CHAPTER II 



GROUPINGS AND GENERAL RELATIONSHIPS OF 

 MICROORGANISMS 



It has already been noted that four distinct groups of 

 microorganisms are to be considered in agricultural bac- 

 teriology, namely, the bacteria, the yeasts, the molds, and a 

 few of the protozoa. It is important that the differences 

 separating these groups from each other should be recog- 

 nized, and that the position of all in their relationship to 

 other plants and animals should be understood. 



Three of these groups, the bacteria, the yeasts, and the 

 molds, are placed in the plant kingdom, the protozoa in the 

 animal kingdom. It is necessary, therefore, first to differ- 

 entiate microscopic animals from plants. 



Differences between Microscopic Plants and Animals. — 

 Bacteria are probably the simplest living plants and proto- 

 zoa the simplest living animals. Both belong near the bot- 

 tom in the evolutionary scheme. It is natural, therefore, 

 that many resemblances are to be found between the lower 

 plants and the lower animals. Some investigators, in fact, 

 group all of the unicellular plants and animals together 

 under the name Protista. The differentiation between bac- 

 teria and protozoa is rendered particularly difficult be- 

 cause there are a few forms which apparently are inter- 

 mediate, and which are grouped by some investigators with 

 the bacteria and by others with the protozoa. Neverthe- 

 less, most forms in the two groups are not difficult of differ- 

 entiation. Plant cells usually possess a firm and well-de- 

 fined cell wall, while the protozoa, at least during their 

 active development, usually do not. The bacteria, in gen- 



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