Didymodon Hedwig, 1801.

Didymodon in California is extremely diverse in morphology with the existing names seeming to be inadequate for the cataloging of that diversity. We predict additional species will be recognized following future monographic study.

Key to Didymodon Etc.

The plants treated in this section are acrocarpous mosses with lanceolate to narrowly lanceolate, entire-margined leaves with densely low papillose quadrate cells.

Species included in this key are in Pottiaceae:
Bryoerythrophyllum columbianum (F. J. Hermann & E. Lawton) Zander
Bryoerythrophyllum recurvirostrum (Hedwig) Chen
Didymodon australasiae (W. J. Hooker & Greville) Zander
Didymodon bistratosus Hebr. & R. B. Pierrot, recognized from CA since 2004
Didymodon brachyphyllus (Sullivant in Whipple) Zander
Didymodon eckeliae Zander
Didymodon fallax (Hedwig) Zander
Didymodon ferrugineus (W. P. Schimper ex Bescherelle) M. O. Hill
Didymodon insulanus (De Notaris) M. O. Hill
Didymodon nevadensis Zander in Zander, Stark & Marrs-Smith, not known from CA
Didymodon nicholsonii Culmann
Didymodon norrisii Zander
Didymodon occidentalis Zander
Didymodon revolutus (Cardot) R. S. Williams
Didymodon rigidulus Hedwig
Didymodon tectorum (C. Müller Hal.) K. Saito, not known from CA
Didymodon tophaceus (Bridel) Lisa
Didymodon umbrosus (C. Müller Hal.) Zander
Didymodon vinealis (Bridel) Zander
Pseudocrossidium obtusulum (Lindberg) H. Crum & L. E. Anderson
Triquetrella californica (Lesquereux) Grout

Didymodon is one of the most widespread and abundant genera in the state of California. The differing levels of interpretation of the genus are discussed under the genus Barbula. Among the unusual features that are encountered in identification of a Didymodon or the similar Bryoerythrophyllum is the reddish coloration of some species when immersed in 10% KOH. Another unusual feature is the shape of the cells (quadrate versus rectangular) on the adaxial surface of the costa. This feature is sometimes difficult to see because of the channeling of the apex of the leaves of many species of Didymodon. We enable such viewing by cutting the leaf into several fragments that can then be placed on a microscope slide with the adaxial surface facing upward. Especially important in the study of Didymodon is distinguishing leaves that are channeled distally. Such leaves will show a V-shaped cross-section with the angle described more than 45 degrees. A common error is to collect Ceratodon imagining it to be a Didymodon, but the toothing and areolation in Ceratodon makes it microscopically easy to identify.

A. Leaves bistratose from edge to edge (not just along margin) ..... A1
A. Leaves bistratose at most along margins ..... A2

A1. Leaves narrow (>5:1) with costa extending into a subula ..... Didymodon rigidulous var. subulatus, variety not recorded for CA in BFNA.
A1. Leaves long-ovate to elliptic lanceolate (<4:1) with costa percurrent .....Didmodon bistratosus

B. Leaves strongly 3-ranked, squarrose spreading when moist from a somewhat sheathing base; plant of shallow soil over rocks, mostly in grassy areas near the coast .....Triquetrella: T. californica
B. Leaves spirally inserted and inconspicuously ranked, closely imbricate and ascending to erect spreading; plant of calcareous seeps .....Didymodon tophaceus

C. Median cells of leaf densely papillose with the papillae obscuring the cell walls at least in distal portion of leaf; adaxial cells of costa mostly quadrate, densely pluripapillose with bifid papillae .....D
C. Median cells of leaf low and bluntly papillose to almost smooth; adaxial cells of costa short-oblong or occasionally quadrate, often nearly smooth .....F

D. Plant conspicuously reddish with coloration obvious in the walls of the laminal cells under the compound microscope; leaf margin recurved with marginal cells not significantly less papillose than interior cells .....E
D. Plant of various colors but with red coloration not obvious in the compound microscope; leaf margin revolute in at least one complete spiral with the marginal cells low papillose to almost smooth relative to the interior cells .....Pseudocrossidium: P. obtusulum

E. Basal cells of normally developed leaves quadrate or nearly so; area of differentiated interior basal cells ovate; costa short excurrent in the acute and entire apex .....Bryoerythrophyllum: B. columbianum
E. Basal cells short rectangular >2:1; area of differentiated basal cells elliptical to oblong, mostly with nearly parallel lateral margins; costa subpercurrent to percurrent in the rather blunt, often dentate apex .....Bryoerythrophyllum: B. recurvirostrum

F. Leaf margins crenulate-papillose in distal 1/2; leaves not keeled; plant fragile with leafy stems breaking from clone .....Didymodon norrisii
F. Leaf margins entire or, if crenulate, leaves keeled; plant not very fragile .....G

G. Adaxial surface of costa at mid-leaf with elongate rectangular (>2.5:1) cells .....H
G. Adaxial surface of costa at mid-leaf with quadrate to very short rectangular cells .....I

H. Moist leaves when detached from stem describing less than 120 degrees of arc; costa broadened at base .....Didymodon fallax
H. Moist leaves when detached from stem describing almost 180 degrees of arc, almost impossible to lay flat on slide without cutting into short portions; costa almost evenly tapered to apex .....Didymodon ferrugineus

I. Margins recurved nearly to apex; leaf apex sometimes obtuse .....J
I. Margins recurved only in the proximal 2/3 or less; apex more narrow .....M

J. Costa medially about 8 cells broad, often branched (spurred) in its distal portion; plant of calcarous rocks .....Didymodon revolutus
J. Costa mostly more narrow, not spurred; plant of more varied ecology .....K

K. Leaf apex mucronate, not at all cucullate; costa very short excurrent in the mucro; leaves deltoid to deltoid lanceolate .....Didymodon tectorum not known from CA
K. Leaf apex obtuse to bluntly acute, cucullate to somewhat concave; costa percurrent to subpercurrent; leaves ovate to ovate-lanceolate .....L

L. Cells of adaxial costa surface bulging and with proportions deeper than wide so that that surface appears convex in hand-lens view; guide cells in more than one layer below those adaxial epidermal cells .....Didymodon nevadensis not known from CA
L. Cells of adaxial costa not bulging; guide cells in only a single layer .....Didymodon brachyphyllus

M. Lamina on each side of costa in mid-leaf erect or nearly so, thus causing the costa to appear to occupy a medial groove; leaf apex usually with a differentiated conical apical cell .....N
M. Leaves not channeled or with only a channeling of the extreme distal portion of the leaf; leaf apex usually without a differentiated conical apical cell .....R

N. Leaf apices bistratose at least in the distal portion; these apices often regularly caducous; basal cells mostly quadrate .....Didymodon occidentalis
N. Leaf apices unistratose and not fragile; basal cells mostly rectangular .....O

O. Leaves evenly crenulate with margins bistratose in 1-2 rows in at least the distal 1/2 .....Didymodon eckeliae
O. Leaves entire throughout with marginal bistratosity appearing only in marginal patches .....P

P. Leaves narrowly lanceolate (>6:1) with the apex often terminated by a cluster of rhizoids; marginal cells of leaf base often short-rectangular; dry leaves usually somewhat crispate .....Didymodon nicholsonii
P. Leaves lanceolate but mostly proportionately broader, without terminal rhizoids; marginal cells of leaf base rectangular; dry leaves not crispate .....Q

Q. Leaves to 2.5 mm long with the apices more or less regularly erect and not very crispate; leaf margins recurved to above the middle .....Didymodon vinealis
Q. Leaves longer with the apices flexuose; leaf apices strongly crispate with leaf margins recurved only in distal 1/3 .....Didymodon insulanus

R. Juxtacostal basal cells not so thin-walled, mostly short-rectangular with the lumen/wall ratio often less than 6:1; leaf not distally channeled .....Didymodon rigidulus
R. Juxtacostal basal cells thin-walled and rectangular, often somewhat inflated, lumen/wall ratio greater than 10:1; leaf usually strongly and abruptly channeled in extreme apex .....S

S. Cortical cells of stem inflated; marginal, basal cells forming a moderately differentiated area of more narrowly rectangular cells; adaxial cells of costa usually somewhat elongate .....Didymodon umbrosus
S. Cortical cells of stem not differentiated; basal cells without a margin of more narrow cells; adaxial cells of costa uniformly quadrate .....Didymodon australasiae