108. Cells inclosed by a gelatinous sheath, with a parietal chloroplast 

 that nearly covers the wall ( plant usually colonial but sometimes 

 occurring as a single cell) — Gloeocystis (in part) 



108. Cells not inclosed by a gelatinous sheath 109 



109. Aerial or terrestrial 110 



109. Aquatic; chloroplast a thin plate, usually without a pyrenoid; 

 reproducing by autospores - Chlorella 



110. Cells with a dense, plate-like and lobed chloroplast (individuals 

 usually gregarious and cliunped ) ; reproducing by 



cell division Protococcus ( in part ) 



( Pleurococcus ) 



110. Cells viath a parietal, cup-like chloroplast; reproducing by 

 swarmers Chlorococcum ( in part ) 



111. Cells pyramidal, triangular, or polygonal 112 



111. Cells ovate, fusiform, or oblong 113 



112. Cells triangular, tetragonal, or pyramidal, with a definite body 

 which may have the angles produced, or 



sharply rounded Tetraedron ( in part ) 



112. Cells triangular or pyramidal, the entire cell composed of radiat- 

 ing processes, the body of the cell not distinct from 



the processes Cerasterias 



113. Cells ovate or moniliform 114 



113. Cells oblong, fusiform, or irregularly globose - — - — 115 



114. Cells with thick, stratified walls, usually showing dark masses of 

 mucilage impregnated with calcium carbonate on either side or 



at the poles; plants usually colonial Gloeotaenium (in part) 



114. Cells broadly ovate or moniliform, without masses of semi- 

 opaque mucilage; rarely solitary, usually arranged in famihes 



of from 2 to 6 within mother cell walls Oocystis ( in part ) 



115. Cells irregularly globose or oblong, with knob-like thickenings on 

 the walls; chloroplast a large massive axial body, with irregu- 

 lar plates at the periphery Kentrosphaera 



115. Cells elliptical, fusiform, or oblong, symmetrical, with a single, 

 parietal plate-like chloroplast; cells usually arranged side by side 



in groups of 4, sometimes solitary Scenedesmus (in part) 



116. Plant an unbranched filament, attached or unattached (some- 

 times short rhizoidal branches may form at points where a 

 filament comes in contact with a substrate ) ; usually not showing 

 basal-distal differentiation 117 



116. Plant a branched filament, composed of adjoined cells or coeno- 

 cytic units, or a cOenocytic filament; usually showing basal- 

 distal differentiation; filaments sometimes forming cushion-like 



thalli, with the branching indistinct - -142 



117. Chloroplast parietal, net-like, ring-like, discoid, or plate-like 



and padded - - - 1^4 



117. Chloroplast axial or, if parietal, in the form of ribbons; usually 



broad, longitudinal, axial bands, or axial star-shaped masses 118 



118. Chloroplast stellate, axial, with radiating processes from a 

 central mass in which there is a large pyrenoid; 2 chloroplasts 

 in each cell, sometimes so dense and with so many starch 



grains present that their shape is discerned with difficulty 119 



118. Chloroplasts not stellate as above 121 



[580] 



