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been emphasized by Orla-Jensen, we sometimes meet with immotile 

 species which unquestionably are closely related to motile ones, where- 

 as, on the other hand, one is struck by the fact that in several large 

 natural groups motility is completely lacking 



Finally one diagnostic character of wide application, viz., the Gram 

 stain, may also be considered as dependent on structural properties of 

 the cell and the many instances of a close correlation of the outcome 

 of the Gram test with several other characters leave no doubt as to its 

 significance also for taxonomic purposes. 



b. Physiological characters 



The behaviour of the various bacteria in relation to temperature and 

 osmotic pressure of the environment has often been used in the char- 

 acterization of species. Recent investigations tend to throw doubt upon 

 the soundness of this procedure, because of the wide range of adapta- 

 tion which is manifested by most bacteria. 



As has already been set forth the physiological characters of prime 

 importance are those which are connected with the metabolism of the 

 organism. Although in this respect the significance of the katabolic 

 properties of the cells has been rightly stressed, yet one should not 

 lose sight of the fact that in some cases the occurrence of special prod- 

 ucts of anabolism can also be made use of. 



A first division of katabolic activity has to be made according to the 

 source of the energy required by the cell. Here we must distinguish be- 

 tween the organisms capable of utilizing radiant energy and those which 

 are dependent upon the chemical energy supplied by one or more con- 

 stituents of the medium. In the latter case, which is the more common 

 with bacteria, a further subdivision imposes itself according to the 

 role of the free oxygen in katabolism. Firstly we find bacteria in which 

 this gas is as indispensable as it is for higher plants and animals. But 

 in addition to this category there exist organisms which can satisfy 

 their energy requirements also in the absence of oxygen. Finally there 

 is a third group of bacteria, which thrive only in its absence, or nearly 

 complete absence. 



In each of these groups a further subdivision can be made both on 

 the basis of the nature of the favoured katabolic substrates and on the 

 basis of the mode of decomposition as evidenced by the nature of the 

 final katabolic products. It may be added at once that this predilection 



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