BLUE-GREEN ALGAE 105 



7 (4) Cells with thin cell walls Synechococcus Nageli. 



Cells comparatively large, cylindrical or ellipsoidal, living 

 usually singly or sometimes forming small families of two 

 to four united in a row or chain. Cell-contents blue-green, 

 sometimes yellowish, pinkish, or pale orange. Free-floating 

 in ponds and pools, or on rocks. 



FIG. 34. Synechococcus aeruginosus Nageli. X 575. (After 

 Kirchner.) 



8(3,11) Cell-division in two planes 9 



9 (10) Cells spherical or oblong, forming flat, plate-like colonies. 



Merismopedia Meyen. 



Cells usually adhering in groups of four, and arranged in reg- 

 ular rows to form a flat, rectangular, plate-like colony. In 

 plankton, in ponds, and lakes. 



FIG. 35. Merismopedia elegans A. Braun. X 450. (After West.) 



10 (9) Cells flat, quadrangular in outline, solitary, or forming small 



colonies. Tetrapedia Reinsch. 



Cells with thin membrane; solitary or united into flat colonies of 2 to 1 6 

 cells. 



11(3,8) Cell-division in three planes 12 



12 (23) Cells united into definite, often comparatively large colonies. 13 



13 (16) Colonies more or less regularly spherical 14 



14 (15) Colonies hollow; cells closely and regularly arranged at the surface. 



Coelosphaerium Nageli. 



Cells globose or oblong, forming on the surface of lakes and 

 ponds microscopically small, hollow, spherical colonies em- 

 bedded in a mass of mucus; reproduction by means of single 

 cells escaping from the colony as well as through the con- 

 striction of old colonies to form new ones. Common in fresh- 

 water plankton. 



FIG. 36. Coelosphaerium kiitzingianum Nageli. X 465. (Original.) 



15 (14) Colonies solid; cells sparsely scattered through the jelly, pyriform 



in shape Gomphosphaeria Kiitzing. 



Cells enclosed by a colorless gelatinous matrix to form micro- 

 scopically small, solid, globular, or ellipsoid colonies; the peripheral 

 cells grouped in pairs and egg-shaped or pyriform, or (during 

 division) heart-shaped. In ponds, stagnant water, etc. 



FIG. 37. Gomphosphaeria aponina Kiitzing. X 465. (Original.) 



16(13) Colonies, when old, generally not spherical 17 



17 (18, 19) Colonies microscopically small, solid, globular, or clustered. 



Microcystis Kiitzing. 



(Probably should be united with Clathrocystis.) Cells spherical, or through 

 pressure somewhat angular; uniting in great numbers to form microscopic- 

 ally small solid colonies. Common in ponds and ditches. 



