Radula mittenii Steph.

[From NSW897201] Forming extensive pure sheets of interwoven pendulous shoots on tree trunks and rocks; living plants opaque yellow-green to glaucous brown-green, fading to milky pale-brown in herbarium; shoot systems regularly pinnate, subdimorphic, with shoots 1.6–2.5 mm wide and up to 80 mm long, branches typically slightly smaller in stature than parent shoot; older shoot sectors becoming ragged in appearance due to irregular leaf fragmentation. Stems 190–250 µm diameter, with cortical cells in a single tier of 30–50 rows; cell walls brown pigmented throughout; cortical cell walls heavily and continuously thickened, at times constricting the cell lumen, dorsally arranged in an oblique zig-zag on young shoot sectors, cell elongation somewhat obscuring this pattern in mature shoot sectors; medulla cells in 80–110 rows, cell walls heavily thickened with coarse nodular trigones that become confluent, and constrict the cell lumen, thin walls occasional. Leaf insertion exceeding dorsal stem mid-line, insertion lines interlocking over two dorsal cortical cell rows, dorsal leaf-free strip absent. Leaf lobes oblong-falcate, 770–1360 µm long by 560–870 µm wide, contiguous to weakly imbricate, acroscopic base sharply deflexed away from stem, otherwise leaves weakly convex, not interlocking over the dorsal stem surface, stem visible in dorsal view, margins may be irregular in outline but always entire, the interior lobe margin curved, not auriculate, antical margin shallowly curved, exterior margin broadly rounded, postical margin straight or substraight. Lobules on leading shoots typically one quarter the lobe area, more or less quadrate, 430–720 µm long by 510–750 µm wide, keel straight to shallowly arched, angle between keel and stem 135°, keel turning through 45–55° at the apex; interior free margin weakly ampliate, acroscopic margin straight or shallowly arched, usually more or less perpendicular to shoot axis, and apices obtuse or broadly acute; attached to stem along 0.4–0.5 of the interior margin, stem insertion gently S-shaped but abruptly revolute at acroscopic end; lobule apex bearing a single papilla, with another papilla situated on the interior lobule margin above the stem insertion; lobules on leading shoots typically larger than those on branches, lobules on branches more rhomboid than quadrate, one fifth to one quarter the lobe area, keel shallowly arched to straight to slightly curved, angle between keel and stem 135°, keel turning through 45–55° at the apex, interior free margin weakly ampliate, acroscopic margin typically S-shaped to shallowly arched, inclined to shoot axis, apex typically broadly acute. Leaf lobe cells rotund to rounded-oblong, 19–28 µm long by 15–23 µm wide, thin walled with concave to triangular trigones, medial wall thickenings absent; cells of lobe margin smaller than those of leaf middle, quadrate to rectangular, 10- 15 µm long by 7–11 µm wide, long axis orientated parallel to lobe margin, exterior cell walls each with a medial wall thickening that bulges into the cell lumen margin; cell surface weakly bulging, bearing heavy verrucose ornamentation. Oil-bodies not known. Asexual reproduction by caducous leaf lobes, sporadic, typically but not always on old shoot sectors, fragmentation scars irregular, shoot primordia forming as irregular buds on leaf lobe margins in older shoot sectors prior to leaf fragmentation. Dioicous. Androecia on lateral branches that continue vegetative growth, androecial bracts in 3–5 pairs, smaller than vegetative leaves, lobes 510–660 µm long and 360–430 µm wide, ovate, imbricate, lobules hypostatic, keel deeply curved, bearing 1–2 antheridia each. Gynoecia terminal on axes, with one pair of female bracts subtended by one (on branches) or two (on leading shoots) full sized subfloral innovations that may again be fertile; archegonia 160–200 µm tall, archegonia neck five cell tiers, cells regularly arranged, 18–20 per gynoecium on a small raised disc of tissue encompassed by the base of the protoperianth; female bracts equal, ovate-falcate, lobes 1200–1310 µm long by 685–870 µm wide, lobules ovate, one half the lobe area, apex rounded to obtuse, keel strongly arched, insertion interlocking both dorsally and ventrally, insertion equitant. Perianths 3000–3500 µm long and 800–900 µm wide at mouth, cyathiform, flaring widely from a narrow base, which is a low stem perigynium 4–5 stratose with brown-pigmented walls, broadest immediately above base, straight-sided, gradually tapering to the mouth, which has irregularly sinuous labia; perianth walls 2–3 stratose at base, unistratose above; calyptral perigynium present, unfertilised archegonia ‘riding’ onto base of calyptra, calyptra 2–3 stratose at base, tapering to unistratose above.

In Australia Radula mittenii is known from a range of localities in the Wet Tropics Bioregion of north-eastern Queensland. Radula mittenii occurs over a broad elevational range, from near sea level to 1500 m, encompassing an array of tropical forest types from lowland mesophyll forests on river floodplains, to cyclone disturbed forests on hillslopes, to notophyll-vine forests on summit peaks. Within these habitats Radula mittenii occupies a range of microsites, from boulders, to tree trunks, and branches, twigs, and liane stems. As a lithophyte on the sides of large boulders Radula mittenii may form extensive pendulous mats of milky yellow-green shoots.