Type:— JAPAN. Ryukyu Islands, Iriomote Island. Mt. Goza, evergreen forest, 300 m (on tree trunk; with F. densiloba Stephani (1911: 549) ex Evans (1906: 157), Lopholejeunea 2 spp., Archilejeunea sp., and Radula sp. All in small quantities on bark of trees), 28 Jan. 1975, M. Mizutani & I. Yoshimura 5506/a (Holotype: NICH 146380!)
Plants rather minute, pale yellowish green or light green when fresh, greenish-brown to slightly reddish-brown in dry condition, loosely adhering to substrate. Shoots up to 10 mm long × 0.50–0.72 mm wide; irregularly branched, branches sparse, similar in size to stem when well-developed, secondary branches usually few; microphyllous branches absent. Stems 46.7–64.4 µm diameter in transverse section, ca. 6 cell rows across, with 14–16 cell rows of medullar cells, 14–15 cell rows of cortical cells, cortical cells as large as the medullary cells, 6–13 µm wide; cell wall light brown or yellowish-brown, exterior cortical wall heavily and evenly thickened, interior cortical cell wall and medullary cell wall heavily thickened, with triangular to bulging trigones, wall between trigones with thin to rather thick continuous thickenings. Rhizoids light brown, arising mostly at lower half of underleaves. Stem dorsal lobes slightly imbricate or contiguous, rarely remote, widely spreading or obliquely spreading, dorsally arched and extending slightly beyond the farther edge of stem, widely ovate, 238–366 μm long × 148–252 μm wide, slightly concave, apices apiculate to acute, occasionally piliferous, incurved, margins entire, antical margins arched toward subtruncate bases, antical margin base not auriculate, extending ca. stem-width across and beyond the farther edge of stem, postical margins nearly straight, rarely arched, postical margin base not cordate. Leaf lobe marginal cells 7.7–13.0 × 9.8–13.3 µm, median cells 13.4– 24.0 × 10.2–17.9 µm, basal cells 21.8–32.6 × 13.8–19.3 µm, thin-walled with moderate to large trigones, triangular to concave, occasionally subnodulose, intermediate thickenings often seen; cuticle smooth. Oil bodies 2–4 per cell, grayish, ovoid or ellipsoidal, 4.0–10.6 × 3.5–5.7 µm, granulate. Ocelli absent. Stem lobules remote, attached to stem by stalk-like base, remote from the stem, distance almost as wide as the width of the lobule, obliquely inserted, widely spreading at angles of 80–90° with the stem, saccate without appendiculum, clavate-cylindrical, inflated throughout, ca. 1/3 of lobe area, 147–191 µm long × 76–106 µm wide, with length/width ratio ca. 1.65–2.24/1, nearly smooth, apex acute-obtuse, often with 1–3 low apical projections, proboscis present at middle of distal portion, without beak, mouth oriented towards stem, mouth valves equal, arched, semicircular, crenulate, distal portion of lobule-mouth with protruding cell margins; styli filiform, 6 cells long, 77.3–88.0 µm long × 14.6–17.7 µm wide, usually appressed to lobules. Stem underleaves one per leaf pair, remote, as wide as or slightly wider than the stem, 93.4–103.4 µm long × 45.4–46.9 µm wide, bifid to ca. (1/2–)2/3 of their length, sinus acute or nearly so, narrow, lobes lanceolate with filiform apical portions, lateral margins plane, transversely inserted. Branch leaves and underleaves similar to those of stem. Asexual reproduction not seen.
Autoicous. Androecia terminal on short lateral branches, usually subglobose (button-shaped), compact or shortly spicate, 345–405 µm long × 327–465 µm wide, with (2–)3 pairs of densely imbricate male bracts; male bracts subequally bilobed, bract-lobes 230–338 µm long × 154–173 µm wide, bract-lobules near the middle usually with small, filiform stylus; androecia with 2– 3 male bracteoles, male bracteoles 105–195 µm long × 35–57 µm wide, free, margins entire, bifid, lanceolate with filiform apical portions, sinus acute or nearly so, upper bracteoles small, 80–92 µm long × 28–37 µm wide. Gynoecia terminal on main shoots or on lateral branches, often with one subfloral innovation; bracts in 2–3 pairs (usually 5 bracts), usually different in size, innermost bract unequally bifid, divided to ca. 2/3 their length, bract lobe narrow elliptical, 665–798 µm long × 200–290 µm wide, apex piliferous, margins irregularly repand with 3–4 large prominent teeth, lobule lanceolate, 540–697 µm long × 178–190 µm wide, apex toothed, margins irregularly repand with 1–3 marginal teeth and a large prominent stylar tooth near the middle of the free margin, consisting of 7–13 cells, 2–4 cells wide at base and ending with one cell at apex, 4–6 cells long, 45.0–83.0 µm long × 36.5–56 µm wide; innermost bracteole elliptical, 596–661 µm long × 176–239 µm wide, bifid to ca. 3/4 of the length, sinus acute, narrow, lobes lanceolate with filiform apical portions, lateral margins plane, often with a blunt or sharp angulation at ca. 3/4 of the length on one or both sides. Perianths exserted to 1/3 of its length, obovate, 728–1085 μm long × 321–520 μm wide at middle, with 4(–5)-keeled (2 lateral keels, 2(–3) ventral keels), lateral keels larger than the ventral keels, surface mammillose; apex distinct, tubular, with beak 75–116 μm long × 58–72 μm wide, ca. 5–7 cells high, cells of beak 14.7–22.3 × 8.5–12.4 μm, inner surface with single-celled protuberances, 23.8–39.0 × 10.8–15.5 μm, mouth crenulate, little expanded only by the mouth margin, mouth open. Sporophytes not seen.
Additional descriptions: — Hattori (1980: 133), Yuzawa (2000: 6, in Japanese).
Additional illustrations: — Hattori (1980: 133, Fig. 1), Yuzawa (2000: 8, Fig. 5).
Distribution and habitat: — Frullania iriomotensis is so far known from the type locality in Japan (Hattori 1980) and Hainan island in China. In China, this species was found on tree trunks in the rainforest at elevations of 1125 m.
Specimens examined: — CHINA. Hainan: Ledong Co., Jianfengling National Forest Park, Main Peak, 18.714189°N, 108.875386°E, 1125 m, 26 Jan. 2021, Rui-Liang Zhu, Chao Shen & Shi-Hui Lu 20210126–101 B (HSNU), Rui-Liang Zhu, Chao Shen & Shi-Hui Lu 20210126–102 (HSNU).