- Phormidium usually forms
flat, slimy mats of tangled filaments, and is similar in morphology
to Lyngbya and Oscillatoria. However,
Phormidium mats do not dissociate as easily as those of
Oscillatoria, and the sheaths on Phormidium filaments
are looser than the rigid sheaths of Lyngbya. The mats are
usually attached to benthic substrates, and can detach and float
to the surface. Occasionally, the filaments may be solitary or
arranged in tufts.
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- The
filaments are long, cylindrical, and may be curved or spiralled.
Thin, firm, colorless sheaths adhere closely to the trichomes.
The filaments move by gliding, creeping, rotating, or oscillating
both inside and outside of the sheaths. The cells are rectangular
and have unconstricted or slightly constricted cross walls. The
apical cells may have calyptra, and are more pointed, narrow,
or rounded than the other cells. Usually, the thylakoids are
arranged radially within the cell, creating a net-like or banded
appearance. Gas vesicles are not present.
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