2 SULPHUR BACTERIA 



the list of Alga?, others to the lower fungal groups, whilst a 

 number would find a place among classes of the animal king- 

 dom. If one compares, for example, the diversity of form 

 and mode of life of Chromatium, Thiobacillus, Thiospirillum, 

 and Beggiatoa, the phylogenetic unsoundness of the grouping 

 becomes manifest. In the present stage of our knowledge, 

 however, the grouping must remain in spite of this defect 

 on account of its convenience for the further investigation of 

 these interesting organisms. 



It is doubtful whether any group of plants excels the sulphur 

 bacteria in the numerous aspects of biological interest that 

 they show. We have in them the best examples of pleomor- 

 phism shown by any group of organisms ; some of the mem- 

 bers hold the first place in their sensitiveness to light. The 

 sulphur metabolism is a subject of transcending interest. 

 In addition, the sulphur bacteria show a great variety in form, 

 in internal structure, and in the different phases of their 

 development. If we regard these plants from the point of 

 view of their more immediate utilitarian aspect there is much 

 that calls for attention. In the first place, as the substance 

 consumed by them is the foul-smelHng sulphuretted hydrogen, 

 and as the chief end product in their metabolism is the com- 

 pound, namely the sulphate, which is absorbed by the higher 

 green plants, their place in the economy of nature is obvious. 

 The higher green plants, from which, either directly or in- 

 directly, all human beings draw their food, are suppHed with 

 a necessary food material. 



Whilst the ultimate result of their activities is productive 

 of great benefit to mankind their more immediate effects may 

 be highly inconvenient, for when growth takes place in water 

 conduits or in stagnant pools, a large volume of sohd organic 

 matter is formed which chokes up the conduit pipes, and 

 renders the water unfit for human consumption. Also, 

 instances are known in which water thus affected has been 

 rendered unfit for use in various industries. 



Indirectly the operations of the sulphur bacteria influence 

 the play of other forces of great economic importance. The 

 role of bacteria in the transformation of muds, sands, and other 



