ENVIRONMENTAL FACTORS 73 



or quality upon form and color, metabolism, rate of growth, etc., 

 are extremely important from a general physiological standpoint. 



Nutrients. The cultivation of fungi upon decoctions or in- 

 fusions of organic substances, or upon solid organic substrata, 

 would afford only through a tedious process of comparative study 

 any fundamental ideas of fungous nutrition. The ease with which 

 fungi may be grown in cultures and the use of synthesized culture 

 media have afforded an opportunity for exact determination of the 

 elements required by these organisms. There may be some specific 

 variations, but it is now generally agreed that the majority of the 

 fungi require nine elements, viz., carbon, hydrogen, oxygen, nitro- 

 gen, sulfur, phosphorus, potassium, magnesium, and iron. 



Carbon. For most culturable fungi, whether primarily parasitic 

 or saprophytic, carbon is available as grape or cane sugar, glycerin, 

 asparagin, peptone, etc., in fact, in almost any soluble or readily 

 convertible nontoxic form. It is to be inferred that the obligate 

 parasite, as well, utilizes the soluble carbohydrates, peptones, etc., 

 of the host cell, but its exact relations cannot well be determined. 

 Owing to indirect needs in respiration, the nutrient solution must, 

 in order to yield a considerable growth of the fungi, contain a 

 relatively large proportion of carbohydrates. 



Nitrogen. Nitrogen may be furnished to the readily culturable 

 fungi in the form of nitrates or ammonia compounds, but in some 

 cases preferably as peptone, casein, or in other organic form. It is 

 probable that the adaptations which result in obligate parasitism 

 have only in part a special relation to the nitrogen food supply. 

 Some fungi may be cultivated only with difficulty, and among these 

 forms certain species are benefited by using as a substratum por- 

 tions of the natural host (steamed), or decoctions prepared from 

 the host plant. It is, however, possible that this relation is con- 

 cerned with special stimuli, and has no bearing on the nitrogen 

 factor. 



The relation of certain parasitic organisms to atmospheric nitro- 

 gen has become unusually interesting. It has been shown by more 

 than one observer that fixation of nitrogen by the various forms 

 of the leguminous tubercle bacteria, Pseudomonas radicicola, may 

 proceed in suitable artificial cultures. It proceeds, therefore, with- 

 out reference to symbiotic association, 



