Environmental Engineering Reference
In-Depth Information
(b) Cells rectangular, touching along
entire length . ...... Fragilaria capucina
a range of waters. Bacillariophyta. (Not
illustrated.)
(see also key No. 28)
191 (176) (a) Cells elongate and narrow, no true
raphe present on either valve. Two elon-
gate chloroplasts. Isopolar with apices
gently narrowing. May be attached to a
surface by a mucilaginous pad or free-
floating Synedra
189 (186) (a) Cells rectangular in shape with bluntly
rounded corners and with a single chloro-
plast. Valve wall thickened towards the
centre . ..................... Diadesmis
Diadesmis has small valves (4-45 μm
long, 2-10 μm wide) with a hyaline area
along the edge of the valve and a clear
area along the axis either side of the raphe
- which can be difficult to see using a
light microscope. Cells can join together
at their valve faces to form small rib-
bons or chains. Widespread in distribu-
tion on damp rocks, soils, mosses and in
ponds and springs. Bacillariophyta. (Not
illustrated.)
Cells of Synedra are elongate - needled-
shaped or fusiform, sometimes with capi-
tate ends. No raphe is present and the axial
clear area is narrow. The central clear area
can be from margin to margin. Fine par-
allel striae are present either side of the
axial area. Cell length from
<
25 μm( S.
parasitica) to
500 μm( S. ulna ), with
cell width ranging from 3 to 10μm. Two
long chloroplasts are present although
only one is usually visible in valve view.
Some species are epiphytic on other algae
(forming colonies with cells radiating out
from a single attachment point) while
other species are free-floating. Generally
widespread and sometimes abundant in
the plankton of lakes and slow flowing
rivers. Can produce odours in drinking
water and block treatment filters when in
large numbers (Palmer, 1962). Bacillario-
phyta. Plate XXVI. Figs. 2.23 and 4.43.
>
(b) Cellswithtwochloroplasts..... 190
190 (189) (a) Cells short, round to elliptical, with
blunt apices, 3-35 μm long and 2-12 μm
wide....................... Staurosira
Cells can either be attached to a sub-
stratum (e.g. sand grains) or be free-
living. Cells contain plate-like chloro-
plasts and can form filaments which may
be zig-zag in shape. Found in shallow
waters and rivers. Bacillariophyta. (Not
illustrated.)
(b) Cells not attached, wider in shape.
True raphe present . ................ 192
192 (191) (a) Two chloroplasts per cell, one either
side of central axis. A series of dots (cari-
nal dots) are visible along one margin
............................ Nitzschia
(b) Cells larger, lanceolate to rhom-
boid, with narrowly rounded apices
..................... Pseudostaurosira
Nitzschia is a large genus whose cells
may be elliptical, narrow linear, spindle-
shaped or sigmoid in valve view. In some
species, the valve centre may be slightly
constricted. The raphe is displaced to one
margin, with the raphe on each valve
diagonally opposite the other. The raphe
Cells elliptical, lanceolate or more rhom-
boid in valve view. Apices narrowly
rounded to almost rostrate. Valve margin
may be slightly undulate. Cells (11-30 μm
long, 2-12 μm wide) contain plate-like
chloroplasts and are usually linked to form
filaments. Found in the epipsammon in
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