84 



Occurrence of Microorganisms in Soil 



Garrett divided the root-infecting fungi into soil inhabitants and soil 

 invaders, the former being primitive or unspecialized parasites with 

 a wide host range and widely distributed in the soil, and the latter 

 including the specialized parasites which depend upon the host 

 plant. 



From an ecological point of view, one may recognize certain spe- 



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fT II 



Fig. 39. Microscopic structure of a soil Penicilliiim (from Thorn). 



cific groups of fungi, depending on the nature of the substrate or the 

 particular nutrients in the substrate which favor their development. 

 Thus, one may speak of (1) "sugar fungi" (comprising largely Phy- 

 comycetes); (2) "cellulose-decomposing fungi" (comprising various 

 Ascomycetes and Fungi Imperfect!); (3) "lignin-decomposing fungi" 

 (comprising some Basidiomycetes ) ; (4) "humus fungi"; (5) "root- 

 inhabiting fungi"; (6) "soil-inhabiting parasitic fungi"; (7) "cop- 

 rophilous fungi"; (8) "predaceous fungi"; etc. When a fresh supply 

 of nutrients is made available in the soil, as by the penetration and 

 subsequent death of plant roots, there is a rapid sequence in the 

 flare up of the various groups of fungi, the "sugar" forms coming 

 first and the "lignin-decomposing" types last (Garrett). 



The effect of lime and manure upon the numbers of fungi in the 

 soil is shown in Table 14. 



