THE ALG.K. 



95 



usually occur isolated, although they are sometimes seen 

 in short filaments, for they grow by the binary division of 

 the cells, and, as in the case of Protococcus and other one 

 celled plants, the cells sometimes remain adherent for a 

 time. The cells are bilaterally symmetrical, i.e., the two 

 halves of each cell are just alike. This fact helps to dis- 

 tinguish some forms from Diatoms. Another distinguish- 

 ing characteristic is the possession of projecting points 



Fro. 54. Desmids. 1, Micrastenas papillifera ; 2, Micrastenas morsa ; 

 3, Cosmarium polygonwn; 4, Xanthidium aculeatum; 5, Staurastrum 

 furcatum ; 6, Euastrum oblongum ; 7, Penium Brebissonii ,- 8, Closterium 

 lunula ; 9, Xanthidium octocorne ; 10, two views of Staurastrum alternant ; 

 11, Cosmarium tetraophthalmum ; 12, Aptegonum desmidium. The figures 

 are all highly magnified. (Kerner and Oliver.) 



or spines. These two facts and the bright green color 

 are great helps in their identification. 



The cell contents are inclosed in a cellulose wall of con- 

 siderable firmness, and a more or less distinct gelatinous 

 covering envelops the cell wall. The protoplasm contains 

 chlorophyl and its product starch, and is arranged in bi- 

 laterally symmetrical patterns in the two halves of^ the 



