360 PHYSIOLOGY OF THE FUNGI 



for germination. Brief reports of a number of observers on oxygen 

 requirements are given by Doran (1922). It is generally agreed that 

 reduced oxygen supply decreases spore germination. Spores germinate 

 better on or near the surface of a li(iuid than when submerged deep in the 

 liquid. In some cases the spores may germinate under water, but only 

 abnormal germ tubes are formed. Aerated water gives better germina- 

 tion than nonaerated water. The spore load in a drop of water, whether 

 all of the same species or of mixed spores, influences greatly the percent- 

 age of germination. This is believed to be due primarily to the competi- 

 tion for the limited supply of oxygen, rather than to toxic substances 

 produced by other germinating spores. 



According to Jones (1923), spore germination of Ustilago avenae is 

 greatest in soil with 30 per cent of water-holding capacity and is greatly 

 reduced at 80 per cent. This was probably due to the amount of avail- 

 able oxygen. The spores failed to germinate in water when exposed to 

 an oxygen-free atmosphere. The " chlamydospores" of Ustilago zeae 

 do not germinate in the absence of oxygen, and at least 5 per cent oxygen 

 must be present to allow germination as high as in the open air (Platz 

 et at., 1927). 



The supply of oxygen may influence the method of spore germination 

 (Uppal, 1926). Germination by zoospores was possible in the absence 

 of oxygen for the sporangia of Phytophthora mfcstans, P. colocasiae, P. 

 palmivora, and P. parasitica. Germination by germ tubes does not take 

 place in these species in the absence of oxygen. However, the presence 

 of oxygen is essential for zoospore formation by sporangia of Alhiigo 

 Candida, Plasmopara viticola, and Sclerospora graminicola. The two 

 methods of germination are different processes, the direct method more 

 nearly resembling vegetative growth. 



Hydrogen-ion concentration. Under natural conditions acidity is not 

 usually a limiting factor for spore germination. In general, spores will 

 germinate within a wide pH range. It seems significant that, in most 

 species of fungi, germination is favored by an acid medium, often at a 

 pH considerably lower than the optimum for vegetative growth or sporu- 

 lation. The effects of acidity of the medium upon a number of species, 

 including Botrytis cinerea, Aspergillus niger, Penicillium cyclopium, P. 

 italicum, Puccinia graminis urediospores, Lenzites saepiaria, Colleto- 

 trichum gossypii, and Fusarium sp., are reported by Webb (1921). The 

 spores of the Fusarium germinated equally well in alkaline and acid 

 media, while CoUetotrichuni gossypii was the only species of the group 

 studied in which germination was better in an alkaline medium. At 

 pH 2.5 spore germination was prevented in all species, and the optimum 

 for most species was 3.0 to 4.0. In sucrose-nitrate (Czapek's) solution, 

 two maxima usually occurred, the primary one at pH 3.0 to 4.0 and a 



