A Key to the More Frequently Occurring Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 231

(b) Colonies star-shaped but valves not
as above. Valve apices wider and more
rounded, slightly heteropolar with the
inner pole a little larger than the outer
one. Non-septate and cells hardly or not
swollen in the centre. Several plate-like
chloroplasts............... Asterionella

Cells contain many small and plate-like
or discoid chloroplasts, and are usually
closely attached to one another to form a
chain or ribbon. Found in lakes and ponds,
frequently in the littoral zone. Bacillario-
phyta Plate XXV.

(b) Cells 12-85 μm long, 4-9 μm wide.
Cell apices subcapitate . Meridion anceps (see also key No. 78)

Widespread and common in the plankton
of meso- and eutrophic lakes. Bacillario-
phyta.

Meridion anceps cells are more rectan-
gular than those of M. circulare (which
are cuneate) and are linear to long ellipti-
cal in girdle view. They also do not tend to
form fan-shaped colonies. Cell apices cap-
itate to subcapitate. Found in more upland
waters low in nutrients. Bacillariophyta.
(Not illustrated.)

182 (177) (a) Valves with costae ............. 183

(b) Valves without costae .......... 186

183 (182) (a) Valves with costae and septa. Cells
square to rectangular in girdle view
........................... Tetracyclus

186 (182) (a) Cells elongate, > 30 μmlong.... 187

(b) Cellsshorterandwider......... 189

(see also key No. 180)

187 (186) (a) Cells narrow but slightly swollen in
centre. Two chloroplasts per cell .... 188

(b) Cells without septa. . . .......... 184

184 (183) (a) Cells square to rectangular in girdle
view, often linked into loose filaments by
means of mucilage pads. ...............
Diatoma mesodon

(b) Cells wider, rectangular in girdle
view, several disc-shaped chloroplasts
...................... Fragilariaforma

Cells of Fragilariaforma are closely
linkedalongtheirlongaxestoformchains,
ribbons or sometimes zig-zag colonies.
Cells more linear in valve view with
subcapitate or rostrate apices, and con-
tain many small discoid chloroplasts.
No transverse costae present. Found in
oligotrophic waters, especially streams,
lake margins and bogs. Bacillariophyta.
Plate XXV.

Diatoma mesodon cells (10-40 μm long,
6-14 μm wide) are rectangular in gir-
dle view, being wider than the cells are
long. In valve view, the cells are slightly
elliptical or lanceolate. Costae are present
within the valve. The chloroplasts are
small and disc-shaped or plate-like. It
occurs in upland flowing waters. Bacillar-
iophyta. (Not illustrated.)

(b) Cells not square in shape but more
elongate. Costae extend near to apices
.................................. 185

188 (187) (a) Cells with slightly swollen cen-
tral region where adjacent cells touch
................. Fragilaria crotonensis

185 (184) (a) Cells linear to lanceolate with bluntly
roundedapices(30-100μmlong,7-13μm
wide)............... Diatoma hyemalis

(see also key No. 28)