72 PHYSIOLOGICAL RELATIONS 



then pasting over the latter a black paper stencil. After an ex- 

 posure to varying durations of sunlight or to the electric arc, 

 the dishes were placed in the incubator. The resulting colonies 

 developing show that an exposure of six hours to sunlight is 

 sufficient to sterilize almost completely the agar in those areas of 

 the dishes to which sunlight was admitted, corresponding to the 

 stencil mark. He also threw a solar spectrum on cultures sim- 

 ilarly made, and upon incubation it was demonstrated that the 

 blue-violet rays are most injurious in their action. Since this kill- 

 ing effect is not evident when the culture is exposed in a vac- 

 uum, it would seem that the deleterious action is probably an 

 oxidation effect. 



It may perhaps be inferred that light is more important in the 

 destruction of the spores of parasitic fungi than are all other 

 agencies combined. Nevertheless, many spores are well pro- 

 tected against these deleterious effects. However this may be, 

 a large number of fungous spores find hiding places under pro- 

 tecting rifts of the bark, beneath the leaf scales, or in the debris 

 on the surface of the soil, so that an adequate proportion survive 

 the resting period, as a rule, to continue the prevalence of all 

 common plant diseases. 



Many fungous forms are wholly independent of the presence 

 of light as a requisite factor in normal development. On the 

 other hand, in darkness the hymenophores of certain species 

 are said to be abnormal in form. 



The results indicate that light has an injurious and retarding 

 influence on the germination of fungous spores. De Bary records 

 that certain members of the Peronosporaceae, notably Phytoph- 

 thora infestans, germinate with difficulty in daylight and not at 

 all in sunlight, and Miss Ferguson and others have confirmed 

 this observation in experiments with Agaricus campestris and 

 many other Hymenomycetes. Very little accurate information 

 is at hand relative to the effects of light in the open upon the 

 development of the fruiting stages of fungi. 



For all practical purposes in culture work with the fungi, the 

 relation of light is not generally an important one. The studies 

 which have been made, however, should be followed up from a 

 quantitative point of view, for the exact effects of light intensities 



