THALLOPHYTA: FUNGI 101 



The cells of bacteria are so simple that they might almost be said to 

 be structureless. A mass of homogeneous protoplasm is surrounded by 

 a thin cell wall, generally composed chiefly of chitin, a nitrogenous sub- 

 stance, whereas the cell walls of green plants are composed mainly of 

 cellulose, a carbohydrate. Commonly the cell wall becomes mucilagi- 

 nous, forming a slimy sheath or capsule. There is no organized nucleus 

 but merely some scattered granules of a chromatin-like material that 

 can be revealed by staining. In some bacteria these granules are aggre- 

 gated to form a distinct central group. Other granules may also be 

 present; these generally represent reserve food. 



8B 0®^) 



i 



D '^ E F 



Fig. 80. Group of common bacteria, X 1,500. A, Sarcina lutea; B, Bacillus subtilis; C, 

 Bacillus typhosus; D, Spirillum cholerae; E, Streptococcus pyrogenes; F, Spirillum undulatum,. 



In some bacteria the two cells separate following division, while in 

 others they remain together in colonies. Spirillum forms are nearly 

 always solitary. In the coccus forms the colonies may be cubical, 

 plate-like, chain-like, filamentous, or irregular. In the bacillus forms 

 the divisions occur only in one plane, and so the colonies are always 

 filamentous. In Beggiatoa, a sulphur bacterium, the filaments are as 

 highly organized as those of Oscillatoria. In Cladothrix, an iron bac- 

 terium, the filaments exhibit false branching. 



As in the Cyanophyceae, cell division takes place by the formation of 

 an inward-growing cell wall. Under favorable circumstances, cell divi- 

 sion in many bacteria may occur as frequently as every 20 minutes, so 

 that, in the course of 24 hours, a single cell may give rise to billions. Such 

 a rate of multiplication is soon checked, however, by the exhaustion of 

 the food supply or by the accumulation of poisonous waste products of 

 metabolism. Although all bacteria are active only in the presence of 

 moisture and other favorable conditions, if these fail, many bacteria can 

 pass into a resting stage and remain inactive for a long time. Bacteria 

 on dust particles in the air are in a dormant state and can resist desicca- 



