304 MICROBIOLOGY OF SOIL 



and others recorded that a number of fungi' are economically impor- 

 tant as wood-destroyers. Spores of a pure culture of Perncillium sown 

 on sterile blocks of spruce wood, germinated and grew normally. 

 Sections of the wood showed that the hyphae had entered the starch- 

 bearing cells of the meduUary rays of the sapwood and consumed the 

 whole of the starch. MacBeth and Scales found that when the medium 

 is slightly alkaline, certain aerobic bacteria will play the principal 

 r61e in the destruction of cellulose. When the medium is acid, molds 

 and higher fungi become the active agents of destruction. They also 

 found that the cellulose-destroying forms multiply with great rapidity 

 in alkaline soils when cellulose in the form of filter paper is added. The 

 power to destroy cellulose is reported for a number of species of Penicill- 

 ium, Aspergilli, Trickodermce and other organisiris which belong to the 

 common soil forms. Though the fungi may play an important part 

 as cellulose destroyers also in alkaline soils, in acid soils where the 

 activity of bacteria is greatly inhibited, fungi proba,bly play a pre- 

 dominant r61e. This fact led Marshall to conclude in 1893 that 

 fungi take an active part in the mineralization of the organic matter 

 in acid humus soils. 



Mycorrhiza. — Apart from the so-called soil fungi, there exists another 

 group known as mycorrhizal fungi. These live syinbiotically on the 

 roots of the higher plants. Many roots of forest trees, when examined 

 carefully, show that there is a union between the mycelium of certain 

 fungi, usually belonging to the fleshy fungi, and the root of the plant. 

 This union is called a "mycorrhiza." The fine filaments of the fungus 

 enter the cells of the root. These organisms were thought at first 

 to supply the roots with water and soluble plant food from the soil. 

 The power to fix atmospheric nitrogen has been ascribed to these organ- 

 isms by several investigators. But aside from these useful so-called 

 endotrophic Mycorrhiza, there are also the ectotrophic Mycorrhiza 

 which probably live only parasitically upon the roots of plants. . 



Actinomyces. — The study of soil Actinomyces is nearly all of very 

 recent origin. Several years ago but two soil Actinomyces had been 

 defimitely described, viz., Act. albus and Act. chromogenus. The work 

 of Krainsky, of Conn, and of Waksman and Curtis has demonstrated 

 that Actinomyces are widely scattered in cultivated soils. The last- 

 named investigators have shown that' while the absolute numbers of 

 Actinomyces decrease with depth of soil, their relative numbers are 



