WISCONSIN PHYTOPLANKTON 49 



KEY TO THE FAMILIES. 



Trichomes without heterocysts* OSCILLATOBIACEAE 



Trichomes with heterocysts. 



Trichomes not attenuated NOSTOCACEAE 



Trichomes attenuated to a hair-like point RIVULARIACEAE 



Family OSCILLATORIACEAE. 



Trichomes free-floating, sessile, epiphytic, or endophytic; of one or 

 mere cells forming simple or branched filaments with one or more tri- 

 chomes within a single sheath. Sheaths of various forms; copious, 

 gelatinous, hyaline or colored, homogeneous to lamellated; firm, nar- 

 row and homogeneous or lamellated; or entirely lacking. Filaments 

 straight or spirally twisted. Cells disciform, cylindrical, or barrel- 

 shaped. Apical cell of trichomes frequently conical and capitate. 

 Heterocysts never present in the trichomes. Cell contents homogene- 

 ous, granulose, or with pseudovacuoles ; variously colored. 



Reproduction by vegetative division of cells and the formation of 

 hormogones. 



KEY TO THE GENERA. 



Trichomes solitary or in flocculent, readily dissociating masses. 



Trichomes containing but one cell SPIRUIJNA 



Trichomes containing more than one cell. 



Sheaths of filaments firm, generally projecting beyond 



the trichome LTNGBYA 



Sheaths delicate or lacking, not projecting beyond 



trichome OSCILLATORIA 



Trichomes laterally united to form free-floating bundles which 



do not dissociate readily TEICHODESMIUM 



SPIRULINA Turpin 1827. 



Trichome consisting of a single, elongate, cylindrical cell that is not 

 attenuated but with rounded ends. Filaments without sheaths and 

 twisted to form loose or compact regular spirals. Cell contents homo- 

 geneous or granulose ; light to dark blue-green in color. 



Reproduction by vegetative division of cells at right angles to the 

 long axis. No hormogone formation possible. 



Cells commonly larger than the vegetative cells and with heavy walls that 

 have a conspicuous nodule at each pole. Cell contents a homogeneous, pellucid, 

 Bon-cytoplasmic substance. 



