THE INTIMATE STRUCTURE OF THE CELL 177 



the same group organisms, some of which show plant-Hke 

 affinities, whilst others are more nearly akin to animals. 



Beggiatoa alba (Trevisan). 



The filaments vary in length, and may be motile or non- 

 motile. The 7nembrane is well defined and stains more deeply 

 than the internal plasma. It is plastic, and offers no resistance, 

 either to the bending of the filament, or to the escape of solid 

 cell inclusions when these are subjected to outwardly directed 

 pressure. Plasmolysis has not been observed. 



The plasma is reticulated, the meshes being irregular in 

 form and variable in size. Very minute granules can be 

 distinguished in its substance which, since they appear to be 

 used up during growth, are probably reserve materials compar- 

 able to the basophile granules of the yeast cell. The plasma 

 consists of strands which extend throughout the cell (Fig. 6^). 



Sheath formation. — The outer layers may undergo trans- 

 formation into a mucilaginous material which first hardens, 

 and then separates from the rest of the organism. It forms a 

 hollow cylindrical covering to the rest of the cell, and is known 

 as the sheath. The formation of a similar sheath has been 

 observed in the life-history of the two Iron bacteria, Cladothrix 

 dichotoma and Crenothrix polyspora. In these it is formed 

 early in development, persists through life, and subsequently 

 hardens, thereby forming a sheath around the cells. In 

 Beggiatoa alba, on the other hand, although sheath formation 

 can be demonstrated in motile threads, its amount is so very 

 small and insignificant that it can be recognized only with 

 difficulty. It reaches full development when conditions of 

 life become unfavourable ; and its formation precedes the 

 dissolution of the cells (see p. 94). When the protoplast 

 under unfavourable conditions breaks up into a string of 

 segments, sheath formation occurs between the segments, 

 and when the latter ultimately disappear a ragged hollow 

 cylindrical sheath is left with occasional transverse bridges 

 of sheath material running across the hollow interior 

 (Fig. 6h). A similar formation of mucilage, which similarly 



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