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).