30 



THE MICROBE AND ITS ACTIVITIES 



bits of life appear to be incapable of existing upon inert material 

 and, so far as is known, can develop only in conjunction with living 

 cells. The presence of such organisms in soil is speculative for 

 the most part. There is, however, a certain kind of ultramicro- 

 scopic substance parasitic upon bacteria — the bacteriophage — 

 which is of common occurrence in soil. It has been found that 

 these organisms are about the size of a protein molecule, or only 

 20 to 30 millimicrons (mm) in diameter. Some appreciation of 

 this infinitesimal size may be gathered from the fact that 1 mm is 

 equal to 1/1,000 n, or 1/10,000,000 of a centimeter. It has also 

 been shown that the ultramicroscopic organism causing mosaic 

 diseases in plants may survive in soil for a short period of time. 



Fig. 17. — Penicillium sp.: (c, d) ter- 

 minal sporulating filaments highly mag- 

 nified; 0', k, m) lower magnifications 

 of sporulating organs (from Thom). 



Fig. 18. — Aspergilhis sp. showing 



various sporulating characteristics 



and differences in sizes of spores 



(from Henrici). 



Activities of Soil Bacteria. — The activities of the bacteria, 

 especially those concerned with the decomposition of organic 

 matter, are frequently referred to by the layman as processes of 

 "putrefaction," "fermentation," and "decay." These terms 

 (with the possible exception of the use of the term "fermentation" 

 as apphed to the decomposition of carbohydrates under anaerobic 

 conditions) are significantly important only historically. In times 

 past they were used in reference to a variety of processes previous 

 to an understanding of the nature of the chemical changes and the 

 biological agents which were involved. These terms will, there- 

 fore, be carefully avoided in the following pages. 



