All Taxa

Achnanthaceae: Achnanthes

Cells elliptical, linear, lanceolate, or with undulate margins in valve view, with rounded, cuneate or protracted apices; frustules bent about the transapical axis to form a shallow 'v' in girdle view, apices sometimes reflexed. Girdle deep or moderately deep, so that cells lie either in valve or girdle view.

Frustules heterovalvar with a concave raphe valve and a convex rapheless (pseudoraphe) valve; isopolar and bilaterally symmetrical in outline, but the rapheless valve generally has an asymmetrical pattern and structure.

Striae uni-, bi- or triseriate, similar in structure on both valves, but sometimes differing in orientation; areolae occluded by complex cribra.

Axial area narrow, straight or slightly curved; central area variable, sometimes a narrow or broad rectangular fascia or stauros. Sternum (pseudoraphe) on the rapheless valve uniformly narrow, never expanded centrally, sometimes clearly displaced towards the valve margin.

Cells of freshwater and subaerial species solitary, occasionally forming short chains; some marine species colonial, forming linear chains. Attached directly to a surface by one valve face, or by a mucilage pad or stalk secreted from the raphe at one valve apex.

Two H-shaped chloroplasts positioned ‘fore and aft’ in the cell (i.e. one towards each apex), each consisting of two girdle-appressed plates linked by a wide bridge containing the pyrenoid; or (in some marine species) many small plastids.