32 TEXTBOOK OF BOTANY 



studied, but they take place in three directions at right angles 

 to each other ; consequently, a colony of four cells forms a 

 plate, and one of more than four usually forms a group of two 

 or more cell layers. 



47. Resting Cells. Sometimes, under very unfavorable circum- 

 stances, the cells of Protococcus stop growing and dividing, and pass 

 into a condition somewhat like that of the spores of bacteria. The cell 

 contents do not shrink away from the wall, but the wall becomes 

 thicker and oil drops appear in the protoplasm. Such cells can return 

 to the ordinary condition when circumstances are once more favorable 

 for growth. 



48. Distribution and Relationships. Protococcus is found in 

 practically all parts of the world. There are also many other widely 

 spread species of one-celled green plants different from Protococcus 

 but probably related to it. These and many other plants containing 

 chlorophyi are grouped under the name of alga (singular, alga). Al- 

 though, as we shall see, many of the algse are larger and more complex 

 than Protococcus, they are all much simpler than the higher green 

 plants, such as the cucumber, wheat and other grains, and the trees. 

 Protococcus and its relatives are of special interest because all the 

 higher plants are undoubtedly descended from one-celled green algag 

 which were something like Protococcus, although probably not exactly 

 like any alga now living. The relationship of the bacteria to the algas 

 is not clear. Probably the bacteria or most of them are degenerate 

 descendants from algae even simpler in structure than Protococcus. 



