PHYSIOLOGY OF THE SULPHUR BACTERIA 287 



period required for the conversion of H,S into SO 3 and the expulsion of the 

 latter from the cell was ascertained by Jegunow to be about five minutes. 



The importance of the sulphur bacteria in the economy of Nature is unmis- 

 takable : in co-operation with the sulphate-reducing bacteria they ensure that 

 the sulphur cycle pursues an uninterrupted course, the element being taken up 

 by the higher plants in the condition of sulphates, and deposited in the cells in 

 the form of organic compounds, from which, in the course of putrefaction, it is 

 liberated as sulphuretted hydrogen, and is finally then reconverted into sulphates 

 by the sulphur bacteria and recommences its course through the higher plants. 



