CLASSIFICATION OF THE SULPHUR-BACTERIA 



147 



other end lying free in the water. The division of the thread into 

 cells is usually hidden by the large quantity of sulphur, but the cells 

 can be demonstrated by washing out the sulphur with absolute 

 alcohol, and then staining with fuchsin or methylene-blue. The cells 

 towards the free end are somewhat larger than those at the other 

 end. Another mark which distinguishes 

 this genus from Beggiatoa is the presence 

 of a common thread-membrane in addi- 

 tion to the membrane which each cell 

 possesses. Thus it follows that Beggiatoa 

 in breaking up divides into separate 

 parts, whereas the Thiothrix cells are 

 more or less held together inside the 

 thread-membrane. Still another distin- 

 guishing mark is that at the free end, 

 single cells loosen themselves and, after 

 becoming separated from the parent FIG. S4. Thiothrix nivea. Group 



,1 i v i j_i i -i i i of young threads containing sulphur 



thread, slightly elongate themselves, and granules. (6) Mucilaginous cushion. 

 then develop a mucilaginous, adhesive 



cushion. As this is done by a large number, and adhesion takes 

 place near the parent thread, a thick cluster of threads of charac- 

 teristic appearance soon results. Three species have been described. 



1. THIOTHRIX NIVEA. Threads 2-2-5 p. thick at the base, 1'7/z 



thick in the middle, and 1 '4-1 -5 /JL at the apex (Fig. 84). 



2. THIOTHRIX TENUIS. Threads equally thick, usually l'0-l'l /*. 



3. THIOTHRIX TENUISSIMA. Very small threads, average of O4- 



0'5 fM thick. 



As these organisms are motionless, and as no sulphur-bacteria can 

 grow without a supply of free oxygen, they are usually found at the 

 bottom of fast-flowing streams, because, if the sulphur-containing water 

 is still, the supply of oxygen in it is scanty and chiefly at the top. 

 Hence it is accessible only to the motile genus Beggiatoa. On the 

 other hand, if the water flows at a rapid pace, the supply of oxygen 

 at the bottom is greater and therefore permits of the growth of 

 representatives of this genus. 



III. Thiophysa. In this group are placed all the colourless sulphur- 

 bacteria which do not form threads. So far back as 1876 Warming 

 described two forms, known respectively as Monas Miilleri and Monas 

 fallax, the former spherical, with a diameter of 5'6-15/x, the latter 

 ellipsoidal, 4 -5 /* long and about 3 //, broad. Two others were dis- 

 covered later by Jegunow, both having a slightly bent, rod-shaped 



