2a) Plant cells without recognizable nucleus or plastids. Coloring matter (usually blue-green) diffused throughout the cell. Figs. 1 to 9. (The Blue-Green Algae)

   2b) Not as in 2a. Plant cells with distinct plastids and of various colors and shapes

   3a) Unicellular or colonial in habit; never forming filaments. Reproduction vegetative only. Figs. 1 and 2

   3b) Cells forming definite filaments, but no endospores. Figs.5 to 9

   3c) Cells usually growing on other plants; solitary, colonial or forming small filaments. Forms endospores. Figs. 3 and 4 . 5

   4a) Cells always in colonies which grow prostrate upon rocks or other bodies and which have erect column like outgrowths. Largely marine.
Family ENTOPHYSALIDACEAE

   Fig. 1. Entophysalls magnollae Farl.
   The family is largely marine though some species grow in fresh water. Rocks between the tide lines form the favorite marine habitat.

   4b) Cells solitary or in colonies but never with column like outgrowths.
Family CHROOCOCCACEAE

   Fig. 2. a) Chroococcus turgidus Nag.; b) Merlsmopedla punctata Mey.; c) Synechococeus aeruginosus Nag.; d) Gleocapsa sp.
   Many known species of diverse form make this a large family. The numerous tiny free floating species make an important group in water contamination.

   5a) Thaltus multicellular as a result of cell division. In spore formation an entire cell is completely divided into endospores.
FamiIy PLEUROCAPSACEAE

   Fig. 3. Hyella fontana H.& J. a, vege­tative thallus; b, cells forming endospores.
   There are a few fresh water forms but most of them are marine. For the most part they grow attached to other plants.

   5b) Cells solitary although often many growing close together. Spores cut off at end of cell one at a time.
Family CHAMAESIPHONACEAE

   Fig. 4. Chamaesiphon lncrustans Grun.
   Rather common as an epiphyte on larger freshwater algae. Most of the family are marine species.

   6a) Thallus a filament of uniform diameter except possibly the terminal cells may be smaller. There are no heterocysts (empty cells). Several filaments often bound together with a sheath.
FamiIy OSCILLATOR IACEAE

   Fig. 5. a) Osclllatorla sp; b) Splrullna major Kutz.; c) Lyngbya contorta Lemm.
   This plant Is a classic among the "blue-greens". Its "oscillations" consist of moving the tip of its filament In a circle. Many folks do but little better.

   6b) Heterocysts (empty cells) present in the thallus. Filaments often tapering whip like though sometimes of uniform thickness. Figs. 6 to 9

   7a) Filaments branched. Figs. 8 and 9

   7b) Filaments unbranched. Figs. 6 and 7

   8a) Filaments nearly the same diameter throughout: covered with a gelatinous sheath.
Family NOSTOCACEAE