34 



THE MICROBE AND ITS ACTIVITIES 



while a number of other forms, including Fusarium, Hormoden- 

 drum and Trichoderma, producing both colorless and black 

 mycelium, belong to the third class. (2) The yeasts, which belong 

 to the Ascomycetes, are found in the soil only to a limited extent. 

 (3) The mushroom fungi, belonging to the Basidiomycetes, are 

 found abundantly in forest and other soils in the form of a very 

 extensive mycelium, sometimes producing fruiting bodies in the 

 form of various mushrooms, toadstools, and puffballs. Many 

 fungi belonging to this group also produce an associative growth 

 with the roots of higher plants, especially forest trees, referred to 

 as mycorrhiza (or fungus root). In such associations, the fungi 

 assist the plants in obtaining their nutrients from the soil. As a 

 rule, the fungi show 

 such differences in 

 structures that their 

 classification into 

 species is almost en- 

 tirely based on mor- 

 phological features ; 

 unlike the bacteria, 

 httle resort to physi- 

 ological tests is used 

 in their differentia- 

 tion. Figs. 17 to 23 

 show a series of typi- 

 cal soil fungi. 



The fungi vary 

 considerably in the 

 nature of the proc- 

 esses which they 

 bring about in the soil, but they are almost all associated with the 

 decomposition of complex organic substances, non-nitrogenous as 

 well as nitrogenous. They have the capacity for producing more 

 cell material per unit of organic substance decomposed than the 

 bacteria. They are usually able to grow only under aerobic 

 conditions, although most yeasts and some Mucors can also live 

 in the absence of free gaseous oxygen, without, however, making 

 much increase in cell substance under such anaerobic conditions. 

 Certain Basidiomycetes may grow extensively in anaerobic 

 environments, providing portions of the growth are exposed to an 



Fig. 23. — Fusarium sp: {n) hyphal segments, 

 (6) spores (after Sherbakoff). 



