A Key to the More Frequently Occurring Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 187

50 μm

Figure 4.43 Synedra . Pennate diatom. Elongate
fusiform cell of a common planktonic species.

quarter the length of normal cells) and these
do not have such obvious swellings at the
poles which can give rise to confusion.

20 μm

(b) Frustules without internal septa but with
thick internal costae. Valve ends may be
swollen but no median swelling present.
Cells almost elliptical (albeit elongated in
some species) in valve view. Bacillariophyta.
................................ Diatoma

Figure 4.44 Actinastrum. Single colony, with cells
radiating out from a central point.

(b) Cells not within a definite mucilaginous
envelope............................. 91

(see also key No. 29)

83 (82) (a) Cells crescent-shaped or fusiform . . . 84

81 (76) (a) Cells elongate, cigar-shaped, without sil-
ica walls, radially arranged in a star-shaped
colony with the cells attached to each other
atoneendonly.............. Actinastrum

(b) Cells spherical, ovoid or other shape
..................................... 85

84 (83) (a) Cells crescent-shaped (lunate), irreg-
ularly arranged in small mucilaginous
colonies.................... Kirchneriella

Cells of Actinastrum join together to form
4-8-16 celled star-shaped colonies. Each
cell (10-25 μm long, 3-6 μm wide) has a
single chloroplast with pyrenoid. Common
in the plankton of lakes and may occur in
rivers,pondsandboggywaters.Chlorophyta.
Plate XI. Fig. 4.44.

Kirchneriella cells have a single parietal
chloroplast with a pyrenoid. Lunate or
crescent-shaped cells are often so curved that
their ends almost touch, although some may
be sigmoid or slightly spiral. Cells (3-8 μm
wide, 10-14 μm long) are often enclosed in
mucilage and irregularly arranged in groups
of 2, 4, 8 (up to 32). Free-floating and
widely distributed in the plankton. Chloro-
phyta. Plate XI.

(b) Cellsnotelongatelikeacigar,oftenmore
spherical, cubical, crescent-shaped or at the
most like a short cylinder. Can form plate-
like colonies. Stellate colonies not formed
..................................... 82

82 (81) (a) Cells of colonies arranged within a defi-
nite mucilaginous envelope. ............ 83

(b) Cells fusiform or wedge-shaped ovals
........................... Elakatothrix