22 



THE MICROBE AND ITS ACTIVITIES 



only through their activities, as in the case of the bacteriophage 

 and filterable viruses. However, they vary considerably in the 

 chemical processes that they bring about in the soil and in arti- 

 ficial cultures. 



The importance of a study of the nature and development of 

 higher plants is accepted without question because these plants 

 are so evident in appearance and in general usefulness to man and 



his domesticated animals. In 

 a similar manner, one has come 

 to recognize the importance of 

 those microorganisms which 

 have been discovered to be 

 the agents responsible for the 

 various diseases of man, domes- 

 tic animals, and plants, and 

 the need for combating them. 

 However, the microscopic 

 forms of life in the soil are so 

 obscured by their environment 

 that they are not given such 

 general consideration. They 

 are frequently given no atten- 

 tion or considered to play an 

 insignificant part in natural 

 processes. The transformations 

 with which they are concerned have been at times completely 

 overlooked or called non-biological. Provided microbes were not 

 generally distributed in soils, their presence in certain localities 

 would be very apparent from the enhanced plant growth. Under 

 the prevailing conditions, however, it might be considered that 

 their very ubiquity tends to conceal their activity. 



These soil organisms are largely saprophytic, acting upon the 

 organic plant and animal residues and the inorganic soil con- 

 stituents. Occasionally certain of the representatives may 

 attack living tissues of the higher plants ; the soil may also harbor 

 a number of animal parasites. As compared, however, with the 

 saprophytes, either in numbers, kinds, or importance, these occa- 

 sional parasites are quite insignificant, except in certain special 

 instances. 



A qualitative survey of the microscopic organisms found in the 



d {)OOoOOO 



Fig. 7.- — Heterotrophic, spore-forming 

 bacterium, Bacillus megalhermm; (a) 

 young cells showing flagella, (b) young 

 cells showing connections into chains of 

 rods, (c) older cells, (d) variations in 

 size and shape of spores (after Conn). 



