Lethocolea Mitt.

Gradstein, S. Robbert, Ilkiu-Borges, Anna Luiza, Cargill, D. Christine, Beckmann, Karen & Vanderpoorten, Alain, 2024, An integrative taxonomic study of the genus Lethocolea (Marchantiophyta: Acrobolbaceae), Plant Ecology and Evolution 157 (3), pp. 375-398 : 375-398

Lethocolea Mitt. ( Hooker 1867: 751, 753) nom. cons.

Podanthe Taylor ( Taylor 1846: 413) – Type species: Podanthe squamata Taylor (= Lethocolea pansa (Taylor) G. A. M. Scott & K. G. Beckm.).

Type species.

Lethocolea drummondii Mitt. , nom. illeg. (= L. pansa (Taylor) G. A. M. Scott & K. G. Beckm. ).

Description.

Plants dioicous, rarely paroicous ( L. naruto-toganensis ), prostrate, pale green to yellowish-green to olive-green to brownish, sometimes reddish or purplish, little-branched, branching ventral-intercalary or (rarely) lateral-intercalary, ventral branches stoloniform. Stems rather fleshy, usually fragile and made up of thin-walled cells, sometimes rigid and with slightly thickened epidermal walls, epidermis cells similar in size to medullary cells, rarely slightly smaller, ventral region of stem sometimes with fungal hyphae inside the cells. Rhizoids scattered to slightly fascicled, hyaline to light brown to pink, rarely purplish ( L. naruto-toganensis ), sometimes with fungal hyphae. Leaves succubous, alternate, imbricate, lamina flat to convex, or concave with convex margins, undivided, suborbicular to ovate to ovate-oblong to lingulate, insertion line reaching dorsal midline of stem, apex rounded to truncate to emarginate, margins entire, sometimes with a border of thick-walled cells, dorsal leaf base not or shortly decurrent, ventral leaf base not decurrent, lower ventral half of the leaf lamina with an area of enlarged, hyaline cells with little or no chlorophyll. Leaf cells isodiametric to elongate, fully thin-walled or with very small to medium-sized trigones, trigones usually hyaline, rarely reddish, cuticle coarsely papillose or smooth; oil bodies large, 1–3 per cell, usually ellipsoid, dark brown to greyish-brown (exceptionally colourless), sometimes persistent in dried material. Underleaves absent. Androecia terminal or intercalary on main shoots, bracts saccate (not saccate in L. naruto-toganensis ), antheridia 1 (– 6) per bract, antheridial stalk short, up to 7 cells long, irregularly biseriate. Gynoecia terminal on main shoots, female bracts in 2–4 pairs, bracts slightly larger than vegetative leaves (except for the inner ones), standing upwards or spreading outwards; perianth absent; archegonia ca 20–30 per gynoecium. Marsupia pendent, shortly to elongate cylindrical, up to 3–10 (– 20) mm long, green and with smooth surface when young, brown and with usually hairy surface when mature, marsupial canal narrow, lined by few or numerous large, elongate, papilliform, mucilaginous cells; archegonia, calyptra and developing sporophyte carried to the base of the marsupium, archegonia with a short neck. Setae whitish, up to 1.2 cm long after elongation, massive, made up of ca 70 rows of cells (Fig. 9 D ), with distinct trigones. Capsules cylindrical, scarcely wider than the seta, tip acute to apiculate (Fig. 9 C ), capsule dehiscence complete or incomplete, splitting to the base into 4 or in only 2–3 valves with two adjacent valves remaining partially connate, capsule wall 2–4 - stratose (Fig. 9 F ), cell walls of the outer layer yellowish to reddish-brown, with small, orange to reddish-brown nodular thickenings evenly spaced along the outer longitudinal wall and turning into conspicuous thickening bands on radial walls, cell walls of the inner layers hyaline, without thickenings (seen in L. congesta only); elaters with 1–2 spirals (seen in L. congesta only), narrowly fusiform, 130–200 × ca 10 µm, surface finely punctate; spores isodiametrical, ca 25 µm in diameter, yellowish to brown, surface finely papillose. Asexual reproduction (subgen. Lethocolea ) via large, disciform (sometimes angular-shaped) gemmae produced near the shoot apex on the dorsal stem surface or on the bases of the dorsal leaf margins, gemmae green to light brown, 1–5 mm in diameter, 4–20 cells across, biconvex, 5–6 cells thick in the middle and 1–2 cells thick at the margin, sometimes with a 1 - celled stalk, with or without transparent, unistratose wing; gemmalings sprouting from the centre or the margin of the gemmae.

Distribution.

Widely distributed in Mediterranean and temperate regions of the Southern Hemisphere and in tropical mountains, extending northwards to Mexico, India, and Japan.

Habitat.

On sandy, loamy, or peaty soils subject to desiccation, in shaded and exposed sites, along trails, on earth walls and on soil over rock, sometimes in swampy, wet habitats, or floating on water, becoming deeply reddish-maroon. The plants sometimes grow partly buried in the substrate and may become brownish, somewhat purplish or carmine when growing in exposed sites.

Notes.

Lethocolea is readily recognized by: 1) creeping, little-branched leafy shoots with rather fragile stems made up of thin-walled cells, without or with one (or more) ventral stolons; 2) rhizoids scattered, hyaline to light brown to pink (exceptionally purplish); 3) leaves succubous, insertion line reaching dorsal midline of stem, alternate, imbricate, undivided, suborbicular to ovate-oblong to lingulate, with a broadly rounded apex, entire margins and an area of enlarged hyaline cells in the lower ventral half of the leaf; 4) leaf cells with or without trigones, trigones hyaline, cuticle coarsely papillose or smooth, oil bodies 1–3 per cell, large, finely granular, greyish-brown to dark brown; 5) underleaves absent; 6) gametoecia on main shoots; 7) sporophyte enclosed by a pendent marsupium at the shoot apex (perianth absent); 8) calyptra present, free, carried down the marsupial canal to the foot of the marsupium together with the unfertilized archegonia; 9) capsule cylindrical, tip acute to apiculate; 10) asexual reproduction by multicellular, disciform gemmae (in subgen. Lethocolea ).

By its undivided succubous leaves, lack of underleaves, and rather large, finely granular oil bodies, one or two per cell, Lethocolea has been confused with Jackiella ( Jackiellaceae ), Odontoschisma ( Cephaloziaceae ), and Solenostoma ( Solenostomataceae ). The latter three genera are distinguished from Lethocolea by the absence of an area of enlarged-hyaline cells in the lower ventral half of the leaf and by the smooth or finely papillose cuticle covered by minute papillae much smaller than in Lethocolea . Jackiella and Odontoschisma , moreover, have dorsal leaf insertions usually not reaching the dorsal midline of stem, gametangia on short ventral branches, and asexual reproduction by small, 1–2 - celled gemmae. The sporophyte is enclosed by a perianth in Odontoschisma and Solenostoma , and by a marsupium in Jackiella .

Based on the results of the morphological and molecular study, four Lethocolea species are recognized in two subgenera, subgen. Lethocolea and subgen. Symphyomitra . The two subgenera are distinguished by the presence or absence of disciform gemmae, the length and shape of the marsupium, and the incidence of papilliform cells lining the marsupial canal ( Grolle 1972). The two groups are fully supported in the molecular analysis (Fig. 2 ). Schuster (2021) treated the two subgenera as sections, but in view of the strong molecular support for the two groups we do not accept Schuster’s classification.

Sporophytes are rare in Lethocolea ; the above description is based on two sporophytic collections of L. congesta , with additions from Spruce (1885) and Schuster (2021). The seta of Lethocolea is massive and the capsule is cylindrical with an apiculate tip (Fig. 9 C, D ), like in other members of Acrobolbaceae ( Schuster 2021) . The capsule of L. congesta is unusual, however, in opening to the base into 2–4 valves (with two adjacent valves sometimes remaining partially connate) and in having a thin, mostly 2 - stratose wall, with limited 3–4 - stratose areas (Fig. 9 F ) and no thickenings on the walls of the inner cell layers (Fig. 9 G, I ) (capsule opening to the base into 4 valves, wall 4–10 - stratose and with thickenings in all cell layers in other Acrobolbaceae ). Moreover, the elaters of L. congesta possess 1–2 spirals (Fig. 9 B ) (elaters bispiral in Acrobolbaceae ). Data on capsule wall thickenings and elaters are still lacking for the other species of Lethocolea . The sporophyte of Lethocolea clearly needs more study.

Figures

Figure 2. Maximum likelihood tree resulting from rps 4 sequences in the liverwort genus Lethocolea. Numbers below the branches correspond to bootstrap support.

Figure 9. Lethocolea congesta. A, E. Female shoot with near-mature marsupium. B. Elater. C. Shoot apex with mature marsupium, female bracts and mature sporophyte before dehiscence, showing apiculate capsule. D. Cross section of seta. F. Cross sections of capsule. G. Outer cells of capsule wall, frontal view. H. Outer cells of capsule wall, lateral view. I. Inner cells of capsule wall, frontal view. J. Spores. K. Male bract with antheridium. L. Male plant with androecia. M. Cross section of stem. A from Onraedt 70. R. 4539; B – J from Halle 139; K – L from Larrain 45507; M from Holz 209. Drawn by Anna Luiza Ilkiu-Borges.