78 SULPHUR BACTERIA 



within the last thirty years than of any other class of plants ; 

 and yet there has been practically no critical examination of 

 the soundness of the characters that have been used to effect 

 the division of these organisms. The classification sponsored 

 by the American Society of Bacteriologists is no exception, at 

 any rate in that part of it which relates to the sulphur bacteria. 

 It is proposed in the present work to attempt a remedy in the 

 limited field of the sulphur bacteria, and to modify Buchanan's 

 classification after elimination from it of what is regarded as 

 of dubious value. 



Review of Previous Classifications. 

 Winogradsky's Classification of the Sulphur Bacteria. 



The predecessors of this grouping may be left out of account, 

 as they were based on very scanty and inaccurate knowledge. 



Winogradsky, 1888, distinguished fourteen genera, in- 

 cluding the genera of previous investigators : — 



I. Colourless Bacteria. 



(i) Beggiatoa. — Threads equally thick, freely motile, and 

 not forming gonidia. 



(2) Thiothrix. — Threads unequally thick, attached, and 



forming motile gonidia. 



II. Coloured Bacteria. 



(A) Cells united in families.* 



a. Division in three directions of space. 



(3) Thiocystis. — Single or several families of small cells, 



surrounded by a slime cyst, and capable of movement. 



(4) Thiocapsa. — Several families of round cells, loosely 



embedded in a common envelope of slime. Not 

 motile. 



(5) Thiosarcina. — Families in packets. 



b. Division at first in three, later in two directions of 



space. 



* The term family is used here in the sense of a cluster or an aggregate of 

 similar cells. 



