A Key to the More Frequently Occurring Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 243

(b) Cells not as above but a shorter, more
rounded cylinder. . ................. 214

groove cannot be seen) rather than from
the side (medial groove clearly visible),
the cells are usually triangular but more
arms may be present. Each semicell has a
lobed chloroplast with a central pyrenoid.
Cells 10-140 μm long. Staurastrum is one
of the more common desmids in the plank-
ton, also occurring on sediments. Present
in nutrient-poor (most species) and mod-
erately nutrient-rich lakes. Chlorophyta.
Plate XXIX. Fig. 4.76.

214 (213) (a) Cells large, 150-240 μm long, cylin-
drical to fusiform in shape with a distinc-
tive notch at each apex . .... Tetmemorus

Cells are solitary rounded cylinders, occa-
sionally with a slight median constriction,
and have apices with a characteristic deep
incision. One chloroplast in each semicell
with several pyrenoids. The cell wall may
be covered with fine granules. Common in
ponds and acid pools and amongst Sphag-
num . Chlorophyta. Plate XXVIII.

(b) Median groove more narrow, or if
wide then cell walls smooth and with a
singlespineatcellapices........... 216

(b) Cells are a short more rounded cylin-
der and only 22-100 μm long, with-
out a notch at each end of the cell
........................ Actinotaenium

216 (215) (a) Median groove marked, acute to
obtuse in angle. Semicells approximately
triangular, each apex with a single spine
........................ Staurodesmus

Actinotaenium cells are relatively short
with only a shallow, wide, constriction
in the centre. Cell wall very finely punc-
tate. Chloroplast stellate with a pyrenoid.
Frequent in more acidic ponds amongst
other algae and mosses. Chlorophyta.
Plate XXVIII.

Staurodesmus cells are usually triradiate
with a spine at the end of each angle.
Cells (up to 60 μm long) are divided
into two semicells with a wide groove or
isthmus in the centre. A common genus
in the plankton of acid, nutrient-poor
waters, although sometimes reported from
nutrient-rich waters, and amongst aquatic
macrophytes Chlorophyta. Plate XXIX.
Xanthidium is similar but with several
spines at all apices and sides. Plate XXIX.

215 (210) (a) Median groove wide, each semi-
cell with horn-like extensions or at
least a polygonal shape. Cell walls may
have granulations and some planktonic
species may be surrounded with mucilage
.......................... Staurastrum

(b) Median groove very narrow, over-
all cell shape ovoid to rounded. Some-
times with slightly flattened sides or
ends. No spines or extended processes
present but cell wall may have markings.
.......................... Cosmarium

Staurastrum is a very large genus of
desmids. Cells are divided into two semi-
cells with a wide groove or isthmus
between them. The outer angles of each
semicell is extended into long horns or
processes (up to 12 in number) which
may at times be short and stumpy (giv-
ing the cell a polygonal appearance) and
may end in short spines. The cell wall may
be ornamented. In apical view (i.e. look-
ing down on the cell so that the median

This is the largest desmid genus and
is very widespread. The cells are nor-
mally rounded (overall shape), divided
into semicells with a narrow groove or
isthmus in the centre between the two
halves.Thesemicellsinsomespecieshave
anangular,polygonalappearance.Thecell