CHAPTER I. 



INTRODUCTION— GEOGRAPHICAL DISTRIBUTION— 

 THE SULPHUR CYCLE IN NATURE— METHODS OF 

 INVESTIGATION— PLEOMORPHISAl. 



Introduction. 



The term Sulphur Bacteria is usually applied to the members 

 of the group which have sulphur globules in their cells. As a 

 result of their activities compounds of the highest importance 

 to the higher green plants are formed ; in fact these organisms 

 constitute a cog in the machinery of life, a breakdown in which 

 would sooner or later result in the elimination of mankind 

 and the higher animals. 



It is inevitable that a great diversity of form should 

 characterize a division of plants the members of which owe 

 their grouping to a physiological trait that does not neces- 

 sarily connote the possession of any other character, either mor- 

 phological or physiological. An examination of the sulphur 

 bacteria supplies sufficient evidence of the unsuitability of 

 physiological classifications to express phylogenetic relation- 

 ships. The sulphur bacteria include almost every variety 

 of size and form that is to be found among the bacteria and 

 allied organisms. Thus there are representatives of the 

 bacillus, the coccus and the spirillum ; and in addition there 

 are ovoid, filamentous and other shaped forms in the group. 

 The size varies from one that is barely visible under the 

 microscope to one that is large enough to permit of micro- 

 tome sections. 



If the bacteria as a group were eliminated from our 

 classifications and a distribution of the sulphur bacteria among 

 the other classes became necessary, some would be added to 



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