Racomitrium Bridel, 1819 [1818].

David Wagner has produced a GUIDE TO RACOMITRIOIDEAE OF OREGON, which we recommend over the 2004 key below: http://fernzenmosses.com/racomitrium_of_oregon/rackey1.htm
It includes all species found in California so far as we know.

Racomitrium are extensively branched plants that, nevertheless, have perichaetia on the ends of erect axes and are therefore acrocarpous (or cladocarpous), not pleurocarpous. The genus is costate and usually with hyaline awns. The diagnostic feature for the genus is in the lateral walls of the laminal cells. All members of the Grimmiaceae have sinuose lateral walls but those walls of Racomitrium are so much more sinuose as to provide a means of automatically recognizing the genus. For the moment, we are using old names, but Racomitrium sensu lato is now viewed as the subfamily Racomitrioideae, and the species have been segregated into various genera (FNA 2007); annotating specimens in the herbarium is in order, after which we need to update to the new names.

Key to Racomitrium Etc.

Species included in this key are all in Grimmiaceae:
Grimmia leibergii Paris
Grimmia ramondii (Lamarck & A. P. de Candolle) Margadant
Racomitrium aciculare (Hedwig) Bridel
Racomitrium affine (Schleicher ex Weber & D. Mohr) Lindberg
Racomitrium brevipes Kindberg in Macoun, not known from CA
Racomitrium depressum Lesquereux
Racomitrium elongatum Ehrhart ex Frisvoll
Racomitrium ericoides (Hedwig) Bridel
Racomitrium fasciculare (Hedwig) Bridel
Racomitrium heterostichum (Hedwig) Bridel
Racomitrium lanuginosum (Hedwig) Bridel
Racomitrium lawtonae Ireland
Racomitrium macounii Kindberg
Racomitrium microcarpon (Hedwig) Bridel
Racomitrium molle Cardot
Racomitrium norrisii Bednarek-Ochyra & Ochyra
Racomitrium obesum Frisvoll
Racomitrium occidentale (Renauld & Cardot) Renauld & Cardot
Racomitrium pacificum Ireland & Spence
Racomitrium pygmaeum Frisvoll, not known from CA
Racomitrium ryszardii Bednarek-Ochyra, not known from CA
Racomitrium sudeticum (Funck) Bruch & W. P. Schimper
Racomitrium varium (Mitten) A. Jaeger

Work with Racomitrium requires leaf cross-sections from approximately the middle of the leaf (excluding the awn). The cross-section should show the number of cell layers in the costa, the pattern of uni- and bi-stratose areas of the lamina, and it should show any flattening or grooving of the abaxial surface of the costa. We are most appreciative of the detailed studies conducted on this genus by Dr. Halina Bednarek-Ochyra. Her interpretations have been completely followed in this work. Those species of Racomitrium that are described as papillose have been described by other authors as "pseudopapillose." The papillae of Racomitrium are patterns of bumps on the surface of the transverse and lateral walls of the leaf cells, and these papillae do not appear over the lumina.

Among the more difficult to understand characters in Racomitrium is the difference between sinuosity and porosity of the lateral cell walls (see R. microcarpum). "Sinuosity" is basically a character of the Grimmiaceae and of few other mosses. The lateral wall is itself sinuose and thus retains a constant thickness on each side. Where one side of the wall curves outward, that portion of the laterally adjacent wall curves inward. "Porosity" is a character of many mosses. It is a feature of thick-walled cells with basically straight lateral walls. The irregularities of those lateral walls are formed from localized thin portions in which the walls of laterally adjacent cells have thin places coinciding on each side of the wall.

While the majority of Racomitrium in California are petricolous, some species also occur on soil and humus, especially in the northwest portion of the state. The genus also displays a wide suite of microhabitats ranging from dry walls to seasonally or regularly submerged rocks in streams and rivers.

A. Leaves with abaxial ridges on costa, never with a hyaline awn .....Grimmia: G. ramondii
A. Costa not ornamented on abaxial surface .....B

B. Leaves strictly muticous .....C
B. At least some of the leaves in a clone with an hyaline awn .....M

C. Leaf cells with low papillae over lumen or over lateral walls .....D
C. Leaf cells smooth .....I

D. Leaves oblong, with a broad, rounded, often dentate to serrulate apex; plant aquatic or in high elevation seepages .....E
D. Leaves lanceolate with the apex acuminate to rounded but always with the apex entire and considerably narrowed from below the leaf middle .....G

E. Margins at mid-leaf bistratose to 10 cells wide; papillosity at mid-leaf very low and inconspicuous; leaf apices primarily acute .....Racomitrium norrisii
E. Leaf unistratose throughout; papillosity at mid-leaf conspicuous even at lower magnifications; leaf apices primarily obtuse .....F

F. Costa ending near mid-leaf, only 2-3 cells wide distally; leaves elliptical to oblong-elliptical, broadly rounded and nearly entire at apex .....Racomitrium molle
F. Costa extending nearly to leaf apex, more than 4 cells wide distally; leaves broadly lingulate to oblong-lanceolate, rounded obtuse or even with a slight acumination, usually erose-dentate at apex .....Racomitrium aciculare

G. Alar cells much inflated; apex on at least some leaves with a hyaline awn; operculum 0.6-0.9 as long as urn .....Racomitrium varium
G. Alar cells not inflated; apex consistently muticous; operculum relatively shorter .....H

H. Median leaf cells elongate, more than 3:1; stems with numerous short, lateral branches .....Racomitrium fasciculare
H. Median leaf cells mostly isodiametric; stems without a close grouping of short lateral branches .....Racomitrium ryszardii not known from CA

I. Leaves somewhat laterally twisted when dry; margin bistratose in 2-4 rows at least from near the apex to below the middle; plants with a red brown cast .....Racomitrium macounii
I. Leaves appressed to falcate when dry; margin unistratose throughout or with rather more limited bistratose streaks; plants mostly without such a red brown cast .....J

J. Leaf apices bluntly rounded, often crenulate; margin unistratose, usually somewhat crenulate distally, inconspicuously recurved, often plane on one side .....K
J. Leaf apices acute to bluntly acute, not at all crenulate; margin often in part bistratose, mostly with a rather long area of recurvature on both sides especially near the leaf middle .....L

K. Leaves concave, mostly longer than 3 mm; alar cells mostly thin-walled, sometimes decurrent, in a poorly demarcated region; plants growing in streams, mostly in high mountains .....Racomitrium depressum
K. Leaves smaller and not concave; alar cells short and thick-walled, often porose, in a well demarcated, often auriculate group; plants growing on wet rocks in lowlands .....Racomitrium pacificum

L. Costa mostly less than 80 µm broad near base, near abaxial apex so convex as to appear almost cylindrical; leaf apex rather gradually acuminate; inner perichaetial leaves only slightly differentiated from adjacent vegetative leaves .....Racomitrium sudeticum
L. Costa mostly broader, near apex not so abaxially convex; leaf apex acute; inner perichaetial leaves very much reduced in size, hyaline or nearly so .....Racomitrium affine

M. Hyaline awn prominently papillose .....N
M. Hyaline awn smooth or absent (note that the awn may sometimes be toothed and appear papillose) .....Q

N. Cells of lamina smooth, with a lumen:wall ratio less than l:1; hyaline awn decurrent a distance almost equal to the length of the rest of the awn .....Racomitrium lanuginosum
N. Cells of lamina papillose; walls thinner; awn decurrency short or absent .....O

O. Costa ending at about 1/2-3/4, frequently forked distally; leaves mostly less than 3:1, not keeled except at extreme apex .....Racomitrium ericoides
O. Costa extending to the base of the acumen or beyond, not forked distally; leaves mostly more than 4:1, strongly keeled in distal 1/3 .....P

P. Leaves with distinct and somewhat inflated alar cells; costa obscured by its position in the deep central keel; abundant plant on most substrata and at most altitudes .....Racomitrium elongatum
P. Leaves with alar cells not so inflated; costa not so obscured; plant rarely present at higher elevations in Washington State .....Racomitrium pygmaeum not known from CA

Q. Costa strongly convex abaxially, distally grooved or ridged; awn terete, strongly spinulose, not flexuose; plant green and sparsely branched .....Racomitrium occidentale
Q. Costa not distally grooved or ridged .....R

R. Median leaf cells with small to rather large papillae over the lateral walls .....S
R. Median leaf cells smooth in lateral or cross-sectional view .....T

S. Plant mostly gray- to glaucous green; alar cells much inflated; costa at mid leaf lying in a poorly defined channel, less than 8 cells wide; operculum 0.6-0.9 times as long as urn .....Racomitrium varium
S. Plants olive-green to fuscous or blackish; alar cells poorly differentiated; costa at mid-leaf in a well-defined channel, about 8 cells wide; operculum less than 0.5 times the length of the urn .....Racomitrium brevipes not known from CA

T. Leaf margins at mid-leaf bistratose in a band two or more cells wide; costa dorsally convex and mostly 4-stratose; plants brown to russet .....Racomitrium macounii
T. Leaf margins at mid-leaf unistratose or with the bistratosity only one cell wide; costa mostly not so thick; plants green to blackish green .....U

U. Basal cells of leaf thick-walled and porose but without sinuosities; basal marginal cells forming a distinct limbidium of several cell rows; costa near base only 3-4 cells wide .....Racomitrium microcarpon
U. Basal cells of leaf not differentiated, not forming a limbidium; costa near base 5-9 cells wide .....V

V. Basal marginal cells quadrate to rectangular with transverse walls markedly thicker than lateral walls; costa near base flattened and somewhat grooved, usually with a small and thin abaxial stereid band .....Grimmia: G. leibergii
V. Basal marginal cells mostly rectangular without transverse wall thickening; costa near the base not strongly flattened or grooved; mostly without stereids .....W

W. Margin recurved from near the base to near the awn; awn of dry leaf mostly not squarrose .....X
W. Margin mostly not recurved above the middle; awn of dry leaf usually at least somewhat squarrose .....Z

X. Leaf margin at mid-leaf mostly bistratose; leaf lamina somewhat contracted at intersection with the terete and rigid awn .....Racomitrium obesum
X. Leaf margin at mid-leaf mostly unistratose; leaf lamina not at all contracted; awn rather broad and flat at base .....Y

Y. Costa adaxially grooved to canaliculate immediately below mid-leaf; costa cross-section mostly only 2 cells thick at mid-leaf .....Racomitrium heterostichum
Y. Costa not at all adaxially grooved, or perhaps slightly so near leaf apex; costa cross-section mostly 3 cells thick at mid-leaf .....Racomitrium affine