358 PHYSIOLOGY OF THE FUNGI 



95 per cent or above. A third group is represented by some of the pow- 

 dery mildews, whose spores are able to pat out short germ tubes under 

 conditions of extremely low relative humidity. Comparatively little 

 careful work has been done to determine the moisture requirements for 

 spore germination. Doran (1922) reviewed some of the earlier reports 

 and gives the results of his own experiments. Among the species whose 

 spores have been reported as requiring contact with liquid water for 

 germination are the following: Sderotinia Jnicticola, Peronospora pygmaea, 

 Phyllosticta antirrhim, teliospores of Gymno sporangium juniperi-vir- 

 ginianae, Cylindrocladium scoparium, and Plasmopara viticola. 



Numerous fungi whose spores may germinate in the absence of liquid 

 water have been reported. Some of these have been germinated on a dry 

 glass slide in a moist chamber, where the humidity is assumed to be at 

 100 per cent, the saturation point. However, Clayton (1942) showed 

 that a humidity of 100 per cent sometimes gave visible condensation of 

 water vapor, w^hereas a relative humidity below 99.85 per cent gave no 

 condensation at constant temperature. The spores of this group of 

 fungi usually show a much higher percentage of germination in liquid 

 w'ater if a plentiful supply of oxygen is present. Some representative 

 fungi reported in this group with the approximate minimum humidity 

 are Puccinia glumarum urediospores, 99 per cent; Venturia inaequalis 

 ascospores and conidia, 98.7 per cent; Ustilago nuda, 95 per cent; and 

 Penicillium glaucum, 84 per cent. The minimum relative humidity for 

 Aspergillus niger is near 70 per cent (Bonner, 1948). Figure 74 shows 

 germination curves. 



The germination of the conidia of certain species of the Erysiphaceae 

 in relative humidity at or near zero has been reported by several investi- 

 gators (Brodie, 1945; Brodie and Neufeld, 1942; Yarwood, 1936; and 

 Clayton, 1942). These species are Erysiphe polygoni, E. graminis, and 

 Microsphaera alni. The mechanism for spore germination under these 

 very dry conditions must be quite different from that of other spores, 

 which require liquid water or high humidity for germination. The 

 "apparent osmotic pressure" of the cell sap of the conidia is reported by 

 Brodie (1945) as about 63 atm. for E. polygoni and 68 atm. for E. graminis 

 hordei. It is likely that these high osmotic pressures may be an aid in 

 absorbing moisture from a relatively dry atmosphere. Brodie believes 

 that the conidia contain little free water but that imbibition may be 

 partly brought about by hydrophilic colloids. 



Brodie and Neufeld (1942) offer a tentative theory to explain "germi- 

 nation" under conditions of per cent humidity. They believe that, as 

 germination begins, free water is released by respiration and by changes 

 in colloidal materials containing bound water. No changes in the length 

 or width of the conidia could be detected during germination. The 



