Appendix D. Key to the Common Algae of the Yellowstone River* 



la Cells encased in a rigid glass shell having radial or bilateral 



symmetry, and distinct ornamentation . . Bacillariophyceae (Diatoms) 

 lb Cells not as above 2 



2a Cells arranged uniseriately (end-to-end) in branched or unbranched 

 filaments 3 



2b Cells solitary or in colonies with two to hundreds of cells; not 

 end-to-end in filaments 14 



3a Filaments branched 4 



3b Filaments unbranched 5 



4a Filaments macroscopic, conspicuous; cells robust, usually greater 



than 50 microns wide; tips of branches rounded . . Cladophora glomerata 



4b Filaments microscopic, inconspicuous; cells delicate, usually less than 

 10 microns wide; tips of branches usually (but not always) attenuated 

 Stigeoclonium 



5a Pigments localized in chromatophores (chloroplasts); cells grass-green, 



contain starch, i.e., turn blue-black with addition of iodine 



(Chlorophyceae) 6 



5b Pigments not localized in chromatophores; cells blue-green, do not con- 

 tain starch (Myxophyceae) 11 



6a Chloroplast a band, not completely filling the cell; may be axial, 



transverse or twisted in a spiral 7 



6b Chloroplast not a band; completely fills cell in healthy specimens . 10 



7a Chloroplast a transverse band, ring-shaped and sometimes open on one 



side; cells usually wider than long 8 



7b Chloroplast an axial or spial band; cells always longer than broad . . 9 



8a Cell walls greatly thickened; cells wider than long; chloroplast a 

 closed ring Ulothrix zonata 



8b Cell walls thin; cells about as long as broad; chloroplast an open 



ring Ulothrix sp. 



9a Chloroplast an axial band; cells several times longer than broad . . 

 Mougeotia 



9b Chloroplast a spiral band (the common Yellowstone species has a siery 

 tight spiral); cells only about two or three times longer than broad 

 Spirogyra 



*This key includes only those taxa found in more than 3 percent of all 

 plankton and periphyton samples. Prescott (7,8) or Smith (10) may be 

 consulted for descriptions and illustrations. 



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