180 WISCONSIN PHYTOPLANKTON 



Cells 5.7-9 fji broad. 



The specimens that I have found in our lakes differ somewhat from 

 those described by West since the cells are not slightly dilate at the 

 poles and the chloroplasts usually contain pyrenoids. The Wisconsin 

 organism does not seem sufficiently differentiated, however, to warrant 

 the description of a new variety or species. This alga is of particular 

 interest since it is the only one of the order which has become established 

 as a true member of the plankton. 



Family CHAETOPHORACEAE. 



Cells united to form branching filamentous, discoidal or pareuchyma- 

 tous thalli. Branches of thallus generally attenuated at apices and 

 sometimes ending in long hair-like processes. Thallus naked or en- 

 closed in a copious, hyaline, homogeneous, gelatinous envelope. Cell 

 shape ordinarily cylindrical with flattened ends. Chloroplast single, 

 parietal and laminate to girdle-shaped; wdth one or more pyrenoids. 

 Nucleus single. 



Asexual reproduction by aplanospores and bi- or tetraciliate zoo- 

 spores. 



Sexual reproduction isogamous, by a fusion of biciliate zoogametes. 



STIGEOCLONIUM Kiitzing 1843. 



Thallus free-floating or attached, with branches scattered and not 

 arising in dense tufts. Thallus without an appreciable difference in 

 size between the main stem and minor branches ; covered with a homo- 

 geneous, hyaline, gelatinous envelope. Cells cylindrical to disciform, 

 gradually decreasing in diameter towards the apex and with terminal 

 cells conical or elongated to hair-like unseptate proeesses. Chloroplast 

 single, parietal ; gridle-shaped and filling median portion of older cells ; 

 completely filling younger cells. Each chloroplast generally containing 

 several pyrenoids. 



Asexual reproduction by aplanospores and tetraciliate zoospores. 



Sexual reproduction isogamous, by the fusion of biciliate zoogametes. 



STIGEOCLONIUM LUBRICUM (Dillwyn) Kiitzing. PL 50, Fig. 7. 



Phycol. germ. 189. 1845. 



Myxonema, lubricum (Dillw.) Fries. Hazen, Mem. Torr. Bot. Cl. 11: 195, 

 p7. 28, figs. 1-2. 1902. 



Filaments forming dark green tufts up to 30 cm. long. Filaments 

 rauch branched and branches usually opposite, rarely single. Cells of 

 main filament at base of branches sub-spherical, shorter than other cells. 



