244 PRINCIPLES OF SOIL MICROBIOLOGY 



in a solution containing 53 per cent glucose. The limiting osmotic 

 pressure, when salts are used, is 17 to 21 atmospheres for Asp. niger 

 and for certain green Penicillia. 10 Fungi grow readily, in pure cul- 

 ture, at a wide range of reaction (see p. 261) and are not injured by 

 high acidity as readily as bacteria; acid reactions, including acid soils, 

 will, therefore, favor the development of fungi, in crude culture. With 

 carbohydrates as sources of energy, the reaction of the medium becomes 

 acid as a result of the growth of many fungi. With proteins and 

 nitrates, however, the reaction will tend to become alkaline. 41 Fungi, 

 with the exception of certain yeasts and certain Mucorales and Dema- 

 tium, are strictly aerobic; certain Mucors are capable of developing 

 anaerobically, especially in the presence of available carbohydrates. 

 Aeration will greatly stimulate the activities of most fungi, because of 

 their strict aerobiosis. The optimum temperature lies at 20 to 30° 

 for the majority of species, in some cases {Asp. niger, Mucor. pusillus) 

 going up to 37°. Some fungi (Penicillium expansion, Botrytis cinerea, 

 Alternaria sp.) germinate slowly at 0°, others (Fusarium radicicola, 

 Cephalothecium roseum) germinate slowly only at 5°, whereas Aspergillus 

 niger will germinate only above 10°. 42 ' 43 In the case of certain 



40 Pringsheim, E. G. tJber den Einflusz der Nahrstoffmenge auf die Entwick- 

 lung der Pilze. Ztschr. Bot., 6: 577-624. 1914. 



41 Bach, M. Variations de la concentration en ions hydrogenes sous 1'influ- 

 ence de l'assimilation des nitrates par {'Aspergillus repens DeBary. Compt. 

 Rend. Acad. Sci., 178: 520-522. 1924. 



42 Brooks, S., and Cooley, J. S. Temperature relations of apple rot fungi. 

 Jour. Agr. Res., 8: 139-164. 1917. 



43 Brown, W. The germination and growth of fungi in various temperatures 

 and in various concentrations of oxygen and CO2. Ann. Bot., 36: 257-283. 1922. 



PLATE XIII 



Soil Fungi— Mucorales 



86. Absidia glauca, showing the runners with sporangiophores, X 3.5 (from 

 Hagem). 



87. Absidia glauca, columellae, X 200 (from Hagem). 



88. Rhizopus nigricans, showing sporangiophores, rhizoids, and columellae, 

 X 40; spores X 360 (from Jensen). 



89. Rhizopui arrhizus: 1, runners with sporangiophores, X 35; 2, swollen 

 sporangiophore, X 35; 3, abnormally divided sporangiophores, X 35; 4, collumel- 

 lae, X 120; 5, spores, X 660 (from Hagem). 



90. Zygorhynchus molleri, showing zygospore formation, X 200 (from Hagem). 



91. Different forms of branching of Mucors (from Lendner). 



92. Different forms of columellae of Mucors: a, spherical; e, spherical with 

 persisting collarette; c, oval; d, oval depressed; e, pyriform; /, panduriform; g, 

 conical; h, cylindro-conical; i, manniform; k and I, spinescent (from Lendner). 



93. Cunninghamella echinulata (from Lendner) . 



