HOW TO KNOW THE FRESH-WATER ALGAE 



66a Cells with distinct sheaths; cells inclosed in layered colonial mu- 

 cilage. See Fig. 41 GLOEOCYST1S 



66b Cells without distinct sheaths; colonial mucilage not layered. .67 



Figure 54 



67a Chloroplast cup-shaped with a conspicuous pyrenoid (a doughnut- 

 shaped, starch-storing body, usually shiny); colony often includ- 

 ing clusters of smaller daughter cells. Fig. 54. . SPHAEROCYSTIS 



Fig. 54. Sphaerocystis Schroeteri Chod. 



This plant, of which there is only 1 

 species in North American records, 

 should be compared with Planktosphae- 

 ria (Fig. 55). There is 1, cup-shaped 

 choroplast, and the colony almost in- 

 variably shows clusters of small cells 

 formed by the division of the parent 

 cell into 4 or 8 daughter-cells. There 

 is a gelatinous sheath but the individual cells do not show the lamel- 

 lated envelope of Gloeocystis, a genus which may be confused with 

 Sphaerocystis. Young cells of Planktosphaeria may have a cup-shaped 

 chloroplast. 



67b Chloroplasts several polygonal plates, each with a pyrenoid (chlo- 

 roplast single when cells are young); colony not containing clusters 

 of daughter-cells. Fig. 55 PLANKTOSPHAERIA 



Fig. 55. Planktosphaeria gelatinosa G. M. Smith. 



This plant, only one species known, occurs most- 

 ly in the tychoplankton (shallow water plankton 

 among other algae) but may appear also in the 

 open water plankton (euplankton). Unless care is 

 used it is easy to confuse this plant with Gloeo- 

 cystis (Fig. 41) or Sphaerocystis (Fig. 54), or other 

 small, green spherical cells. When mature the 

 cells are recognizable by their angular, 5-sided 

 chloroplasts, each of which contains a pyrenoid. 

 The colonial sheath is often very thin and diffi- 

 cult of determination. 



Figure 55 



68a (65) Cells lunate or sickle-shaped, tapering and pointed at the 

 apices 69 



68b Cells bean-shaped, oval or spindle-shaped, the apices sometimes 

 tapered but rounded or blunt ' 



48 



