North America, Eurasia, mostly temperate zone.
Genera 4, species 7 (2 genera, 4 species in the flora).
Helodiaceae are generally 1-pinnate from a single axis, have a single costa to mid leaf or beyond, and have elongate medial laminal cells, (5–)7–10(–18):1. Abaxial portions of the stem leaves are smooth or with a single, central papilla over the lumina; all are characterized by papillae located at or near the distal end of the cell, or the cells are prorulose, or both. The stems are somewhat to densely tomentose with slender smooth-walled paraphyllia and smooth to proximally 1-papillose pseudoparaphyllia whose apical cell is also sharply pointed and smooth on surface and margins. The most obvious difference between Helodiaceae and Thuidiaceae in North America is the more elongate, linear-rhomboidal cells in the stem leaf and paraphyllia in the former, especially in the apical and medial proximal regions of the leaves. Laminal cells in Thuidiaceae are subquadrate-isodiametric, especially in the stem paraphyllia, which are (long-)rectangular with smooth surfaces (R. Ochyra 1989). M. R. Crosby et al. (1999) cited four genera for the family, Actinothuidium Brotherus, Echinophyllum (as Bryochenea C. Gao & G. C. Zhang), Helodium, and Palustriella Ochyra.
|1||Sexual condition autoicous; stems regularly ± 1-pinnate; stem leaf medial laminal cells elongate; costa surfaces smooth or with 1 or 2 teeth near insertion; laminal cells with abaxial surface roughened, papillae small, delicate; stem and branch leaf apical cells 6-7: 1.||Elodium|
|1||Sexual condition dioicous; stems 1-pinnate or sometimes irregularly 2-pinnate; stem leaf medial laminal cells subquadrate to short-rectangular; costa surfaces with coarse, spinose teeth over proximal 1/2; laminal cells with abaxial surface echinate, papillae coarsely obliquely blunt-conic; stem leaf apical cell 20:1, branch leaf apical cell 7:1.||Echinophyllum|