SECTION IX. 

 OXIDISING FERMENTATIONS. 



CHAPTER XXXIV. 



THE IRON BACTERIA. 



197. Morphology of the Genera Crenothrix and 

 Cladothrix. 



THE group of Schizomycetes known as thread bacteria can be 

 divided into two sub-groups. To the one of these belong the two 

 genera (described in the following chapter) which store up globules 

 of free sulphur in their cells. The appearance of these organisms 

 is so characteristic that a skilled eye can detect them with ease. 

 On the other hand, the five genera of the second sub-group lack 

 this peculiarity. In three of these latter, viz., Streptothrix, Lepto- 

 thrix, and Cladothrix, the fission of the cell takes place in one 

 direction only ; consequently, rods are formed. In the other two, 

 however, viz., Crenothrix and Phragmidiothrix, the extremity of 

 the thread is broken up, by division in all three directions of space, 

 into coccus-like cells. No perfectly satisfactory morphological de- 

 scription, or definite separation of these five genera into species, has 

 yet been drawn up on this basis. This defect to which reference 

 has also been made by C. SAUVAGEAU and M. RADAIS (I.) there- 

 fore restricts us to the consideration of individual examples. A 

 typical one is afforded by Crenothrix polyspora. This thread 

 bacterium was first described by F. COHN (XL) in 1870, and was 

 subsequently also named Crenothrix Kuhniana. It is illustrated 

 in Fig. 65, which shows that the threads are sessile, each of them 

 consisting of a single row of short cells held together by a common 

 tubular integument, called a sheath. This latter is formed by the 

 splitting of the cell membrane into two layers, the outer one of 

 each cell becoming merged into the adjacent membranes above 

 and below, and thus forming a uniform tube in which the cells 

 can move up and down. The threads are not cylindrical, but in- 

 crease in diameter from about half way along their length towards 

 the free end, so that the cross-sectional diameter varies from 1.5 

 to 5 ju.. Moreover, as may be seen from the figure, the various 

 segments of the thread are of unequal length. As the cells 



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