| Specialized structures |
The true branches of this
organism are uniseriate and composed of cells that are
generally longer than broad, particularly those distal
from the base. The axis (primary trichome) from which
they arise is mainly uniseriate as well but may become
multiseriate in part, with divisions in more than one
plane. The axis in these regions, however, is seldom more
than 2-3 cells in thickness. In addition, the cells of
the axes become enlarged, often semispherical in shape.
The older cells of a main axis may become separated from
each other by sheath material and may act as akinetes
(Martin and Wyatt 1974). Most of the widened cells of the
axial trichome, however, possess a true filamentous
nature with only a peptidoglycan septum separating cells
(Nierzwicki et al. 1982b; Balkwill et al. 1984). The main
axis forms when a hormogonium comes to rest, cells
enlarge, and some cell divisions begin that are parallel
to the long axis or diagonal (oblique); some of the
resulting cells elongate to form branches (secondary
trichomes) (Balkwill et al. 1984; Nierzwicki-Bauer et al.
1984) (These narrow secondary trichomes (which may taper)
become progressively longer with cell elongation and
transverse divisions. Fischerella
may eventually differentiate series of spherical,
thick-walled cells that are akinetelike. Typical
cyanobacterial akinete types are not easily recognizable.
In some cases, the widened cells may divide in a second
plane and form new branches (i.e. secondary trichomes).
The hormogonium is a gliding, rotating trichome composed
of few (about 11-16), narrow, morphologically uniform
cells that are cylindrical or slightly barrel-shaped
(Hernndez-Muniz and Stevens (1987) They are formed as the
distal portion of branches. No heterocysts differentiate
until after hormogonia have ceased motility. Heterocysts
of Fischerella (Mastigocladus laminosus) are elongate,
spherical or even compressed (shorter than broad) and are
lateral, terminal or intercalary. Heterocysts of
thermophilic strains are, at least, different from
typical heterocysts of the Nostocales, in that they
possess only one additional wall layer (homogeneous type)
and have densely stacked lamellar membranes
(Nierzwicki-Bauer et al. 1984).
In field populations of Fischerella in
flowing hot springs, almost the entire mass is composed
of tufts or streamers of secondary trichomes several
centimeters in length, and no branching is seen except in
the prosterate attached mass or primary trichomes (main
axes).
Besides the true branching habit, the
great diversity of form as described by Frmy (1936),
Schwabe (1960), Rippka et al. (1979) and Balkwill et al.
(1984) for Fischerella (Mastigocladus laminosus) also
applies to these nonthermophilic representatives. In the
nonthermophilic strains, hormogonia were not always
formed under the expected conditions, and sometimes the
multi-seriate condition of the axis was rare or lacking.
Thurston and Ingram (1971) have also studied the
morphology and fine structure of another nonthermophilic
species of Fischerella in which they show details of
synaptic connections, with microplasmodesmata joining
adjacent cells in the branch trichomes
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