CHAPTER III 



PROTOCOCCUS 



43. General Appearance. — On the sides of fences or of 

 old buildings, especially in spots that are shaded much of 

 the time, we often see a layer of a dark-green, powdery sub- 

 stance which becomes bright green when moist. A similar 



green layer is found on many 

 other objects, such as flower- 

 pots, trees, rocks, and stone 

 walls. If we examine a small 

 bit of this green coating under 

 the microscope, we shall prob- 

 ably find in it several different 

 kinds of minute plants, but the 

 one that is likely to be most 

 abundant appears in the form 

 of single green cells or of two, 

 four, or more cells joined to- 

 gether. This little plant is 

 Profococcus viridis. 



44. Structure. — A single cell 

 of Protococcus (Fig. 12, A) re- 

 but it is green and is larger 

 than most bacteria. The average diameter of a Protococcus 

 cell is said to be about ^Vir inch. It is surrounded by a wall 

 that is thicker than that of a bacterium. Its green color 

 is due to a substance called chlorophyl ; this is the same 

 substance that gives the green color to the cucumber and to 

 other familiar plants. It is never present in bacteria. 



30 



Fig. 12. — Protococcus. A, a 

 single cell ; B, a dividing cell ; 

 C, the division completed, the two 

 daughter cells being still within 

 the wall of the mother cell ; D, a 

 colony of three cells. 



minds us of a bacterial cell ; 



