THE MICRO-ORGANIC POPULATION OF THE SOIL 257 



in Switzerland, have confined themselves mainly to particular 

 types, and have not attempted a comprehensive survey of the 

 whole field. Others have tried to ascertain how far facultative 

 parasitic fungi could live saprophytically in the soil : apparently 

 certain disease-producing Fusaria and Pythium can do this. 

 Pratt 1 has isolated fungi which cause disease in potatoes from 

 virgin desert lands and also from Idaho soils that have never 

 been cropped with potatoes. 



The more serious problem of studying the fungus flora of 

 the soil as a whole has been attempted in Holland by Ou de- 

 mans and Koning (221), in the United States by Waksman 

 (292^:), and in England by Miss E. Dale (75). It is now 

 under investigation at Rothamsted by W. B. Brierley and 

 Miss S. T. Jewson. 



The great difficulty is the lack of suitable methods of in- 

 vestigation : neither the morphologists nor the physiologists 

 have yet developed precise, simple methods. The fungi are 

 invariably isolated from the soil by means of cultural media, 

 but there is no means of ensuring that the medium used is 

 suitable for the development of all forms that may be present : 

 many of the parasitic forms and others in addition are doubt- 

 less missed. Further, the morphological characters vary 

 with different media : 2 hence it is impossible to describe the 

 organisms as they exist in the soil. Nor are the physiological 

 properties any more definite : they depend on the condition of 

 the organism the newly germinated fungi behaving differently 

 from the older mycelium. 



In these circumstances rigid identification is a matter of 

 great difficulty and it is impossible to reprehend too strongly 

 the practice, so tempting to pioneers, to describe forms as new 

 species, unless careful examination of their behaviour on 

 different media has shown that they really are new. 



Again, there is no method of estimating even approxi- 



1 O. A. Pratt, Journ. Ag. Res., 1918, 13, 73-100. 



2 For a discussion of the phenomena see W. B. Brierley, Some Concepts in 

 Mycology An Attempt at Synthesis (Trans. Brit. Mycol. Soc., VI., part 2, 

 1919). 



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