HOW TO KNOW THE FRESH-WATER ALGAE 



The phyla of freshwater algae herein recognized are as follows: 



1 Chlorophyta (Green Algae). 



Plants unicellular, colonial, or filamentous; swimming, floating, or 

 attached and stationary; cells containing plastids in which chlorophyll 

 (grass-green) is predominant, and in which there is usually a shiny, 

 starch-storing body, the pyrenoid; pigments are chlorophyll, xantho- 

 phyll, carotene; starch test with iodine positive (in almost every in- 

 stance); nucleus definite (although often small and inconspicuous); cell 

 wall usually relatively thick and definite, composed of cellulose and 

 pectose; swimming cells or motile reproductive elements furnished 

 with 2-4 flagella of equal length attached at the anterior end; sexual 

 reproduction by iso-, aniso- and by heterogametes. 



2 Cyanophyta (Blue-Green Algae). 



Plants unicellular, colonial, or in simple or branched (sometimes 

 falsely branched) filaments; without chloroplasts but with pigments in 

 solution and coloring the entire protoplast; variously colored with a 

 combination of chlorophyll, xanthophyll, carotene, phycocyanin, and 

 phycoerythrin; cell wall thin, a membrane which usually has a gela- 

 tinous outer sheath; contents often with false (pseudo-) vacuoles which 

 refract light and obscure the true color of the cells; definite nucleus 

 lacking but occurring as a cluster of granules in the mid-region (central 

 body) of the cell; motile cells and sexual reproduction wanting; asexual 

 reproduction by cell division (fission) or rarely by spores (akinetes); 

 food storage questionably glycogen, possibly floridean starch; iodine 

 test for starch negative. 



3 Chrysophyta (Yellow-Green, or Yellow-Brown Algae). 



Plants unicellular or colonial, rarely filamentous; pigments contained 

 in chromatophores in which yellow or brown often predominates, 

 chlorophyll, carotene and xanthophyll also present (some chromato- 

 phores appearing pale green or yellow-green); food storage in the 

 form of oil or leucosin, the latter often giving the cell a metallic lustre; 

 starch test with iodine negative; wall relatively thick and definite, 

 pectic in composition, often impregnated with silicon (especially in 

 the diatoms), and sometimes built in 2 sections which overlap in the 

 mid-region; motile cells and swimming reproductive cells furnished 

 with 2 flagella of unequal length, or with but a single flagellum; rhi- 

 zopodial (pseudopodial or amoeboid) extensions of the cell not uncom- 

 mon in some families. 



4 Euglenophyta. (Euglenoids). 



Cells solitary, swimming by one (usually) or by 2 (rarely 3) flagella; 

 a gullet present in the anterior end of the cell in many members, as is 

 also a red pigment (eye) spot; chloroplasts few to many variously 

 shaped green bodies (a few relatives colorless); a chlorophyll-like 

 pigment predominating, but with carotene also present; nucleus large 



