A Key to the More Frequently Occurring Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 218

147 (145) (a) Cells triradiate in valve view with a
small chloroplast near to the base of each
arm....................... Centronella

brackish water species whereas Pleu-
rosigma has its markings diagonally and
is predominantly marine. Bacillariophyta.
Plate XXII. Fig. 4.63 ( Pleurosigma illus-
trated).

Centronella has characteristically
triradiate- shaped cells with a small
chloroplast at the base of each arm.
It occasionally occurs in the plankton
of meso- and eutrophic lakes although
it is quite rare. Bacillariophyta. (Not
illustrated.)

(b) Cells narrow and sigmoid, only
slightly tapering towards the ends. Lon-
gitudinal margins of frustule more or less
parallel. One chloroplast or if two one
either side of the centre . ........... 150

(b) Cellsnottriradiate.............. 148 150 (149) (a) Cells long (up to 200 μm) and narrow

(6-9 μm). Single long plate-like chloro-
plast. Around the cell margin may be seen
alineofdots............. Stenopterobia

148 (147) (a) Cells S-shaped (sigmoid) in outline
.................................. 149

(b) Cells not S-shaped in outline.. . . . 151 Stenopterobia occurs in oligotrophic,

often upland, pools but is rarely abun-
dant. Cells are sigmoid in some species
(e.g. S. sigmatella ), linear in others
(e.g. S. delicatissima ). Cells have one
chloroplast which is divided into two
parts. As with other members of the
Surirellales, the raphe is supported on a
raised wing around the margin of the cell.
Bacillariophyta. Plate XXII (linear form
illustrated).

149 (148) (a) Cells sigmoid in outline, wide in
the central region and narrowing towards
the apices. Usually quite large ( 70 μm

long). Two large plate-like chloroplasts,
one either side of the longitudinal axis
........................... Gyrosigma

>

Gyrosigma cells are sigmoid in shape
(although some species are only slightly
so) with rounded poles and S-shaped
raphe. The striae on the valve sur-
face are both parallel to the raphe and
transverse. They are generally fine but
may be coarser in some species. Central
areas round or cylindrical. Two plate-like
chloroplasts are present lying either side
of the girdle. Pyrenoids may be present.
Gyrosigma acuminatum and G. attenu-
atum are widespread species. Found in
flowing or standing waters on sediments
and rocks (epipelic) usually in waters with
high electrolyte content.

(b) Cells long but wider ( 10 μm

wide). Two plate-like chloroplasts, one
above and one below the central area.
................... Nitzschia sigmoidea

>

As with all members of the genus
Nitzschia , N. sigmoidea has its raphe
raised on a keel along one margin. The
keel supports show up along that mar-
gin as a series of dots (carinal dots).
Nitzschia sigmoidea cells are large, 90-
over 200 μm long, 8-14 μm wide. Two
plate-like chloroplasts are present, one
above and one below the central area.
Common and widespread in meso- to
eutrophicwaters.Abenthicspecies.Bacil-
lariophyta. Plate XXII.

Pleurosigma and Gyrosigma are very
similar but Gyrosigma has markings in
lines from the apical and transapical
directions and is generally a fresh to