WISCONSIN PHYTOPLANKTON 55 



disciform, with straight or markedly convex sides. Cell contents occa- 

 sionally homogeneous and pale to bright blue-green in color ; generally 

 with numerous pseudovacuoles. Trichomes without heterocysts or 

 spores. ( Euplanktont) . 



Cells 5-7 fi broad, 5-7 //. long. Colonies up to 1 mm. in length. 



Amicoy (rrr), Beaverdam (r), Island (rr), Lake of the Woods (rrr), Little 

 Bass (1) (rr), Mud (1) (s), Nicaboyne (rr), Rice (1) (rr), Wild Goose (rrr). 



The general appearance of the colony at once suggests an Aplianizo- 

 menon species with cells shorter and more rounded than the familiar 

 A. flos-aqiMe. I have examined a sufficient number of colonies to feel 

 confident in confirming Klebahn's statement that there are no spores 

 or heterocysts in the alga under consideration. This being the case it 

 is impossible to refer the species to Aphanizomenon, but it should be 

 considered a freshwater species of the common Trichodesmium of tropi- 

 cal seas. 



Family NOSTOCACEAE. 



Filaments free-floating, sessile, epiphytic, or endophytic ; solitary or 

 in colonies of microscopic to macroscopic size that are definitely or in- 

 definitely shaped. Trichomes always unbranched ; straight, flexed, spi- 

 rally twisted, or irregularly interwoven. Sheath rarely firm and nar- 

 row; generally copious, gelatinous, hyaline or colored, and distinct or 

 confluent with the colonial envelope. Trichomes generally the same 

 thickness throughout and with spherical to cylindrical cells. Hetero- 

 cysts either terminal or intercalary, solitary or catenate. Resting 

 spores variously shaped, adjacent to or remote from the heterocysts; 

 formed in all genera. Cell contents homogeneous, granulose, or with 

 pseudovacuoles ; grey or variously colored. 



Reproduction by fragmentation of colony, hormogones, and resting 

 spores. 



KEY TO THE GENERA. 



Trichomes solitary or interwoven to form flocculent masses of 



small size ANABAENA 



Trichomes laterally joined to form small plate-like colonies APHANIZOMENON 



ANABAENA Bory 1822. 



Filaments free-floating or forming a delicate mucous layer; solitary 

 or in floccose colonies. Triehomes the same thickness throughout or 

 slightly attenuated at the apices; straight, spiral, circinate, or aggre- 

 gated without definite order. Sheaths delicate or copious, gelatinous, 

 hyaline, homogeneous, rarely confluent with the colonial envelope ; some- 



