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.		Didymodon johansenii
+	Distal laminal cells 8--10 µm wide.		Didymodon anserinocapitatus
4 (2)	Leaves 2--3 mm, leaf margins broadly crenate in distal 2/3--3/4.		Didymodon eckeliae
+	Leaves usually less than 2 mm, leaf margins entire.		(5)
5 (4)	Cells of leaf apex smooth, apex merely fragile.		Didymodon rigidulus var. icmadophilus
+	Cells of leaf apex weakly conic-mamillose; apex caducous.		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 midleaf, laminal papillae absent or hemispherical and several per lumen, peristome absent.		Didymodon norrisii
+	Plants in nature red- to black-brown, costa thin, 2–3 cells wide above midleaf, 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.		Didymodon subandreaeoides
+	Leaves monomorphic.		(9)
9 (8)	Plants often fruiting, leaf apices acute, propagula absent.		Di
+	Plants sterile, leaf apices obtuse, clusters of unicellular propagula almost always present in the leaf axils.		Didymodon perobtusus
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 whip-like, very long-acuminate.		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.		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.		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.		Didymodon fallax
+	Leaves usually 2.5--4 mm, distal cell walls irregularly thickened and trigonous, trigones as large as the lumens or nearly so.		Didymodon giganteus
16 (14)	Stems to 2.5 cm, leaves usually 0.8--2 mm.		Didymodon ferrugineus
+	Stems usually more than 3 cm, leaves mostly 2--2.5 mm.		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 midleaf, 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.		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 midleaf 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.		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.		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.		Didymodon tectorum
22 (19)	Leaves 1-stratose, or 2-stratose in small areas or patches marginally.		Didymodon vinealis
+	Leaves 2-stratose marginally or medially or both.		(23)
23 (22)	Leaves long-lanceolate, apex narrowly acute, margins evenly and broadly crenulate above leaf base, 2-stratose in 1--2 rows to near base.		Didymodon eckeliae
+	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.		Didymodon nicholsonii
24 (24)	Axillary gemmae present.		(24)
+	Axillary gemmae absent.		(27)
25	Propagula all multicellular, leaf apex acute.		Didymodon rigidulus var. rigidulus
+	Propagula mostly unicellular, leaf apex broadly obtuse.		(26)
26 (25)	Costa narrow, usually 2 cells wide at midleaf, not strongly spurred.		Didymodon perobtusus
+	Costa broad, 6--8 cells wide at midleaf and often strongly spurred by rows of lateral cells ending abruptly in the lamina.		Didymodon revolutus
27 (24)	Distal lamina 1-stratose or occasionally 2-stratose in small patches.		(28)
+	Distal lamina 2-stratose totally or just along margins.		(31)
28 (27)	Plants flagellate, leaves strongly appressed when dry, linear-lanceolate, costa long-excurrent.		Didymodon rigidulus var. ditrichoides
+	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 short-ovate.		Didymodon revolutus
+	Leaves lanceolate.		(30)
30 (29)	Leaf base rectangular and gradually widened, costa percurrent to short-excurrent, unbroken, basal leaf cells short-rectangular		Didymodon rigidulus var. gracilis
+	Leaf base ovate and often abruptly widened, costa usually long-excurrent, often fragile, basal leaf cells quadrate.		Didymodon rigidulus var. icmadophilus
31 (27)	Distal lamina entirely 2-stratose.		Didymodon rigidulus var. subulatus
+	Distal lamina 2-stratose along margins.		(32)
32 (31)	Basal laminal cells with firm, weakly to strongly thickened walls, differentiated usually only medially.		Didymodon rigidulus var. rigidulus
+	Basal laminal cells thin-walled and usually somewhat inflated, often bulging-rectangular, differentiated across leaf base.		(33)
33 (32)	Leaves long-lanceolate, usually smooth or weakly papillose, marginal basal cells narrowly rectangular in 2--4 rows, adaxial superficial cells of costa usually elongate.		Didymodon umbrosus
+	Leaves short-lanceolate, smooth to strongly papillose, marginal basal cells not or weakly differentiated from the medial, adaxial superficial cells of costa quadrate.		Didymodon australasiae