A Key to the More Frequently Occurring Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 245

25 μm

25 μm

Figure 4.77 Cosmarium. Thetwosemicellsarejoined
by a narrow isthmus (arrow) at the cell equator.

Figure 4.76 Staurastrum . Single cell, composed of
two prominent semicells.

Tetraedron has angular cells which may
or may not have spines at the apices.
Typically a single chloroplast (sometimes
more) with one to many pyrenoids. Com-
mon but not abundant in the plankton.
Chlorophyta. Plate XXVIII.

walls may be ornamented with granules
either scattered over the surface or in reg-
ular rows. Pyrenoids are present in the
chloroplasts. Over 1000 species with sizes
ranging from

10 to 200 μm. Common
in upland areas (e.g. acid, low-nutrient
bogs) although some species do occur
in more alkaline or nutrient-rich sites.
Chlorophyta. Fig. 4.77.

<

(b) Cells elongate, crescent or cigar-
shaped .. . . ........................ 219

219 (218) (a) Cells with a single chloroplast almost
filling the whole cell, with or without a
pyrenoid. Cells needle-like or fusiform in
shape . ............................ 220

217 (212) (a) Cells usually in groups of 2-4-
8 or more, united along some, if not
all, of the lateral walls. Spines some-
times present especially on the end cells
......................... Scenedesmus

(b) Cells with two axial chloroplasts, one
in each half of the cell and each one having
several pyrenoids in a row along its length
.................................. 223

(see also key No. 101)

(b) Cellsnotunitedasabove....... 218

220 (219) (a) Cells lunate to arcuate often in aggre-
gates of 4-8-16.. .......... Selenastrum

218 (217) (a) Cells polygonal or tetragonal,
sometimes with spines at the angles
........................... Tetraedron

Selenastrum cells are strongly curved
and frequently occur in aggregations
but without a mucilaginous surround

(see also key No. 211)