i6o 



FRESH-WATER BIOLOGY 



i8$ (183, 184) Cells four together, never forming larger plates. From two to 

 five spines on the external margin of each cell. 



Tetrastrum Chodat. 

 Schmidle regards those forms with spines simply as different species of Staurogenia. 



186 (182, 177) Cells four, lying in two planes Tetradesmus Smith. 



This coenobium resembles a Scenedesmus rolled up, and in the size, shape, 

 and structure of the cells they are the same. 



FlG. 207. Tetradesmus wisconsiensis Smith. X 1500. (After Smith.) 



Xd 



187 (176) Coenobium a coarse net or a concentrically-arranged circular disc of 



cells, formed by the joining together of zoospores while within 

 the mother-membrane, or still within the liberated inner 

 lining of the same. . . Family HYDRODICTYACEAE . . 188 



1 88 (189) Coenobium a free-swimming circular plate of cells, one layer in 



thickness Pediastrum Meyen. 



The cells arranged either with intercellular spaces or not; marginal 

 cells with one or two pointed projections; inner cells angled or concave; 

 chromatophore parietal, with one pyrenoid, and perforated at one side. 

 Reproduction by means of zoospores which are cast out together with 

 the inner lining of the mother-membrane, and within which they form 

 a new coenobium. 



An alga which greatly resembles a two-celled Pediastrum was formerly 

 described as Euastrum by Schmidle, but Lagerheim places it in a new 

 genus Euastropsis. The mode of reproduction is the same as for Pedi- 

 aslrum; the zoospores, however, arrange themselves in pairs instead of 

 in a single plate, and form a number of new individuals which are set 

 free while within the inner layer of the mother -membrane. 



FIG. 208. Pediastrum boryanum Meneghini. X 600. (Original.) 



189 (188) Coenobium a coarse net Hydrodictyon Roth. 



Nets large, each mesh bounded by five or six 

 cylindrical cells; the chromatophore reticulate, 

 parietal, with numerous pyrenoids; asexual re- 

 production by zoospores, those from each cell 

 forming a new net; sexual reproduction by 

 many isogametes. The zygospore produces 

 two to five large zoospores which in turn give 

 rise to a new net when they germinate. 



In the early stages the nucleus is single, but 

 later divides rapidly so that the cell is multi- 

 nucleate. As the nets are formed within the 

 cylindrical mother-membrane they are cylin- 

 drical in shape for some time, but later become 

 torn and irregular. The nets occur as a very 

 thick light green scum on the surface of ponds 

 exposed to the direct rays of the sun. The dif- 

 ferent modes of reproduction have been proved 

 by Klebs to depend largely on the condition in 

 environment, and that by varying these condi- 

 ditions the different phases to development can 

 be produced. 



FIG. 209. Hydrodictyon reticulatum Lagerheim. 

 X loo. (Original.) 



190 (68, 249) Plant of septate filaments, or of closely-arranged cells, forming 



plates or cylinders, one or more layers thick; attached or 



free-swimming Order Confervales . . 191 



Reproduction asexual, sexual, or both in the same species. 



191 (196, 246) Plant in adult form a macroscopic, free-swimming plate or 



hollow cylinder of cells; in early stages often filamentous 

 and attached Family ULVACEAE . . 192 



