CHAPTER V 



INFLUENCE OF THE PHYSICAL STRUCTURE OF THE 



MEDIUM. 



The physical structure of the medium has often a remarkable in- 

 fluence upon the development of microorganisms. Very little attention 

 has been paid to this fact, as yet, and only a few instances are known where 

 the structure of the medium is a deciding factor in the life of microorgan- 

 isms. A few examples have already been mentioned. Saccharomyces 

 will only exceptionally form spores on the surface of liquids while they 

 are abundantly formed on the surface of a moist gypsum block. This 

 may be accounted for by the better aeration of the yeast cells on gypsum. 

 Another example, mentioned before, is the influence of the medium, on 

 which bacteria are dried, upon their resistance against desiccation. 

 Chemical action of the medium upon the bacteria cannot explain the 

 differences since the same stratum in different structures gives different 

 results. The great advantage of porous materials (soil, starch, charcoal, 

 etc.) over smooth surfaces may be due to the very rapid and complete dry- 

 ing in the porous material and possibly to the power of absorbing gases of 

 all kinds. 



Recently, attention has been called to another example of considerable 

 importance, namely, the difference of development of bacteria in soil and 

 in solution. The processes of ammonia and nitrate formation do not run 

 parallel in solution and in soil ; this is not due to certain chemical com- 

 pounds in the soil, since the result remains the same if the soil is sub- 

 stituted by the insoluble quartz sand ; quartz has no absorptive powers, so 

 this cannot account for the difference. The most probable explanation 

 is the enormously increased surface of the water surrounding the soil 

 particles. The surface exposed to oxygen is many times larger in a soil 

 of a normal water content than in the same amount of water in a flask. 

 In agreement with this explanation, nitrification, as a strict oxidation 

 process, is greatly increased in soils with moderate moisture and is checked 

 if the soil is saturated with water. Ammonia formation is usually favored 

 in well -aerated soils, but occasionally it is decreased. This is probably 

 due to the different types of ammonia-producers, some of them growing 

 better with air, others growing better without it. Details will be given 

 in the division on Microbiology of Soil. 



