Lejeuneaceae

Gametophyte Form and Habit

Plants are typically small (often only a few mm wide), delicate, and form creeping or appressed mats, sometimes pendent. Color ranges from pale or whitish-green to bright green or yellowish-green, rarely brownish or reddish. Branching patterns are diverse, often pinnate or irregularly branched, with specific types (e.g., Lejeunea-type, Frullania-type) having taxonomic significance. Stems usually show limited differentiation between cortex and medulla.

Leaves (Phyllids)

Leaves are typically arranged in three rows (two lateral, one ventral/underleaves), but underleaves are absent in some large genera (e.g., Cololejeunea). Lateral leaves are usually incubous or sometimes transversely inserted, rarely succubous. They are almost always bilobed:

• Dorsal Lobe: Usually the larger part, shape varies widely (ovate, oblong, lanceolate).
• Ventral Lobule: Highly variable in size and shape; can be inflated, sac-like, flattened, or reduced to just a few cells. Often possesses an apical tooth or papilla, and sometimes a basal or marginal hyaline papilla (a clear, thin-walled cell), the position of which is critical for identification. A fold or keel connects the lobule to the lobe. In some groups (e.g., Cololejeuneoideae), a stylus (a small cellular appendage) may be present near the lobule base.

Underleaves (Amphigastria)

When present, underleaves are located on the ventral side of the stem. They are typically smaller than the lateral leaves and are most commonly bilobed, but can be entire (undivided) in some groups. Their shape, size, depth of division, and insertion line are important taxonomic characters.

Reproductive Structures

Sexual condition varies; many are autoicous (male and female organs on the same plant but separate branches) or dioicous (separate male and female plants).

• Antheridia occur in the axils of modified bracts, usually on specialized short branches (androecia).
• Archegonia are typically terminal on short specialized branches (gynoecia), protected by larger, often modified bracts and bracteoles, and a perianth.
• The perianth is highly characteristic in many Lejeuneaceae: typically emergent, inflated or flattened, and commonly possesses 5 keels (ridges) – two lateral, two ventral, one dorsal – though the number can vary (0, 3, 4, or more). It is usually contracted at the apex into a distinct beak.

Asexual reproduction is common via specialized means, including caducous (easily detached) leaves or branches, or multicellular gemmae produced on leaf surfaces or margins.

Sporophyte

The sporophyte develops within the perianth and remains dependent on the gametophyte. It generally conforms to the typical liverwort structure but often shows reductions:

• Foot: Anchors and absorbs nutrients.
• Seta: Often very short and delicate, consisting of few cell rows. Elongates briefly to push the capsule out.
• Capsule: Typically spherical, pale or light brown. The wall is thin (often 2-layered) and splits into 4 valves upon maturity. Contains spores and elaters. Elaters are usually unicellular with 1-2 spiral thickenings.

Spores

Spores are typically small, unicellular, greenish, and often have irregular shapes or surface ornamentations (papillose, baculate) which can be taxonomically useful.

Cellular Features & Oil Bodies

Microscopic features are crucial for identification. Leaf cells typically have trigones (wall thickenings at cell corners), which vary in size and shape. Cells contain oil bodies, whose number, size, shape (fusiform, spherical), and internal structure (homogeneous, granular, segmented) are highly diagnostic at the genus and species level but require fresh material to observe.

Chemical Characteristics

Lejeuneaceae produce a diverse array of secondary metabolites, including terpenoids and aromatic compounds, often concentrated in the oil bodies. These contribute to defense and potentially allelopathic interactions. Chemical profiles vary significantly across the family.