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Homalothecium W. P. Schimper in Bruch & W. P. Schimper, 1851.

The genus Homalothecium is here viewed as including Camptothecium. Dixon and Jameson (1924) pointed out the variation in caspule form in the usually symmetric-capsuled Homalothecium as opposed to the usually arcuate-capsuled Camptothecium. We concur in that observation. The members of both Homalothecium sensu stricto and Camptothecium sensu stricto share an overall field appearance: large mats of golden-green plants with strongly plicate, costate leaves. The dry plants of Homalothecium sensu lato consistently have the leaves somewhat twisted around the stem so as to give the leafy stem a corded appearance like that of a cotton rope. While we have lumped Homalothecium and Camptothecium into a single genus, we have followed other workers in recognizing two other separate genera on the basis of rectangular, thick-walled and pitted alar cells: Tomentypnum and Trachybryum. The seven California species in our definition of Homalothecium are easily recognized in the field. H. arenarium is the one representative of the genus without the characteristic golden-green coloration. North of the San Francisco Bay area, H. arenarium is primarily found on sand-dunes but, to the south, it moves out on to the soil of open forests and scrub. The remaining species of California Homalothecium occur on tree trunks and rock outcrops.

Key to Homalothecium Etc.

Mosses treated in this section are costate pleurocarps of mesic habitats. They have strong costae that extend into the base of the acumen. The leaves are plicate with several plicae on each side of the costa.

Species included in this key are in Brachytheciaceae, except Tomentypnum (Campyliaceae):
Homalothecium aeneum (Mitten) E. Lawton
Homalothecium arenarium (Lesquereux) E. Lawton
Homalothecium californicum, recognized from CA since 2004
Homalothecium fulgescens (Mitten ex C. Müller Hal.) E. Lawton
Homalothecium nevadense (Lesquereux) Renauld & Cardot
Homalothecium nuttallii (Wilson) A. Jaeger
Homalothecium pinnatifidum (Sullivant & Lesquereux) E. Lawton
Tomentypnum nitens (Hedwig) Loeske
Trachybryum megaptilum (Sullivant) W. B. Schofield

Preparation of materials believed to be Homalothecium requires a rather concise protocol. Most of our species of the genus are rather closely branched, often pinnate. One should take a closely branching stem and spray it with water. At that point the branches should be plucked leaving the main stem, and this main stem should have leaves carefully pulled from the stem for microscopic examination. A section of the now almost denuded stem should also be placed on the slide. Observation of the base of the leaves still inserted on the stem will show the pattern of decurrency.

A. Alar cells rectangular and thick-walled; stems plumose and loosely ascending; leaves 3-5 mm long .....B
A. Alar cells at least in part quadrate or isodiametric; main stem prostrate; leaves smaller .....C

B. Plants of high-elevation calcareous bogs; older portions of stems densely cloaked with clusters of brown rhizoids that are inserted on the abaxial surface of the leaf near the costa .....Tomentypnum: T. nitens
B. Plants of soil in mesic forests; rhizoids not so densely arranged, inserted on stem .....Trachybryum: T. megaptilum

C. Quadrate alar cells reaching costa; costa with a number of spines closely arranged on the distal portion; costa extending to near the base of the acumen; dry branches not markedly hamate .....Homalothecium arenarium
C. Alar cells not reaching costa; spines on costa inconspicuous, mostly single; length of costa usually shorter; dry branches mostly hamate .....D

D. Differentiated alar cells quadrate to short-rectangular, up to 15 on margin and 10 along the base; quadrate alar cells extending farther along margin than along immediate inframarginal regions .....E
D. Differentiated alar cells fewer; immediate margin of alar region with elongate cells .....F

E. Leaves long-decurrent; main stems up to 15 cm long .....Homalothecium californicum
E. Leaves not or very little decurrent, plant smaller .....Homalothecium pinnatifidum

F. Larger stem leaves often with a somewhat auriculate base; alar cells mostly elongate but with a few quadrate cells; urn 2.4-2.8 mm long with a short-rostrate operculum over l mm long; mostly on trees near the coast .....Homalothecium fulgescens
F. None of the stem or branch leaves at all auriculate; alar cells mostly quadrate; urn shorter with operculum conic and shorter; ecology various .....G

G. Leaves with several marginal teeth near junction of alar region with more distal region; plant usually very regularly pinnate, often with flagelliform caducous branchlets; stem leaves mostly less than 2 mm long; plant epiphytic on trees near the coast .....Homalothecium nuttallii
G. Leaves without such peribasal teeth; plant pinnate but often obscurely so, without caducous branchlets; stem leaves larger; range and geography various .....H

H. Alar cells rather obscure under low magnifications of the compound microscope with alar region more opaque than surrounding cell regions .....Homalothecium nevadense
H. Alar cells with outlines clear under low magnigication; alar region not so opaque .....Homalothecium aeneum

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