34 MEDICAL MYCOLOGY 



sources and concluded that amino acids and compounds nearly resembling 

 them, are most fully and easily utilized, since the principal use of nitrogen in 

 nutrition is in the production of proteins. Lutz (1905) extended and con- 

 firmed these observations. Ritter (1909, 1912, 1914) studied the effect of the 

 acid produced when ammonium salts were used. Kossowicz (1912, 1914) ex- 

 tended his studies to purines and related compounds. Brenner (1911) extended 

 his observations to include many compounds which proved toxic. 



While these and many other later papers have much interest for the prob- 

 lem of the mechanism of protein synthesis in fungi, they have little direct 

 bearing on the problems of cultivation. 



Few studies have been made on the vitamin requirements of fungi, and 

 the evidence is not very conclusive at present. (See excellent review of litera- 

 ture by Sergent 1928, Peskett 1933.) However, the possibility of such fac- 

 tors should be borne in mind in nutritional studies (see Freedman & Funk 

 1922; Robertson & Davis 1923). 



The problem of nutrition of fungi is closely bound up with the varying 

 degrees of parasitism. The division of fungi into saprophytes and parasites is 

 rather artificial since there are so many intergrading forms. However, it is 

 often convenient to speak of facultative parasites, species which normally are 

 saprophytic, but under especially favorable conditions may develop in the 

 tissues of other organisms, usually as secondary invaders. Other fungi may 

 develop during part of their life cycle as parasites and reproduce sexually 

 only on the dead tissues of the host, or, vice versa, with, a reproductive cycle 

 in the living host and saprophytic vegetative existence outside, as in Cocci- 

 dioides immitis. A few, such as the plant rusts and the Laboulbeniales on living 

 insects, complete their whole life cycle on the living host, and have not been 

 successfully cultivated apart from their hosts. 



Even more artificial and confusing are the terms employed for symbiosis 

 and related phenomena to express varying degrees of the interaction of two 

 organisms which may vary all the way from parasitism at one extreme to 

 epiphytism at the other. 



Hydrogen Ion Concentration. — Although biologists have ceased attempt- 

 ing to explain nearly all phenomena by hydrogen ion concentrations, yet these 

 do play an important part in metabolism and should be considered. At this 

 point it might be well to review briefly the underlying concepts and the 

 methods of determining this concentration of hydrogen ions. 



In a given ionizing solvent, of which water is the only one which can concern us 

 here, solutions of various substances are observed to conduct an electric current to a greater 

 or lesser degree. Pure, distilled water is for aU practical purposes a nonconductor. Solu- 

 tions of sugar for instance, are quite as unaffected by electric current as pure water. Acetic 

 acid in solution conducts a given current slightly, while substances such as HCl, NaCl, and 

 NaOH are strong conductors when dissolved. 



It is quite reasonable, then, to assume that in conducting solutions there are present 

 conducting units and that the quantity of current conducted is in some way proportional to 

 the numbers of these units. Now, since most of these substances that are good conductors 



