71. ACAULON Plate 102.

Acaulon C. Müll., Bot. Zeit. 5: 99, 1847. Lectotype: Acaulon muticum (Hedw.) C. Müll. see Limpr., Laubm. Deutschl. 1: 178. 1885.

Sphaerangium Schimp., Syn. 12, 1860, nom. illeg. incl. gen. prior.

Phascum subg. Acaulon (C. Müll.) Wils. in Hook. f., Fl. Nov. Zel. 2: 58, 1854.

Acaulon subg. Alaticosta Stone, J. Bryol. 9: 213, 1976, nom. inval. holotyp. non cit.

Acaulon sect. Sphaerangium C. Müll., Gen. Musc. Fr. 20, 1900.

Subg. Alaticosta Stone, J. Bryol. 9: 573, 1977. Type: Acaulon chrysacanthum Stone.

Subg. Acaulonopsis Stone, J. Bryol. 15: 746, 1989. Type: Acaulon robustum Broth. ex Roth.

Plants very small, gemmate, gregarious or scattered, reddish or occasionally yellowish brown above, brown below. Stems not branching, very short, to 0.5 µm in length, transverse section rounded, central strand absent, sclerodermis absent, hyalodermis absent; axillary hairs ca. 5 cells in length, proximal cell walls sometimes thickened; sparsely radiculose. Leaves strongly appressed and apices usually reflexed when dry, appressed to weakly spreading when moist, ovate, small, 0.5–1.75 µm in length, lamina broadly channeled and usually deeply concave, margins plane, entire to serrulate or dentate; apex broadly acute; base not differentiated in shape; costa excurrent in a sharp apiculus or stout mucro, occasionally only percurrent or as a short, sometimes dentate awn, costa with lamina inserted laterally, superficial cells elongate and smooth ventrally, dorsally elongate and usually smooth, 3–4 rows of cells across costa ventrally at midleaf, costal transverse section round, stereid bands ventrally generally absent, dorsally present (but usually weak) and rounded in shape, ventral and dorsal epidermises present, guide cells 2–4 in 1 layer or rarely absent, hydroid strand usually present, occasionally apparently centered in the stereid band, costal outgrowths sometimes present, of ca. 2 longitudinal lamellae formed on ventral surface of the costa; upper laminal cells rounded-quadrate to rhomboidal, ca. 13–15 µm in width, 1–4:1, walls evenly thickened, occasionally highly thickened on dorsal walls, superficially convex on both sides of la mina; papillae absent or occasionally large and simple, 1(–2) per lumen; leaf base not differentiated in shape, basal cells rectangular, little wider than upper cells, 3–4:1, walls thin. Dioicous and perigoniate plants smaller than the perichaetiate (often much reduced), or else monoicous and usually paroicous. Perichaetia terminal, inner leaves somewhat enlarged. Seta very short, to 0.2 µm in length, 1 per perichaetium, light brown; theca cleistocarpous, spherical, apiculus lacking, ca. 0.4–0.7 µm in diameter, light brown, exothecial cells quadrate to rhomboidal, 25–50 µm in width, 1:1, thin-walled, stomates phaneropore, occurring at base of capsule. Operculum absent. Calyptra mitrate, often lobed, smooth, ca. 0.1–0.4 µm in length. Spores rather large, ca. 25–35 µm in diameter, spherical to weakly elliptical, light brown, lightly papillose to irregularly warty or spiculose. Laminal KOH color reaction red. Reported chromosome number n = 26.

Found on soil, a widely distributed genus mainly found in temperate regions of low rainfall.

Acaulon is clearly similar to Microbryum, differing in the even smaller habit size, capsules spherical and lacking an apiculus (Pl. 102, f. 28), upper laminal margins plane, and papillae lacking in most species. Acaulon schimperianum (Pl. 102, f. 29) has upper laminal papillae somewhat like those of Microbryum vlassovii and M. floerckeanum (one to two over each lumen, simple or occasionally branching apically, hollow, often rather tall). Those species of Microbryum with short-ovate leaves and no papillae also, like Acaulon, have rectangular ventral quadrate cells. Acaulon may be an end member of a series including Tortula sect. Tortula and Microbryum through reduction of gametophyte and sporophyte size and complexity, and differentiation of red coloration. This is supported somewhat by Cladograms 12 and 14 to the extent that Tortula is lower on the tree than the other genera.

In A. eremicola (Pl. 102, f. 14–15), A. muticum and A. schimperianum, the perigoniate plants are about a quarter to a third the size of the perichaetiate plants, and are situated near the base of the perichaetiate plants (possibly rhizautoicous).

The costa may appear, in section, to have two stereid bands in species (e.g. A. chrysacanthum, Pl. 102, f. 11–13) that develop strong awns. Stone (1976b, 1977b, 1979, 1988, 1989) has discussed the morphology of Australasian Acaulon species in a particularly incisive manner. Casas et al. (1990) gave a key to the five species of the genus on the Iberian Peninsula, while Sérgio (1992) noted that the European A. piligerum belongs in the lamellate subgenus Alaticosta, previously known only from Australia.

Additional literature: Bryan (1956), Crdenas (1988), Crum and Anderson (1965), Grout (1945), Hill (1982), Sérgio (1972a, 1992).

Number of accepted species: 15.

Species examined: A. chrysacanthum (NY), A. eremicola (MELU), A. integrifolium (BUF, NY), A. leucochaete (BUF, NY), A. muticum (BUF), A. robustum (NY), A. schimperianum, A. triquetrum (BUF), A. uleanum (BM).