrolyzed prima- 

 rily to maltose, really good growth on starch is accompanied by the 

 ability to grow well on maltose (142), although a Chytridium sp. 

 unable to utilize maltose makes limited growth on starch (45). 



