Brachythecium W. P. Schimper, 1853.

Brachythecium is a difficult genus in California. The genus primarily includes plants of mesic but not inundated sites. Brachythecium frigidum is an exception, being emergent from the edge of streams and seeps. Species in the genus Brachythecium may have significant differences between the shape and cell areolation of the stem and branch leaves. Brachythecium and more generally the Brachytheciaceae is being revisited by several bryologists (e.g. Ignatov and Huttunen 2002).

Key to Brachythecium Etc.

The mosses keyed in this section are prostrate to laxly decumbent pleurocarps mostly of mesic sites. Gametophytes differ widely in degree of leaf plication, in length of the usually weak costa, in length of the usually long cells, in alar cell size and shape and in the serration of the leaf margins. Sporophytes are, however, quite distinctive in the short and arcuate capsules with apiculate opercula and reddish coloration.

Species included in this key are in Brachytheciaceae except Leptodictyum (Amblystegiaceae):
Brachythecium albicans (Hedwig) Bruch & W. P. Schimper
Brachythecium asperrimum (C. Müller Hal.) Sullivant
Brachythecium bolanderi (Lesquereux) A. Jaeger
Brachythecium calcareum Kindberg
Brachythecium collinum (Schleicher ex C. Müller Hal.) Bruch & W. P. Schimper
Brachythecium erythrorrhizon Bruch & W. P. Schimper
Brachythecium fendleri (Sullivant) A. Jaeger
Brachythecium frigidum (C. Müller Hal.) Bescherelle
Brachythecium holzingeri (Grout) Grout
Brachythecium hylotapetum N. Higinbotham & B. Higinbotham
Brachythecium laetum (Bridel) Bruch & W. P. Schimper, not known from CA
Brachythecium leibergii Grout
Brachythecium nelsonii Grout
Brachythecium occidentale (Hampe) A. Jaeger, not known from CA
Brachythecium oedipodium (Mitten) A. Jaeger
Brachythecium plumosum (Hedwig) Bruch & W. P. Schimper
Brachythecium populeum (Hedwig) Bruch & W. P. Schimper
Brachythecium reflexum (Starke in Weber & D. Mohr) Bruch & W. P. Schimper
Brachythecium rivulare Bruch & W. P. Schimper
Brachythecium rutabulum (Hedwig) Bruch & W. P. Schimper
Brachythecium salebrosum (Weber & D. Mohr) W. P. Schimper in Bruch & W. P. Schimper
Brachythecium starkei (Bridel) Bruch & W. P. Schimper
Brachythecium velutinum (Hedwig) Bruch & W. P. Schimper
Brachythecium venustum De Notaris
Cirriphyllum cirrosum (Schwägrichen in Schultes) Grout, not known from CA
Leptodictyum riparium (Hedwig) Warnstorf
Pseudoscleropodium purum (Hedwig) Fleischer in Brotherus

For identification it is essential to look primarily at stem leaves, and unmodified reference to leaves in this key can be understood to refer to stem leaves. Most of our species have a costa reaching only somewhat past mid-leaf, and that costa almost invariably ends in one or several abaxial spines. The leaf margin is usually somewhat toothed.

Among the features required in the study of Brachythecium, the pattern of plication may be especially difficult. The leaf margins of Brachythecium are essentially always recurved at the base, and this confers an impression of plication. A plication which defines a recurvature must extend beyond the area of recurvature in order to be considered as a plication.

Alar cells of Brachythecium may be rectangular or they may be almost quadrate. The latter alar pattern is confounded by the pattern of opaque versus pellucid cells. Opaque alar cells under low and medium power of a compound microscope are difficult to visualise as to the pattern of cell outline. Pellucid cells can be seen even under low power. Opaque alar cells are usually rather irregular in cell outline; pellucid ones are very regularly quadrate.

Field notes should accompany a collection suspected of being Brachythecium. Main stem axes as compared with higher order branches show different degrees of prostrate versus decumbent versus nearly erect orientation, and this orientation cannot usually be known from herbarium specimens. Often the sexuality of the plant can be seen in the field. The presence of sporophytes emerging from the perichaetia allows one to look along the stems of the same plant for the bud-shaped perigonia. Laboratory examination for those same features may require floating a plant in water and spreading individual stems.

A. Costa reaching into acumen, often nearly percurrent; seta papillose; plant autoicous .....B
A. Costa shorter, usually ending at 1/2-3/4 of the leaf length .....D

B. Apex twisted on at least some leaves; leaves cordate-deltoid, 1.5-2:1 .....Brachythecium starkei
B. Apex never twisted; leaves ovate-lanceolate, 3-5:1 .....C

C. Leaves somewhat spreading and rather distantly inserted so that the stems can be seen without dissection; decurrency extending several cells down stem .....Brachythecium reflexum
C. Leaves more appressed and imbricate, typically obscuring the stems, almost not decurrent .....Brachythecium populeum

D. Leaves plicate, often with plicae defining the alar region and thus ignored but with those plicae extending more than 1/4 up the leaf .....E
D. Leaves not plicate or plicae restricted to leaf base .....M

E. Alar cells quadrate to irregular, in a large group extending more than 5 cells up the margin; branches mostly prostrate to decumbent .....F
E. Alar cells fewer, mostly rectangular, somewhat inflated and decurrent; branches erect or ascending .....J

F. Leaves contracted to a linear, nearly entire awn, when dry imbricate and homomallous .....G
F. Leaves acuminate but with that acumen neither filiform nor entire; acumen serrulate to serrate .....H

G. Acumen filiform with several of its terminal cells arranged in a uniseriate fashion; alar region rather dense .....Brachythecium calcareum
G. Acumen narrowly linear but not uniseriate distally; alar region pellucid .....Brachythecium albicans

H. Alar region opaque and appearing darker than adjacent lamina .....Brachythecium laetum not known from CA
H. Alar region pellucid with constituent cells of larger diameter than those of adjacent lamina .....I

I. Leaves strongly falcate-secund; decurrent with cells of the decurrency rectangular, abruptly differentiated from the quadrate cells of the alar region; leaves with acumen well-demarcated; plant dioicous .....Brachythecium erythrorrhizon
I. Leaves straight or only obscurely falcate; short decurrent with cells of the decurrency only gradually different from cells of the alar region; leaves mostly only gradually contracted to apex; plant autoicous .....Brachythecium salebrosum

J. Leaves lightly to strongly falcate, sometimes with this falcation best seen as asymmetry when detached; upper leaf margin serrulate with the teeth comprised primarily of thickening of the walls on the distal end of the cells; leaves mostly less than 2.25 mm long .....Brachythecium leibergii
J. Leaves not falcate; upper leaf margin various but with teeth not so incrassate; leaves mostly more than 2.5 mm long .....K

K. Plant sparingly branched with leaves closely appressed, concave between the plications; alar and basal cells little or not at all inflated, not extending across the leaf base; margin serrate near apex, serrulate nearly to base; plant autoicous; seta mostly smooth .....Brachythecium occidentale not known from CA
K. Plants with leaves erect-spreading to loosely imbricate, not keeled along costa; alar cells inflated in the decurrent auricle; margin serrulate to serrate; plant dioicous; seta papillose .....L

L. Leaves acute to bluntly acute, mostly only lightly plicate; inflated cells; region of inflated cells often occupying a well-demarcated basal auricle and the very long decurrency; seldom with inflation of interior basal cells .....Brachythecium rivulare
L. Leaves acuminate with apex of some leaves laterally twisted, strongly plicate; region of inflated cells including both the decurrency and one or more rows of basal cells extending inwards to costa .....Brachythecium frigidum

M. Alar cells, especially within the decurrency, strongly inflated in several series .....N
M. Cells of alar and decurrency region only gradually or not at all inflated, seldom more than twice as wide as upper laminal cells .....O

N. Leaves broadly ovate to oblong-ovate, abruptly contracted to a blunt apiculus; alar region not extending to costa .....Brachythecium rivulare
N. Leaves deltoid-triangular, gradually contracted to a short or long acumen; alar region broad, reaching to costa .....Brachythecium nelsonii

O. Plant small with leaves less than 1.5 mm long; margin serrate to serrulate throughout .....P
O. Plant mostly larger; alar group poorly defined with cells short-rectangular to irregular; leaves mostly entire to low serrulate below the middle .....T

P. Leaves deeply concave with the plants somewhat julaceous; alar cells in a large group extending more than 10 cells up the margin .....Brachythecium collinum
P. Leaves not concave, or the plants larger .....Q

Q. Leaf cells mostly less than 6:1; plant dioicous; seta papillose throughout .....Brachythecium bolanderi
Q. Leaf cells longer; plant monoicous; seta variable .....R

R. Leaves appressed, not at all plicate; plant synoicous; seta smooth .....Brachythecium fendleri
R. Leaves erect-spreading to loosely ascending; plant autoicous; seta papillose .....S

S. Seta rough throughout .....Brachythecium velutinum
S. Seta smooth, or somewhat papillose at immediate base .....Brachythecium venustum

T. Leaves falcate-secund with short basal plications; alar cells short-rectangular, decurrent in a rather broad, triangular group .....Brachythecium erythrorrhizon
T. Leaves not falcate-secund; decurrency various .....U

U. Stem leaves deeply concave, abruptly narrowed to an apiculus or an acumen, often with the apices of some or all leaves twisted or reflexed .....V
U. Stem leaves not abruptly apiculate from a concave base; apices of leaves not twisted .....X

V. Stem and branch leaves abruptly narrowed to a very narrow acumen which is at least 1/4 as long as the rest of the leaf .....Cirriphyllum: C. cirrosum not known from CA
V. Leaves without such a narrow acumen .....W

W. Plant strongly julaceous with branching regular and in a single plane; leaves with an abrupt short and reflexed acumen; plant restricted to lawns and similar disturbed areas .....Pseudoscleropodium: P. purum
W. Plant neither julaceous nor with planar branching; leaves with acumen twisted; plant of medium to high elevations in the Cascades .....Brachythecium hylotapetum

X. Alar region poorly defined with the cells not at all inflated, quadrate to short-rectangular at least some of the plant often with a red-brown cast .....Y
X. Alar region moderately well-defined with the cells short rectangular, often somewhat inflated plants green .....Z

Y. Leaves with a well-defined acumen filled with elongate cells, ascending and somewhat homomallous; plant mostly mesic, on soil, logs or rocks .....Brachythecium plumosum
Y. Leaves acute to bluntly acute, without a defined acumen, with distal cell typical very short; leaves obliquely inserted, appearing somewhat complanate; plant aquatic or on very wet soil .....Leptodictyum: L. riparium

Z. Some leaves 3-5 cells decurrent with rectangular cells; median cells mostly with straight walls .....AA
Z. Leaves less decurrent with cells of the decurrency slightly inflated; median cells vermicular .....AB

AA. Stem leaves ovate lanceolate, broadest at about 1/3; acumen long but only gradually defined; seta nearly smooth .....Brachythecium holzingeri
AA. Stem leaves deltoid, broadest near the base; acumen mostly rather abruptly defined; seta coarsely papillose .....Brachythecium oedipodium

AB. Plant dioicous; dry leaves little distorted, appearing acuminate; leaves typically with a single short plica on each side .....Brachythecium asperrimum
AB. Plant autoicous; dry leaves with the margins ascending and thus appearing apiculate at apex; leaves often irregularly wrinkled but not plicate even at the base .....Brachythecium rutabulum