THE FUNGI OF THE SOIL 



33 



of soil microbiology may be interpreted as embracing a study of 

 the microscopic soil population which is responsible for the numer- 

 ous transformations occurring in the soil environment. How- 

 ever, many of the soil microorganisms are not limited to this 

 environment alone but may exist also in a variety of other 

 habitats. 



The Fungi of the Soil. — Fungi are characterized by their 

 filamentous structure. This is termed mycelium, and consists 

 of numerous hyphae, either unicellular or multicellular. This 

 mycelium frequently attains considerable dimensions and forms 

 a complicated, profusely branched, vegetative growth with 

 specialized, spore-forming, fruiting bodies. Although numeri- 

 cally, as determined by the number of single cells, they are fewer 

 in the soil than the 

 bacteria, their actual 

 abundance, as meas- 

 ured by the amount 

 of cell substance pro- 

 duced, may be con- 

 siderably greater than 

 that of bacterial 

 growth. They vary 

 considerably in struc- 

 ture and size from the 

 simplest yeasts and 

 molds to the more 

 complex forms, as the 



mushrooms and ^ ^ , . , , 



, , f. . 1 Fig. 22. — Mucor s\>: (a) young sporulating head, 



bracket tungl Wiiose ^^^ mature sporulating organ, (c) spores becoming 

 sporulating bodies can liberated from sporangium, (d) columella after the 

 be seen with the naked scattering of the spores, 



eye. Fungi are devoid 



of chlorophyll, and consequently cannot obtain their carbon from 

 the carbon dioxide of the air as green plants do. 



Fungi can be classified into three principal groups on the basis 

 of their morphology: (1) Filamentous fungi, which are fre- 

 quently spoken of as molds. These comprise various Phycomy- 

 cetes, Ascomycetes, and Fungi Imperfecti or Hyphomycetes. The 

 important soil genera, Mucor, Rhizopus, and Zygorhynchus are 

 found in the first class; Aspergillus and PenicilHum in the second, 



