Branched filaments: |
(without
chloroplasts = branched filamentous cyanobacteria) |
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Click on the thumbnail for a larger image |
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Cells
rounded, arranged in series within a thick sheath: |
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Filament becoming
multiseriate as it increases in size. True branches
arise by lateral cell division: branches of various lengths stick
out at right angles to the main axis. Plant brown; cells olive-green
or brown. Heterocysts may be present.
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Stigonema |
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True branches
stick out at right angles to the main filament: filament usually uniseriate.
Cells blue-green.
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Hapalosiphon
: Fischerella
is rather similar
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Looks similar
to last, but branching is false and cells have chloroplasts with clearly
visible pyrenoids; arranged in series (usually single) in a broad sheath.
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Asterocystis
(Chroodactylon): a Rhodophyte |
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cells
short-cylindric, forming smooth filaments (trichomes), with a sheath |
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Filaments more
or less even thickness throughout:
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(a) false branches arise at a heterocyst:
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(i) each trichome
has its own sheath
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Tolypothrix: a heterocyst lies at the base of each branch
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(ii) several trichomes
share a common sheath
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Coleodesmium: looks like a rather bushy Tolypothrix (not evident from these images):
(cf. Dichothrix and Rivularia, which have tapering filaments)
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(b) false branches arise between heterocysts
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Scytonema:
there is no heterocyst at the point of branching; branching is usually
mid-way between heterocysts. This double form of branching is typical,
but single branching also occurs: |
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Filaments taper
towards tip
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(a)
no basal heterocyst
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Homoeothrix:
compare very carefully with (a) Tolypothrix and Scytonema,
to ensure that the branches do taper as in the preceeding illustration,
and (b) with Calothrix to check there are no basal heterocysts.
The sheath is firm over the basal part of the trichome but becomes thinner
and may splay out towards the tip.
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(b)
basal heterocyst
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(i) filaments
single or in a small group: false branching is infrequent and
may not be seen.
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Calothrix:
sheath is firm, covering the thicker, basal part of the trichome, but
does not usually cover the heterocyst
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(ii) filaments
branching frequently, false branches with a basal heterocyst,
arising in a clump or separately
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Dichothrix:
sheath is firm over the basal part of the trichome but becomes thinner
and may splay out towards the tip
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(iii) Filaments
forming a clump:
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radiating
out from the centre, not branched
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Gloeotrichia:
may form spherical planktonic colonies |
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forming
a clump attached to a surface, radiating or running parallel;
false branching frequent with a heterocyst at the base of the
branch, occasional intercalary heterocysts
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Rivularia:
(no illustration available) |