The Dichotomous Key...

Is the cell sheathed...?

The presence of a sheath is one of the most difficult characteristics to establish. A true sheath is a clear structure (hence, hard to observe) exterior to the cell wall. Sheaths can be seen best in unstained preparations when they are empty of the cells, or when some of the cells are missing. In the latter case the outline of the sheath can be seen continuing along either side of the empty space. Several features of filamentous microorganisms can be confused with sheaths. A yellowish "halo" observed around filaments under phase contrast observation is not a sheath, but an artifact of phase contrast illumination. Short, empty spaces in a trichome or at the trichome apex should be used to indicate the presence of a sheath. The cell wall of some filamentous microorganisms may remain after cell lysis; however, this can be distinguished from a sheath because some evidence of pre - existing cross - walls usually remains. This is commonly observed for Type 021N.
Presence of substantial attached growth generally indicates the presence of a sheath. Staining wet mounts with "Crystal Violet" may aid in sheath detection.