Environmental Engineering Reference
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obtained, however, by sharply focussing on
the cell wall. If a regular pattern of markings
can be made out (e.g. Fig. 1.13), then it is
quite probable that the wall is made of silica.
If an empty cell can be found then it may be
easier to see any markings. With some pen-
nate diatoms, a distinctive gliding movement
may be observed in live (fresh) material
10 μm
(b) Cell wall not made of silica.........
30
25
(24) (a) Cells embedded in a gelatinous tube but
separate from each other within the tube . . .
Frustulia
in part see 203
Cymbella
and
Encyonema
in part see 164
(b) Cellsnotasabove.................
26
26
(25) (a) Cells joined together to form a contin-
uous filament not surrounded by extensive
mucilage. Filaments and cells circular in
crosssection..........................
27
(b) Cells forming a ribbon or chain. Cells
elongate and not circular in transverse sec-
tion..................................
28
15 μm
27
(26) The former genus
Melosira
consisting of fil-
aments of centric cells has now been sub-
divided into
Melosira
and
Aulacoseira
(see
(a) and (b) below)
Figure 4.7
Melosira
. Top: Single cell showing numer-
ousyellow-browndiscoidchloroplasts.Bottom:Straight,
unbranched filaments typical of this colonial diatom.
Inset: Characteristic paired arrangement of cells along
filament.
(a) Cell walls with no obvious markings.
Cells linked in pairs (may be difficult to see
without cleaning) ...............
Melosira
smaller, eutrophic waters.
Melosira
dickiei
(10-20 μm in diameter, 7-
10 μm deep) is found on damp rocks
and river banks and
M. nummuloides
(9-42 μm in diameter, 10-14 μm deep) is
found in brackish or marine environments,
often attached to the substratum.
Melosira
varians
can grow in water treatment filters
open to sunlight and cause clogging prob-
lems. Bacillariophyta. Plate IV. Fig. 4.7.
Cells of
Melosira
are rectangular (
M. vari-
ans
)orovoid(
M. nummuloides
) in shape
with relatively thin walls or rectangular
with quite thick walls (
M. dickiei
). Chloro-
plasts are small discs or plates and may
be golden-brown to dark-brown.
Melosira
varians
(cells 8-35 μm in diameter,
4-14 μm deep) is common and some-
times abundant in shallow, frequently
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