28 CELL STRUCTURES 



Students are apt to confuse vacuoles with spores (page 32). 

 Staining is the surest way to differentiate (Chapter XIX, 

 page 2()G). If vacuoles have any special function, it is an 

 unimportant one. 



Capsule.— The capsule is a second covering outside the cell 

 wall and probably developed from it (Fig. 20). It is usually 

 gelatinous, so that bacteria which form capsules frequently 

 stick together when growing in a fluid, so that the whole 

 mass has a jelly-like consistency. The term zoogloea was 

 formerly applied to such masses, but is a poor term and 

 misleading (zoon = an animal) and should be dropped. The 

 masses of jelly-like material frequently found on decaying 



Fig. 20.— Bacteria seen within Fig. 21.— Metachromatic gran- 



capsules, ules in bacteria. The dark round 



spots are the granules. The cells 

 of the bacteria are scarcely visible. 



wood, especially in rainy weather, are in some cases masses 

 of capsule-forming bacteria, though a part of the jelly is a 

 product of bacterial activity, a gum-like substance which 

 lies among the capsulated organisms. When these masses 

 dry out they become tough and leathery, but it is not to be 

 presumed that capsules are of this consistency. On the 

 contrary, they are soft and delicate, though they certainly 

 serve as an additional protection to the organism, doubtless 

 more by selective absorption than mechanically^ Certain 

 bacteria which cause disease form capsules in the blood of 

 those animals which they kill and not in the blood of those 

 in which they have no effect (Bacillus anthracis in guinea- 



