20 



AGRICULTURAL BACTERIOLOGY 



in bunches of fours. Such grouping is called a tetracoccus. 

 Again, the successive planes of division may be in three 

 dimensions of space, resulting in packets of cells, to which 

 the name sarcina is applied. 



The spirilla exhibit the same cell grouping as do the ba- 

 cilli, although, as a rule, it is less pronounced. 



Cell structure. The bacterial cell wall is a relatively 

 firm membrane, through which all food must pass by dif- 

 fusion. Lining the 

 cell wall is a layer of 

 protoplasm, the ecto- 

 plast, which has a se- 

 lective action on sub- 

 stances in solution in 

 the cell sap, or in the 

 liquid in which the 

 cell occurs. Since 

 the action of this cell 

 structure determines 

 what substances en- 

 ter or leave the cell. 

 Fig. 12. A Tetracoccus it is one of the most 



It will be noted that the cells tend to occur in 

 aggregates of four 



After Orla-Jensen. 



cells are placed in a strong sugar or salt solution, the proto- 

 plasm shrinks and the cell is said to be plasmolyzed, a con- 

 dition in which growth is impossible. 



The nucleus of the cell of the higher plant and animal 

 is a structure of the utmost importance, since it governs 

 for the most part the physiological activities of the cell. It 

 plays the most important role in cell division. Its impor- 

 tance is such, in the cells of the higher forms, that it would 

 seem impossible for any cell to function without a nucleus. 

 A definite nucleus is not found in the typical bacterial cell. 



important elements 

 of the cell. If the 



