Acaulon C. Müller Hal., 1847.

These plants are among our smallest ephemeral mosses. Most appear as scattered individual plants, but a few produce turfs of contiguous plants. The gametophyte is almost stemless, and the few leaves allow no effective distinction between the vegetative leaves and the perichaetial leaves. Placement of Acaulon in the Pottiaceae may be difficult because the median cells of the leaves are elongate hexagonal and smooth (in our two species). Instead, one identifies the plant to genus by noting the erect leaves that completely sheath and hide the nearly sessile sporophytes.

Key to Acaulon Etc.

In this key, we have included all the essentially stemless California plants mostly with costate leaves and cleistocarpous, sessile sporophytes. All these are winter-growing species on soil that is dry during the summer.

Species included in this key are in the Pottiaceae except Archidium (Archidiaceae), Ephemerum (Ephemeraceae), Aphanorrhegma, Physcomitrella and Physcomitrium (Funariaceae), and Lorentziella (Gigaspermaceae):
Acaulon rufescens A. Jaeger
Acaulon triquetrum (Spruce) C. Müller Hal.
Aphanorrhegma serratum (W. J. Hooker. & Wilson in Drummond) Sullivant in A. Gray, not known from CA
Archidium alternifolium (Dickson ex Bridel) Mitten
Archidium donnellii Austin, not known from CA
Ephemerum serratum (Schreber ex Hedwig) Hampe
Lorentziella imbricata (Mitten) Brotherus
Phascum cuspidatum Hedwig
Physcomitrella patens (Hedwig) Bruch & W. P. Schimper
Physcomitrella readeri (C. Müller Hal.) Stone & G. A. M. Scott
Physcomitrium immersum Sullivant, not known from CA
Stegonia hyalinotricha (Cardot & Thériot) Zander

A. Leaves ecostate and strap-shaped with margins coarsely dentate; plant with a very conspicuous felt-like protonema .....Ephemerum: E. serratum
A. Leaves costate, lanceolate to ovate-lanceolate with margins entire to serrulate or serrate, rarely dentate; protonema seldom obvious .....B

B. Median leaf cells mostly papillose with C-shaped papillae; abaxial walls of median laminal cells not markedly different in thickness from the adaxial walls; leaf margins mostly rather broadly recurved near the middle .....Phascum: P. cuspidatum
B. Median leaf cells smooth; leaf margins plane or minutely recurved near the extreme base .....C

C. Spores larger than 100 µm in diameter .....D
C. Spores markedly smaller, mostly about 20 µm in diameter .....F

D. Costa cross-section without stereid bands; leaves appearing bleached and hyaline, at least above the middle; plant with inconspicuous rhizomatous branches arising from basal portions of the upright stem .....Lorentziella: L. imbricata
D. Costa cross-section with a well-developed abaxial stereid band; leaves of healthy plants not bleached; plant without rhizomatous growths .....E

E. Median cells of larger leaves extremely variable in size and shape, often with quadrate cells adjacent to long elliptic cells; perigonia on lateral branches with fully defined bracts .....Archidium: A. donnellii not known from CA
E. Median cells of larger leaves uniformly short rectangular to rhombic; antheridia usually naked in axils of lower perichaetial leaves .....Archidium: A. alternifolium

F. Leaves subtending the sessile sporophyte spreading and easily revealing the sporophyte whether moist or dry; median laminal cells rectangular and thin-walled .....G
F. Leaves erect around the sessile sporophyte hiding most or all of it without dissection; median laminal cells otherwise .....I

G. Capsules irregularly and tardily dehiscent; exothecial cells very thin-walled throughout .....Physcomitrella: P. readeri
G. Capsules with a well-defined operculum; exothecial cells with collenchymatous or regular thickenings .....H

H. Exothecial cells strongly collenchymatous; operculum conic-apiculate to rostellate .....Aphanorrhegma: A. serratum not known from CA
H. Exothecial cells thick-walled, not collenchymatous; operculum rostrate .....Physcomitrium: P. immersum not known from CA

I. Leaves with a prominent awn, often somewhat hyaline; sporophyte immersed but not completely hidden by the enclosing perichaetial leaves .....Stegonia: S. hyalinotricha
I. Leaves without an awn or with that awn short and not hyaline; sporophyte so immersed as to require removal of perichaetial leaves to enable visibility .....J

J. Upper (perichaetial) leaves keeled but slightly spreading so that some part of the urn of the mature sporophyte can usually be seen without dissection; plant tristichous in vertical view; seta arcuate .....Acaulon triquetrum
J. Upper leaves closely enclosing and hiding the mature sporophyte; plant roughly pentastichous in vertical view; seta straight .....Acaulon rufescens