A Key to the More Frequently Occurring Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 191

92 (91) (a) Cellsofcolonyformalatdisc...... 93

(b) Cellsformingasphericalorovoidcolony
or globular groups of 4-16 cells ........ 96

93 (92) (a) Colonyfree-loating....... Pediastrum

Pediastrum forms characteristic flat circu-
lar, plate-like colonies which are common in
nutrient-rich lakes, ponds and slow flowing
rivers. The cell walls are often quite tough
and persist for some time after the contents
have disappeared. Can, if present in large
numbers, impart unwanted odour to drinking
waters (Palmer, 1962). Chlorophyta. There
are many species, differing in overall colony
appearance and the shape of the marginal
cells - examples shown in Plate XII. Figs.
2.18 and 4.47.

25 μm

(b) Colony attached to a surface. ....... 94

94 (93) (a) At least some of the cells bear setae (see
Glossary)............................. 95

(b) Cells without setae ....... Protoderma

Protoderma forms a flat cushion or disc one
cell thick (sometimes becomes a little thicker
with age) that is composed of irregularly
branched partially radiating filaments. This
is more obvious towards the edge of the disc.
Cells quadrate to cylindrical up to 15 μm
in length. Chloroplasts are parietal with a
pyrenoid. Commonly attached to submerged
aquatics. Chlorophyta. Plate XII.

20 μm

Figure 4.47 Pediastrum . Top: Open colony with fret-
work of cells. Reproduced with permission from G. Ji.
Bottom: Closed plate-like colony, with prominent cell
walls and nuclei (DIC image). Separate species. In both
cases, the colony is one cell thick - with distinctive
peripheral and central cells.

95 (94) (a) Setae with sheaths, cells up to 40 μmin
length. ...................... Coleochaete

(see also key No. 12)

(b) Setae not sheathed, frequent. Cells up to
30 μmlong.................. Chaetopeltis mm in diameter) on submerged aquatic

plants. Chloroplast cup-shaped. Some
cells with pseudocilia. Chlorophyta. (Not
illustrated.)

Colonies composed of radiating rectangu-
lar cells forming a flat disc (up to 1