MICRASTERIAS. 61 



longitudinally under the eye of the observer, a 

 dark line is seen to cross alternately from one side 

 to the other this is caused by the twisted form 

 of the filament, and the line is the third corner of 

 the triangle. It separates into single joints or 

 cells, and then shows its triangular shape, with the 

 green substance within forming a star of three 

 points. This is a species of common occurrence. 

 Desmidium guadrangulatum is more rare, and 

 only seems to differ from the last in the number 

 of sides. There are other genera of these thread- 

 like Algae, but this is the most striking. 



The genus Micrasterias contains many sin- 

 gular forms ; they are more or less round, and 

 flat, deeply divided into two lobed segments, or 

 cells, and notched or cleft at the edges, sorne- 



G 



