44 Fresh-water Biology 



{Continued from page 42) 



16 — Only a single filament with one sheath 17 



— Usually several filaments in a more or less thick sheath. Several genera. 



17 — Sheath thick, slimy filaments often bent, agglutinated (fig. 1) Phormidium 



— Sheath firm, not slimy. Filaments not in bundles, Lyngbya, Hypheothrix; filaments 

 in crest or prostrate bundles Symploca 



18 — Filaments contorted, within a definite gelatinous tegument (fig. 11) Nostoc 



— Filament more or less straight, free or in a formless slimy mass, without tegument. . 19 



19 — Heterocysts terminal ; spores long and cylindrical Cylindrospermum 



— Heterocysts not terminal. Filaments aggregated without order (fig. 4), Anabaena; 

 filaments in bundles of plate like masses (fig. 3) Amphanizomenon 



20 — Organism green, not yellowish green; if reddish then unicellular 21 



— Organism yellowish green, red or brown 53 



21 — Organism with whorls of leaves. Ch.ajiaceae Chara, Nitella 



— Organism smaller, without whorls (see figs.) Chlorophyceae — 22 



22 — Thallus tubular, multinuclear, cell division not apparent; usually branched. Sipho- 



nales Vaucheria 



— Thallus filamentous and septate, or unicellular, or expanded 23 



23 — Thallus expanded, membranous Ulvales 



— Thallus neither expanded nor membranous 24 



24 — Thallus not filamentous; no conjugation Protococcales — 25 



— Thallus filamentous, though filaments may unite in a plane; if unicellular then con- 

 jugation takes place 42 



25 — Unicellular or of a definite number of ciliated motile cells. — Volvocaceae. (see 



figs.) 26 



— Cells not ciliated or motile, (see figs.) 30 



26 — Composed of colonies of many cells; cells with 2 cilia 27 



— Composed of single cells with 2 or rarely 4 cilia. Contents of cell close to cell wall 

 (fig. 4), Chlamydomonas ; contents of cell connected to cell wall by threads (fig. 2) 

 Sphaerella 



27 — Colonies spherical or circular 28 



— Colonies flat, cells 4-16, angles rounded in a colorless sheath (Compare also Ulvella 

 fig. 14). (Fig. 10) Gonium 



28 — No gelatinous cover. Many cells in a hollow globe (fig. 13), Volox; cells 16, arranged 



in 4 rows Spondylomorum 



— With a gelatinous cover 29 



29 — Colonies ovate or spherical. Cells 16-32, globose, not crowded (figs. 7, 8), Eudorina; 



cells, 8, 16, 32, or 64, globose, crowded (figs. 13, 16) Pandorina 



— Colony of 8 cells in an equatorial zone in a spherical or ellipsoidal investment 



Stephanosphaera 



30 — Cells formed in plates or network. Hydrodictyaceae, cells in a flat plate (fig. 1, 2), 



Pediastrum; cells form a net work (fig. 25) Hydrodictyon 



— Cells not in a plate or network 31 



31 — Unicellular and solitary; cell with differentiation of base and apex (fig. 5). Char- 



aciaceae Characium 



— Cells without differentiation of base and apex 32 



32 — Unicellular and globular or consisting of short, few celled filaments; firm cell wall. 



Often in damp situations (figs. 22, 23). Pleurococcaceae Pleurococcus 



— Not as above 33 



33 — Cells spherical and indefinite in number, embedded in a copious gelatinous envelope 



(fig. 21). Paliiellaceae Tetraspora, et. al. 



— Colonies free or colonial without copious gelatinous envelope; forming autospores 

 Protococcaceae — 34 



