vi PREFACE 



primarily written as a contribution to botanical literature. 

 The author's claim to make the effort rests on a study of the 

 sulphur bacteria over a period of fourteen years. Some of 

 the work has already appeared in various scientific periodicals, 

 but a large portion is here published for the first time. 



On the morphological side of the subject all the known forms 

 are sketched and described with sufiicient fullness to secure 

 identification. A scheme of classification is drawn up which 

 avoids the defects of its predecessors. Numerous schemes of 

 classification of the sulphur bacteria have appeared, but, with 

 one or two exceptions, these are parts of a general scheme 

 embracing the whole of the bacteria, and have not followed a 

 special study of the sulphur bacteria. As a consequence the 

 later schemes have merely perpetuated the defects of the older 

 classifications. 



There are many gaps in our knowledge of the physiology 

 of the sulphur bacteria which cannot be adequately filled by 

 the biologist, and which await the attention of the biochemist. 

 It is hoped that the present work will form a basis for investiga- 

 tions on biochemical lines. 



I desire to acknowledge my especial indebtedness to my 

 colleagues, Mr. W. G. Burrell, M.A., Dr. J. A. Cranston, Mr. J. 

 Muil Leitch, B.Sc, and Dr. Blodwen Lloyd, for their assistance 

 in the preparation of the manuscript and in the revision of the 

 proofs ; and to Mr. Leitch for the preparation of the Indices. 



DAVID ELLIS. 



Department of Bacteriology, 



The Royai- Technical College, 

 Glasgow. 



