226 SULPHUR BACTERIA 



Nomenclature of the Thionic Acid Bacteria. 



All the members of this group are rod-shaped, and therefore 

 may be included either in the genus Bacillus, or the genus 

 Pseudomonas. It is unfortunate that Beijerinck should have 

 used the term Thiohacilliis to designate these organisms, for 

 the prefix thio in all morphological classifications is used to 

 designate the bacteria with sulphur contents, and the thionic 

 acid bacteria either do not secrete sulphur, or if they do it is 

 eliminated from the cell. The cumbrous name Thionsdure- 

 hakteriiim (Eng. Thionic acid bacterium) has been suggested 

 and used as a generic title by Issatchenko ; and lastly the 

 name Sulfonionas has been proposed. In both these changes 

 it would appear as if each innovator had held in view only 

 the needs of his own particular branch of research without 

 reference to the effect which the innovations would have on 

 the efforts of the systematists who have to pass in review 

 all the groups of bacteria. The generic terms Bacillus and 

 Pseudomonas are sufficiently clearly defined to be used without 

 fear of confusion, and all the known thionic acid bacteria can 

 readily be included in one or other of them. A loose nomen- 

 clature based on unstable physiological functions can only 

 lead to ultimate confusion. 



The Phylogeny of the Sulphur Bacteria. 



The most primitive members of the group are contained in 

 the genus Lankesteron, and of these Lankesteron roseo-persicina 

 is probably the most primitive. The majority of the sulphur 

 bacteria are probably derived from some such form. This 

 species reproduces by simple fission, and has not an external 

 plasmatic membrane differentiated from the rest of the cell, 

 and in addition it is highly pleomorphic. The more various 

 the number of pleomorphic phases in an organism, the greater 

 its chances of success in a changing environment. Probably 

 the first stage in the advance of such an organism would be the 

 assumption of a stable form under a new set of conditions. 

 If the new conditions persisted for a sufficiently lengthened 

 period a morpliologically stable organism could evolve, which 



