PHYSIOLOGY OF FUNGI 33 



deposited in the cells or hydrogen sulphide may be produced, although these 

 phenomena are rare under ordinary cultural conditions. 



Carbon. — The possible sources of carbon are almost infinite, since there 

 are many possible combinations from which it may be utilized in dilute solu- 

 tion. For practical purposes the carbohydrates and organic acids are em- 

 ployed most. Of these, glucose has the widest use although the other hexoses, 

 especially mannose and fructose, are easily utilized, and also sucrose (cane 

 sugar) by organisms which secrete invertase. The other sugars and aldehydes, 

 celluloses, starches, etc., are variable in effect, those with low molecular 

 weights generallj^ being toxic. The lower alcohols are also generally toxic, 

 although some of higher molecular weight, such as glycerol (glycerin) and 

 mannitol (mannite), are very readily assimilated. In general, substances hav- 

 ing a three- or six-membered carbon chain are the most assimilable. Among 

 the organic acids, the amino acids and those of the aliphatic series having a 

 low hydrogen ion concentration, are readily utilized. Acids of the cyclic 

 (benzene) and heterocyclic series are rarely metabolized unless there is a side 

 chain of at least three carbon atoms, as in the amino acids, phenylalanine, 

 tyrosine, and tryptophane. There are also scattered observations on utiliza- 

 tion of depsides, tannins, alkaloids, amines, etc. The proteins and their de- 

 composition products often furnish a suitable carbon source, although they 

 are primarily regarded as a nitrogen source. 



In many of the compounds assimilability depends upon the ability of the 

 organism in question to secrete some enzyme which will hydrolyze or so break 

 down the higher compound that one or more of the resulting products will be 

 utilizable while none will be toxic, e.g., in depsides and tannins, usually the 

 hexose is utilized and the galloyl compounds are not attacked. 



Many of the older authors attempted to arrange carbon compounds in the 

 order of decreasing availability, but this is rather futile, as the exact order 

 varies Avith the organism in question and with other environmental factors. 

 (Pasteur 1860, 1862 ; Naegeli 1879, 1882 ; Reinke 1882 ; Duclaux 1885, 1889 ; 

 Linossier & Roux 1890; Went 1901; Ekman 1911. For action of cyclic carbon 

 compounds, see Waterman 1913, Bokorny 1920.) 



Nitrogen. — While from time to time atmospheric nitrogen has been indi- 

 cated as a source of nitrogen for fungi, the inadequacy of methods of deter- 

 mination of total nitrogen has tended to discredit this source for most fungi. 

 Duggar and Davis (1916) have carefully summarized the literature and dis- 

 cussed the methods employed. Klotz (1923) has adequately summarized the 

 literature for other sources. Older authors attempted to group fungi as 

 nitrate, ammonium, amino acid, peptone or protein organisms, but in view of 

 the large number of variables involved and the difficulties of adopting a suit- 

 able criterion for growth these groupings are quite inadequate. While some 

 fungi are capable of utilizing nitrates or ammonium salts, in the groups in 

 which we are most interested here, they are so few that they need not be con- 

 sidered. Czapek (1902) made a very extensive study of substances as nitrogen 



