Plants small (main shoots 400–700 µm wide), often dark brown to almost black.  Leading stem 15–30mm long and 75–100µm in diam. [Schuster 1970: to 125µm], 6–8 cells wide, 15–18 cortical cells and 15–19 medullary cells, both with firm walls, and irregularly-shaped lumen; cortical cells slightly smaller than medullary cells, and the walls of the former often slightly thicker and deeper pigmented than those of the medullary cells; [Engel 1978: the stems in surface view have conspicuous irregular thickenings].  Branching often regularly pinnate, occasionally bipinnate.  Frullania-type branching only.  First branch underleaf (BUL1) with three distinct segments, the ventral lamina divided for ca. ½ its length into two unequally sized lobes + 1 dorsal saccate lobe; occasionally BUL1 with three explanate segments.  The first branch leaf (BL1) characteristic of those of the main stem (i.e. 1 dorsal, explanate, segment +1 saccate ventral segment + a stylus).  Stemleavesof main branchflat when dry and wet, dorsal lobes contiguous to loosely imbricate, rarely remote [of Schuster 1970 leaves remote], and broadly ovate to asymmetrically elliptical; to 450 µm long × 325 µm wide (ca. 25 cells × 20 cells); antical base extending beyond the farther edge of the stem (by 1–1.5× the stem width, ca. 75–150µm); tapering to subtriangular, incurved and acute apices, non-auriculate at the base, and with entire margins and smooth dorsal cell surface.  Lobules ± remote from the stem (lobule attached to stem by 2–3 cells) and at angles of (25) 45–75º with the stem so that lobules tilted outwards; lobules slightly darker or similar in colour to other organs (in part due to heavily pigmented cell walls); uniformly inflated, smooth, obovoid, (orbicular in cross-section with up to 18 cells in circumference) and ca. 1.25–1.75× long as wide, to 160µm long × 100 µm wide (ca. to 10 cells in length); lobuli small in proportion to the leaf-lobe (lobule area obscuring no more than 0.25× the exposed area of the dorsal lobe), somewhat dorsiventrally compressed near mouth as compared to gibbous upper half, the opening extending along the abaxial lobule margin; immediately above the slit of the lobule mouth there is usually an enlarged and elongated cell, to 20 µm long (cell cavity 1.5–2× long as wide), the free wall forming an angular protuberance that projects outwards to 7.5 µm; the free margin of lobular mouth inconspicuously crenulate-sinuate, hyaline only at marginal cells of the mouth, lobule apex obtuse.  Stylus, distinct, subtriangular to triangular, up to 80 µm long, 3–4 cells wide at base (6–10 cells total), often with a slime papilla at apex, especially on those leaves towards the stem apex (Engel 1978: terminates in a unicellular row of two cells with the terminal cell a hyaline slime papilla which eventually collapse).  Underleaves of leading stems, ± small, at most only ¼the size of leaf lobes; at best only contiguous with lobules, underleaves otherwise distant from each other; usually long as wide, or occasionally slightly wider than long, suborbicular to obovate in outline, 150–225 µm long × 125–220 µm wide, (1.0) 1.2–2.0 (3.0) × as wide as the stem, broadest at middle (10–15 cells wide); entire lateral margins or at most with minor angulations; apex of underleaf bilobed to 0.4–0.5 its length, lobes separated by a very narrow U to V-shaped sinus, lobes 4–6 (8) cells wide at base, with acute to subacute apices.  Rhizoid-initial area present near base of underleaf, rhizoids rarely seen, subhyaline, in short bundles [Engel 1978: rhizoids simple or branched very thick-walled tips].  Not strictly microphyllous, lobules of secondary stems ± similar in size to those of the main stem, but lobes and underleaves of secondary branches markedly smaller than those of leading stems.

     Lobe marginal cells rectangular to quadrate, subhyaline walls subequally thickened, small, 10–13.5 (15) µm long × 9–12.5 µm wide; median cells ± subquadrate, rectangular to distinctly 5–6-sided, hyaline walls subequally thickened, with intermediate thickening rare to occasional, wall thickness to ca. 2 µm wide (without intermediate thickenings), cell cavities of median cells brownish red, (10) 12–17 (20) µm long × (9) 10–15 (18) µm wide; cells becoming gradually larger basally, cavities of the basal median group of cells to 26–36 µm long × 15–22 µm wide; walls of basal cells increasing in thickness, and with rare to occasional, indistinct, intermediate thickenings.  Median cells of underleaves generally ± subequally thickened so that the hyaline trigones and intermediate thickenings become indistinct, but the trigones becoming more distinct towards the margins, cell cavities to 16 µm long X 14 µm wide.  Median cells oflobuleca. 2× longer than wide, cell cavities to 18 µm long × 10 µm wide, walls flexuose throughout the lobule, with indistinct trigones and small nodulose intermediate thickenings (becoming increasingly flexuose towards the lobule mouth).  Oil-bodies : no records.  Asexual reproduction;none known

     Autoicous.  Androecia usually subspherical (to 600 µm) to occasionally shortly spicate (to 700 µm long × 550 µm wide), 2–4 pairs of closely imbricate bracts, sessile or terminal on very short-stalked Frullania-type branches, stalk with 1 (2) vegetative leaf lobes.  Gynoecia terminal on main or leading stem or terminal (almost sessile) on short secondary branches with the stalk bearing 1–2 vegetative leaf lobes.  The androecial branches usually some distance, typically 1–3 vegetative branches below the gynoecia.   Gynoecial branches with mature perianths typically with the first vegetative branch developing 2 (3) vegetative leaves posterior to the ♀ bracts and bracteoles; gynoecial branches with gynoecia that remain unfertilised and occasionally develop subfloral innovations.  ♀ bracts and bracteoles in 1–2 closely imbricate cycles.  Innermost bract unequally bilobed; bract-lobe asymmetrically elliptical-lanceolate to ovate, ca. to 850 µm long × 500 µm wide, tapering to an acute to obtuse apex, the margins entire; bract-lobule subequal in length, but ca. 0.5× the size of the lobe, broadly lanceolate, with an acute to subacute apex, marginal teeth absent.  Innermost bracteole free from bracts, oblong to suborbicular in outline, about ½ bilobed, the sinus narrowed; lobes ovate-triangular, with acute apices, and without marginal teeth.  Marginal cells of bract and bracteole ± subequally thickened, but towards the median cells, trigones becoming large and bulging.  One archegonium per gynoecium.  Perianth freely emergent, oblong-clavate, to 1200 µm long (excluding perianth beak) [Schuster 1970: to 1400 µm long] × 650 µm wide, exterior smoothly trigonous, keels obtuse, tapering towards the apex into a short (to 50 µm long) cylindrical, beak; perianth beak with a smooth rim or mouth, but the inner beak surface densely covered with large single-celled protuberances (10–20 µm long), becoming almost instantly reduced in size beyond the beak, where the cells are raised into hill-like elevations throughout the entire inner surface of the perianth.  Median cells of perianth with large bulging, convex trigones, intermediate thickening rare or absent.

     No sporophyte seen.  The following description of the sporophyte is compiled from data outlined by Engel (1978) and Schuster (1970, 1992).  Seta ca. 225–245 in µm diam., with 16 rows of epidermal cells that are rather smaller in diam. than the 4 internal cells; the epidermal cells not distinctly tiered.  Capsule spherical, wall yellow-brown, 2-stratose.  Epidermal cells hyaline, with (in distal half of valves) coarse “nodular” (vertical) but hyaline thickenings, chiefly confined to loci where 2–3 (4) radial walls intersect, there forming a lobed, radiate, nodular thickening complex.  Inner cells with an ill-defined, sub fenestrate, yellow-brown pattern of thickenings on the tangential free walls.  The inner capsule wall has, in surface view, 3 ridges parallel with the long valve axis.  Elaters usually 5 per valve, 14.5–17.5 µm in diam. medially, with a single, yellow-brown, well defined thickening band ca. 7–7.5 µm broad.  Spores variable, obscurely angular to subspherical, up to 66–69 × 70–72 µm, densely covered with round, ill-defined clusters of tubercles and bearing on one end a smooth operculum-like lid from which the germ tube escapes.

Morphological similarity and chemical evidence suggests that the South American, F. microcaulis, is allied to F. truncata of New Zealand.  Both species share similar branching appendages, cell anatomy, and the distinctive projecting cell above lobule mouth.  However, F. microcaulis can be differentiated from F. truncata by the following: 1) plants monoicous; 2) lobules small in proportion to the dorsal lobes; 3) the stylus gradually tapering to a rounded or subacute apex; and 4) interior cells of the perianth elevated in to small hill-like projections.  In contrast, plants of F. truncata are dioicous, have more or less large lobules in comparison to the dorsal lobes, styli with subtruncate apices, and smooth internal perianth surfaces.  Chemically, Frullania microcaulis and F. truncata are unusual in that they can not be classified according to the current scheme of eight broad chemical types because they contained neither sesquiterpene lactones nor bibenzyls - both common constituents for many Frullania species.  Therefore, chemically, F. microcaulis and F. truncata are very different from all other Frullania species studied to this date (Asakawa, Toyota, von Konrat & Braggins 2003)