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FNA | Family List | FNA Vol. 27 | Pottiaceae

16. Didymodon Hedwig, Sp. Musc. Frond. 104. 1801.

[Greek didymos, double or twin, and odon, tooth, alluding to paired peristome teeth]

Richard H. Zander

Barbula sect. Acutae Steere; Barbula subsect. Acutiformes Kindberg; Barbula sect. Asteriscium Müller Hal.; Barbula sect. Fallaces (De Notaris) Steere; Barbula subsect. Fallaciformes Kindberg; Barbula sect. Graciles Milde; Barbula sect. Rubiginosae Steere; Barbula sect. Vineales Steere; Barbula subsect. Vinealiformes Kindberg; Dactylhymenium Cardot; Didymodon sect. Asteriscium (Müller Hal.) R. H. Zander; Didymodon sect. Fallaces (De Notaris) R. H. Zander; Didymodon sect. Graciles (Milde) K. Saito; Didymodon sect. Vineales (Steere) R. H. Zander; Geheebia Schimper; Husnotiella Cardot; Trichostomopsis Cardot

Plants forming turfs or cushions, light to blackish, olive or reddish green distally, brown to reddish brown or tan proximally. Stems to 2(-9) cm; hyalodermis occasionally present, sclerodermis usually present, central strand usually distinct; axillary hairs ca. 5 cells long, proximal 1-2 cells brownish. Leaves crowded, appressed-incurved, occasionally twisted or curled when dry, spreading to strongly reflexed and occasionally keeled when moist; ovate to long-lanceolate or long-triangular, adaxial surface usually broadly concave, occasionally narrowly channeled, 0.4-3(-6) mm; base weakly differentiated in shape to oblong and half-sheathing the stem, proximal margins occasionally decurrent; distal margins plane to recurved or revolute, entire or occasionally weakly dentate or crenulate, occasionally 2-stratose in patches or entirely; apex narrowly acute to rounded, occasionally fragile or caducous; costa ending several cells before apex to short-excurrent, adaxial cells quadrate to elongate, in 2-4(-8) rows; transverse section ovate, semicircular or reniform, adaxial epidermis present or seldom absent (rarely bulging as a 1-stratose pad of cells in D. nevadensis), adaxial stereid band usually weak, occasionally absent in small plants or entirely in D. australasiae, D. revolutus and D. umbrosus, guide cells 2-6 in 1(-2) layers, hydroid strand occasionally present, abaxial stereid band weak to strong, crescent-shaped or ovate in sectional shape, abaxial epidermis present but usually weak; basal laminal cells not or weakly differentiated to strongly differentiated across leaf or rising higher medially, quadrate to rectangular, seldom bulging, usually little wider than distal cells, ca. 2-4:1, walls of basal cells usually rather thin, occasionally porose or transversely slit (D. umbrosus), smooth to papillose; distal medial cells subquadrate to hexagonal or rounded-angular, occasionally shortly rectangular or rhomboid, usually 8-13 µm wide, 1:1, 1-stratose, papillae usually simple to 2-fid, usually solid, with 3-5 projections, occasionally absent or multiplex, cell walls thin to thick, lumens sometimes angular, occasionally trigonous (D. giganteus), weakly to strongly convex on both surfaces. Specialized asexual reproduction by gemmae occasional, usually spheric to elliptic, of 1-10 cells, usually borne in leaf axils, occasionally on adaxial surface of costa, or rarely as small tubers borne on rhizoids in the soil. Sexual condition dioicous (occasionally possibly rhizautoicous). Perichaetia terminal, interior leaves not or occasionally sheathing in proximal 1/2, ovate to long-lanceolate, occasionally somewhat enlarged, laminal cells rhombiodal-rectangular in proximal 1/2. Seta yellowish to reddish brown, mostly 0.3-2 cm, twisted clockwise proximally, occasionally counterclockwise distally. Capsule stegocarpous, theca elliptic to cylindric, ca. 1-3 mm, annulus of 1-3 rows of hexagonal, often vesiculose cells, often deciduous in pieces or revolute; operculum short- to long-conic or conic-rostrate, ca. 0.5-1.2 mm, cells in straight rows or twisted counterclockwise; peristome teeth 32, or 16 and grouped in pairs, occasionally absent or rudimentary, oblong to linear or long-triangular, often medially perforated or cleft, usually straight or weakly twisted counterclockwise, teeth to 700(-1300) µm, sometimes rudimentary. Calyptra cucullate. Spores ca. 7-18 µm. KOH laminal color reaction red, occasionally red-orange or yellow.

Species ca. 122 (26 in the flora): worldwide.

The genus Didymodon is distinguished (K. Saito 1975; R. H. Zander 1978d) from its closest relative, Barbula, by its usually lanceolate to long-lanceolate leaves, axillary hairs with 1 or more brown proximal cells, basal laminal cells usually little differentiated from the distal, abaxial costal cells usually quadrate, laminal papillae absent or simple or only occasionally multiplex, gemmae composed of only 1-10 cells, and peristome teeth seldom twisted, occasionally absent or rudimentary. Recent treatments of the genus or its sections in the flora area or Mexico include those by H. Robinson (1970), W. C. Steere (1938b) and Zander (1978d, 1981b, 1994k). The leaves of most species of Didymodon are yellow or orange in nature (before application of KOH), unlike those of Bryoerythrophyllum, which are red.


Bartram, E. B. 1926. Notes on the genus Husnotiella Cardot. Bryologist 29: 44-46. Robinson, H. 1970. A revision of the moss genus Trichostomopsis. Phytologia 20: 184-191. Steere, W. C. 1938b. Didymodon. In: A. J. Grout. 1928-1940. Moss Flora of North America, North of Mexico. 3 vols. in 12 parts. Newfane, Vt. and New York. Vol. 1, pp. 185-190. Williams, R. S. 1913b. The genus Husnotiella Cardot. Bryologist 16: 25. Zander, R. H. 1978d. New combinations in Didymodon (Musci) and a key to the taxa in North America north of Mexico. Phytologia 41: 11-32. Zander, R. H. 1981b. Didymodon (Pottiaceae) in Mexico and California: Taxonomy and nomenclature of discontinuous and nondiscontinuous taxa. Cryptog. Bryol. Lichénol. 2: 379-422. Zander, R. H. 1994k. Didymodon. In: A. J. Sharp et al., eds. The Moss flora of Mexico. Mem. New York Bot. Gard. 69: 299-319. Zander, R. H. 1998. A phylogrammatic evolutionary analysis of the moss genus Didymodon in North America north of Mexico. Bull. Buffalo Soc. Nat. Sci. 36: 81-115.

1 Leaf apices caducous or very fragile   (2)
+ Leaf apices intact or only occasionally broken   (6)
2 (1) Leaf apices apically swollen as a propagulum   (3)
+ Leaf apices not swollen, usually evenly narrowing   (4)
3 (2) Distal laminal cells 13-15 µm wide.   2 Didymodon johansenii
+ Distal laminal cells 8-10 µm wide.   3 Didymodon anserinocapitatus
4 (2) Leaves 2-3 mm, leaf margins broadly crenate or weakly notched in distal 2/3-3/4   12 Didymodon eckeliae (in part)
+ Leaves usually less than 2 mm, leaf margins entire   (5)
5 (4) Cells of leaf apex smooth; apex merely fragile.   1 Didymodon rigidulus (in part)
+ Cells of leaf apex weakly conic-mammillose; apex caducous.   17 Didymodon murrayae
6 (1) Leaves not keeled when moist, not highly recurved, margins finely crenulate by bulging cell walls   (7)
+ Leaves sometimes keeled or highly recurved when moist, margins usually entire or occasionally dentate or scalloped-crenate but not minutely crenulate   (10)
7 (6) Plants in nature red-brown, occasionally deep brick or rose red, costa thick, 6-10 cells wide above mid leaf, laminal papillae absent or hemispherical and several per lumen, peristome absent.   7 Didymodon norrisii
+ Plants in nature red- to black-brown, costa thin, 2-3 cells wide above mid leaf, laminal papillae absent or low, massive and lens-shaped, peristome present or capsules unknown   (8)
8 (7) Leaves dimorphic: cochleariform, epapillose leaves present on fragile branchlets or portions of some stems.   10 Didymodon subandreaeoides
+ Leaves monomorphic   (9)
9 (8) Plants often fruiting, leaf apices acute, propagula absent.   8 Didymodon nigrescens
+ Plants sterile, leaf apices obtuse, clusters of unicellular propagula almost always present in the leaf axils.   9 Didymodon perobtusus (in part)
10 (6) Costa with elongate superficial adaxial cells   (11)
+ Costa with quadrate or occasionally short-rectangular superficial adaxial cells, or, if elongate, then distal laminal cells 2-stratose   (17)
11 (10) Leaf base auriculate or weakly winged at insertion, apex often whiplike, very long-acuminate.   21 Didymodon leskeoides
+ Leaf base gradually or quickly narrowed to the insertion, not flaring, apex obtuse to acuminate   (12)
12 (11) Leaves ligulate to ovate-lanceolate, apex often obtuse, costa often ending before the apex.   24 Didymodon tophaceus
+ Leaves short- to long-lanceolate, apex always acute, costa subpercurrent to short-excurrent   (13)
13 (12) Plants with axillary gemmae, leaves mostly 0.9-1.1 mm, catenulate- incurved when dry.   20 Didymodon maschalogena
+ Plants lacking gemmae in leaf axils, leaves usually (0.6-)1.2-4 mm, appressed-incurved to weakly spreading when dry   (14)
14 (13) Leaves when moist spreading to weakly recurved, usually lying flat, costa usually distinctly widened at base   (15)
+ Leaves when moist strongly recurved and keeled, lying on their sides, costa little widened at base   (16)
15 (14) Leaves 0.6-2(-2.5) mm, distal cell walls little thickened or irregularly thickened, trigones absent or weakly developed.   18 Didymodon fallax
+ Leaves usually 2.5-4 mm, distal cell walls irregularly thickened and trigonous, trigones as large as the lumens or nearly so.   23 Didymodon giganteus
16 (14) Stems to 2.5 cm, leaves usually 0.8-2 mm.   19 Didymodon ferrugineus
+ Stems usually more than 3 cm, leaves mostly 2-2.5 mm.   22 Didymodon maximus
17 (10) Leaves adaxially with a narrow medial channel about the width of the costa at least at leaf apex, apex often apiculate by one or more conical cells, costa usually percurrent, margins usually recurved, often to near the apex, laminal color reaction to KOH usually brick-red, occasionally orange   (18)
+ Leaves adaxially very widely channeled medially or merely slightly concave across leaf, apex seldom apiculate by a conical cell, costa percurrent or excurrent as a multicellular, stout mucro, margins plane to recurved below mid leaf, laminal color reaction to KOH usually negative, yellow or orange, occasionally brick-red in lower parts of plant, especially in D. rigidulus var. icmadophilus   (24)
18 (17) Leaves strongly reflexed and keeled when moist, papillae when present simple, stem central strand usually absent   25 Didymodon asperifolius
+ Leaves spreading to weakly reflexed and occasionally weakly keeled when moist, papillae when present 2-fid to multiplex, stem central strand present   (19)
19 (18) Leaves deltoid to short-lanceolate or ovate, to 1.5 or rarely to 2 mm, margins recurved or revolute to near apex, propagula sometimes present   (20)
+ Leaves short- to long-lanceolate or long-triangular, to 4 mm, margins recurved near base or up to proximal 2/3 of leaf, propagula rare   (22)
20 (19) Costal section showing adaxial epidermal cells thin-walled, remainder of costa thick-walled; costa blunt apically, costa wider at mid leaf than below, with a bulging adaxial surface forming a long-elliptic 1-stratose pad of cells, guide cells in 2(-3) layers, leaf margins loosely revolute.   16 Didymodon nevadensis
+ Costal section showing all cells about equally thickened; costa often with an apical conical cell or costa short-excurrent, costa gradually narrowing distally, adaxial surface nearly flat and not forming a wide pad of cells (but costa occasionally thickened and bulging adaxially), guide cells usually in 1 layer, leaf margins narrowly to loosely recurved   (21)
21 (20) Leaves ovate or ovate-lanceolate, 0.7-1 mm, base ovate or weakly differentiated, apex cucullate or weakly concave, margins weakly recurved, costa percurrent or very weakly excurrent from an obtuse or acute apex in 1-3 cells; lamina red in nature and with KOH, seldom green and KOH negative.   14 Didymodon brachyphyllus
+ Leaves deltoid to deltoid-lanceolate, 1-1.5(-2) mm, base squared, apex straight or somewhat reflexed, margins strongly recurved to revolute, costa excurrent from an obtuse apex as a several-celled blunt mucro; lamina green in nature and with KOH.   15 Didymodon tectorum
22 (19) Leaves 1-stratose, or 2-stratose in small areas or patches marginally.   11 Didymodon vinealis
+ Leaves 2-stratose marginally or medially or both   (23)
23 (22) Leaves long-lanceolate, apex narrowly acute, margins 2-stratose in 1-2 rows distally to near base, often evenly and broadly crenulate beyond leaf base or weakly notched.   12 Didymodon eckeliae (in part)
+ Leaves long-ovate to broadly lanceolate, seldom narrowly acute, apex usually blunt to broadly acute, margins smooth and 2-stratose marginally in 1-several rows or rarely across leaf in distal leaf half or occasionally only in patches.   13 Didymodon nicholsonii
24 (17) Axillary gemmae present   (25)
+ Axillary gemmae absent   (27)
25 (24) Propagula all multicellular, leaf apex acute.   1 Didymodon rigidulus (in part)
+ Propagula mostly unicellular, leaf apex broadly obtuse   (26)
26 (25) Costa narrow, usually 2 cells wide at mid leaf, not strongly spurred.   9 Didymodon perobtusus (in part)
+ Costa broad, 6-8 cells wide at mid leaf and often strongly spurred by rows of lateral cells ending abruptly in the lamina.   6 Didymodon revolutus (in part)
27 (24) Distal lamina 1-stratose or occasionally 2-stratose in small patches   (28)
+ Distal lamina 2-stratose totally or just along margins   (30)
28 (27) Plants flagellate, leaves strongly appressed when dry, linear-lanceolate, costa long- excurrent   1 Didymodon rigidulus (in part)
+ Plants not flagellate, leaves appressed-incurved to weakly twisted and weakly spreading when dry, short- to long-lanceolate, costa short- to long-excurrent   (29)
29 (28) Leaves lanceolate.   1 Didymodon rigidulus (in part)
+ Leaves short-ovate.   6 Didymodon revolutus (in part)
30 (27) Distal lamina entirely 2-stratose   (31)
+ Distal lamina 2-stratose along margins   (32)
31 (30) Leaves long-lanceolate, costa excurrent as a subula.   1 Didymodon rigidulus (in part)
+ Leaves long-ovate to elliptic-lanceolate, costa percurrent.   26 Didymodon bistratosus
32 (30) Basal laminal cells with firm, weakly to strongly thickened walls, differentiated usually only medially.   1 Didymodon rigidulus (in part)
+ Basal laminal cells thin-walled and usually somewhat inflated, often bulging-rectangular, differentiated across leaf base   (33)
33 (32) Leaves short-lanceolate, smooth to strongly papillose, marginal basal cells not or weakly differentiated from the medial, adaxial superficial cells of costa quadrate.   4 Didymodon australasiae
+ Leaves long-lanceolate, usually smooth or weakly papillose, marginal basal cells narrowly rectangular in 2-4 rows, adaxial superficial cells of costa usually elongate.   5 Didymodon umbrosus

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