PLEUROCOCCUS 179 



CLASS II. THE GREEN ALGLE, OR CHLOROPHYCE^ 



212. The green algae. The green algae comprise a large and 

 varied assemblage of groups, many of which are widely different 

 from one another. Some forms of the Chlorophycece are believed 

 to stand rather close to what was the main line of ascent from 

 the algae to the liverworts and mosses. Consequently the class 

 has an important place in an account of the evolution of the 

 plant kingdom. The green algae illustrate better than any other 

 group the origin and evolution of sexual processes in plants. 

 Since the more familiar algal growths of fresh water are green 

 algse, a more extended treatment of the Chlorophycece will be 

 given than of the less familiar groups of the CyanopJiycece, 

 Phceopliycece, and Rhodophycece, and the following six orders will 

 be considered together with a " Summary of the Green Algae." 



Order 1. The one-celled green algae, or Protococcales. 



Order 2. The confervas, or Confervales. 



Order 3. The pond scums and desmids, or Conjugates. 



Order 4. The diatoms, or Diatomales. 



Order 5. The siphon algae, or Siphonales. 



Order 6. The stoneworts, or Chorales. 



ORDER 1. THE ONE-CELLED GREEN ALG.E, OR PROTOCOCCALES 



213. The one-celled green algae. This order contains almost 

 all of the one-celled green algae excepting the large but very 

 special groups of the desmids and diatoms. We can only 

 describe briefly five types. 



214. Pleurococcus. Pleurococcus (family Pleurococcacece) forms 

 the green coating or stain that is very common on the north 

 sides of tree trunks, fences, and stone walls. The cells (Fig. 175) 

 may be solitary, but they usually remain associated in small 

 groups for some time after the cell divisions. The protoplast 

 contains generally a single chromatophore of irregular shape 

 which, as a rule, fills the greater part of the cell. The chroma- 

 tophore is, however, variable in size and may or may not have 



