A Key to the More Frequently Occurring Freshwater Algae - Freshwater Algae: Identification, Enumeration and Use as Bioindicators - page 166

47 (46) (a) Trichomes tapering to a fine point .. . 48

(b) Trichomes not tapering to a fine point
but possibly showing some slight narrowing
towards the trichome end .............. 50

48 (47) (a) Tapered trichomes that are solitary or in
smalltufts..................... Calothrix

Individual trichomes are tapered, 5-10 μm
wide at their base, and have a solitary basal
heterocyst. The sheath is firm and often
dark straw coloured. False branching may
rarely occur. Hormogonia may form towards
the end of the filament. Most frequent in
hard water streams attached to a substra-
tum, growing as mats or tufts of trichomes
over the submerged surface. Cyanophyta.
Plate VI.

25 μm

Figure 4.21 Tolypothrix. Detail from a web of fila-
ments showing the false branching in this alga. Cells at
the base of branches (arrows) have a distinctive flattened
appearance.

(b) Tapered trichomes always in a colony,
which may be globular or spherical . . . . . 49

49 (48) (a) Basal heterocysts usually with an akinete
immediately above. Usually planktonic in
spherical colonies of radiating trichomes
............................ Gloeotrichia

NB. In some genera, false branching is rare
but may occur occasionally. This can be true
in Homoeothrix , Calothrix , Gloeotrichia
and Rivularia . In this key, these are sepa-
rated out on the basis that false branching
will not normally be seen.

Colonies of Gloeotrichia are usually
free-floating and spherical. The radiating
sheathed trichomes are embedded in soft
mucilage. They taper towards the end and
have a basal heterocyst. An akinete is fre-
quently present immediately above the hete-
rocyst and is cylindrical in shape. Trichomes
4-10 μm in diameter, colonies up to 5 mm
in diameter. Although usually free-floating,
the colonies may be attached to a substratum
when young before breaking free. Hormogo-
nia may be formed at the trichome ends.
Can occur in both brackish and freshwaters.
Cyanophyta. Plate VII. Fig. 4.22.

(b) False branches arise in pairs
............................. Scytonema

Scytonema has false branches that appear to
ariseinpairs.Thisisdistinctiveforthegenus.
The branches usually occur between hetero-
cysts. Trichomes sheathed. Cells 5-30 μm
wide. Hormogonia may form at the apex
of the trichome. Akinetes may be present.
Occurs in lakes and on damp rocks and soil,
often forming dark tufted mats. Cyanophyta.
Plate VI.

(b) Basal heterocysts present on trichome
but no akinetes present. Usually grows as
attached globular colonies embedded in firm
mucilage...................... Rivularia

46 (44) (a) Heterocysts present (although some-
times infrequent) . ..................... 47

(b) Heterocystsabsent................. 53