Frullaniaceae

Gametophyte Form and Habit

Plants are small to medium-sized, occasionally larger, typically forming flat, closely appressed mats or patches adhering tightly to the substrate. Color is highly characteristic, often reddish-brown, purplish-black, or dark olive-green, less commonly bright green. Stems are prostrate, usually regularly 1-2 times pinnately branched (Frullania-type lateral branching).

Leaves and Underleaves

Leaves are arranged incubously and are closely overlapping. They are distinctly complicate-bilobed, divided into a large, typically ovate to orbicular dorsal lobe (often with a rounded apex) and a smaller ventral lobule. The key feature is that the lobule is typically modified into a helmet-shaped or cylindrical sac (water sac), oriented obliquely or parallel to the stem. A small, often filamentous or lanceolate stylus is usually present between the lobule base and the stem. Underleaves are consistently present, usually smaller than the leaves but distinct, and characteristically bifid (divided into two lobes), although the lobes and margins can vary (e.g., toothed, entire).

Rhizoids

Rhizoids are usually colorless or brownish, arising in a tuft from the base of the underleaves.

Cellular Structure

Leaf cells are typically small, isodiametric (polygonal). Cell walls are often thickened, with distinct, often bulging or nodulose trigones. Cell walls and sometimes the entire cell contents are frequently pigmented brown or red. Ocelli (specialized cells containing a single large oil body, appearing as eye-spots) are present in the leaves and underleaves of some species. Oil bodies are typically present, granular or homogeneous.

Asexual Reproduction

Asexual reproduction via gemmae is generally rare or absent, though caducous leaves or branches occur in some species.

Reproductive Structures

Plants are mostly dioicous, sometimes monoicous.

• Antheridia are borne in the axils of saccate bracts on specialized, short, often capitate (head-like) lateral branches.
• Archegonia are terminal on short lateral branches, surrounded by 2-5 pairs of bracts and bracteoles that are usually larger and sometimes differently shaped than vegetative leaves/underleaves.
• A prominent perianth is present, typically exserted. It is characteristically pyriform (pear-shaped) to oblong, sharply 3-keeled (trigonous), sometimes with additional smaller folds (plicate), and abruptly contracted to a distinct tubular beak or rostrum at the mouth. Marsupia are absent.

Sporophytes and Spores

The sporophyte develops within the perianth. The seta is short and thick, elongating rapidly via a unique explosive cell elongation mechanism just before spore release. The capsule is spherical, dark, splitting into four valves that often recurve. The capsule wall is typically composed of two layers of cells. Spores are large; elaters are typically attached to the inner capsule wall and valve surfaces.