Nos ' toc . M.L. n. Nostoc origin uncertain, supposedly invented by Paracelsus from the old English word Nosthryl (nostril) and the corresponding German word Nasenloch = Nostoch . Cyanobacteria / Subsection IV / Subsection IV.I / Form‐ Nostoc

Mi.cro.co' le.us . Gr. adj. mikros small (or thin); Gr. n. koleos sheath; M.L. masc. n. Microcoleus small or thin sheath. Cyanobacteria / Subsection III / Incertae Sedis / Form‐ Microcoleus In members assigned to this form‐genus, two to several trichomes oriented parallel , often spirally and tightly interwoven, are enclosed by a common homogeneous sheath (Figure 1). Cells (diameter 3–6 µm) composing the trichomes are longer than wide (or isodiametric); visible constrictions at the cross‐walls (but usually much less than 1/4 of the cell diameter); mature end cells are conical ; the bundles of filaments generally exhibit slow gliding motility, not involving rotation . Based on analysis of a limited number of isolates, a radial arrangement of thylakoids (in cross‐section, T.E.M.) seems to be typical .

Lep.to.lyng' by.a . Gr. adj. leptos delicate or thin; lyngbya named after H.C. Lyngbye, Danish botanist, 1782–1837; Lyngbya another genus of cyanobacteria; M.L. fem. n. Leptolyngbya delicate or thin Lyngbya. Cyanobacteria / Subsection III / Incertae Sedis / Form‐ Leptolyngbya Cylindrical trichomes less than 3 μm in diameter; cells isodiametric or longer than wide; constrictions between cells are generally absent or shallow , seldom exceeding 1/8 of the trichome diameter; trichome breakage may be trans‐ or inter‐cellular; apical cells rounded (or exceptionally conical) (Figure 1); persistent sheaths may be produced but are not of firm consistency (and not laminated); in some members, several trichomes may be surrounded by a “mucous‐like” matrix. Trichomes nonmotile (or inconspicuously motile as seen by light microscopy, <0.1 μm/s), except for transient motile hormogonia when free of any sheath or coalescent “mucous” material . Some strains or natural specimens may undergo false branching, which may depend mainly on sheath strength.

Cha.mae.si' phon . Gr. adv. chamai dwarf; Gr. masc. n. siphon tube; M.L. masc. n. Chamaesiphon microbial tube. Cyanobacteria / Subsection I / Incertae Sedis / Form‐Chamaesiphon Unicellular cyanobacteria that reproduce exclusively by successive unequal binary fission (budding) at the apical pole of ovoid, club‐shaped, or pyriform cells. The small spherical buds are produced in succession at one pole of the mother cell. This mode of reproduction confers an intrinsic heteropolarity on the cell. The mol % G + C of the DNA is : 47.

Ca' lo.thrix . Gr. adj. kalos beautiful; Gr. n. thrix hair; M.L. fem. n. Calothrix beautiful filament. Cyanobacteria / Subsection IV / Subsection IV.II / Form‐Calothrix Mature ensheathed trichomes exhibit a pronounced degree of tapering. The motile hormogonia are generally very short, exhibit a significantly reduced cell diameter, and are sheathless (or less ensheathed). They give rise to trichomes that differentiate a basal, one‐pored heterocyst. False branching frequent. Akinetes lacking in most members. All are freshwater, soil, or hot spring inhabitants.

Bor' zi.a . M.L. fem. n. Borzi named after A. Borzi, Italian cyanobacteriologist. Cyanobacteria / Subsection III / Incertae Sedis / Form‐ Borzia This genus is characterized by very short trichomes of 2–8 (16) cells , with conspicuous constrictions at crosswalls, of various diameters; motility slight or lacking; cells usually shorter than broad . This last characteristic seems to be the main feature separating this genus from Pseudanabaena (as represented in this volume), since many strains of Pseudanabaena consistently exhibit comparably short trichomes. Brevitrichomy may also occur in other filamentous cyanobacteria (Pinevich et al., 1997), and mutants with short trichomes have been isolated from heterocystous forms such as Anabaena (Bauer et al., 1995). Thus this property may be trivial, which could be demonstrated by genetic analyses on typical representatives of this “genus”. Hormoscilla Anagnostidis and Komárek, 1988 is very similar to Borzia and will not be considered here. A species isolated by E.G. Pringsheim and conforming to the description of Borzia trilocularis (Pringsheim, 1968) exists in culture strain SAG B 1407–1, but has been assigned to Hormoscilla pringsheimii (Schlösser, 1994).

Chro . o . coc ' cus . Gr. fem. n. chroa skin; Gr. n. kokkos grain or kernel; M.L. masc. n. Chroococcus kernel in skin. Cyanobacteria / Subsection I / Incertae Sedis / Form‐ Chroococcus Large cells (22–32 µm in diameter) that reproduce by binary fission in two successive planes at right angles; the cells occur in aggregates, generally of four cells, held together by multilaminated sheath material. Due to the constraints imposed by the tightly appressed sheath layers, a single, spherical, predivisional cell gives rise to hemispherical and quadrantal cells after the first and second cell divisions, respectively.

Cy . an . o . the ' ce . Gr. adj. kuanos blue color; Gr. n. theke case, envelope; M.L. fem. n. Cyanothece blue (green) sheath. Cyanobacteria / Subsection I / Incertae Sedis / Form‐ Cyanothece The form‐genus Cyanothece is a provisional assemblage of strains loosely defined as unicellular coccoid to rod‐shaped cyanobacteria that divide by transverse binary fission in a single plane. The cells are ~3 µm or greater in width and lack structured (e.g., laminated) sheaths. The mol % G + C of the DNA is : 41–49.