Plagiothecium Bruch & W. P. Schimper, 1852.

The genera included here (except Dacryophyllum and Vesicularia) have been placed by various authors in Plagiothecium, but this is now generally viewed as the single member of the family Plagiotheciaceae with the others placed in the large family Hypnaceae. The genus Plagiothecium in the older, broad sense included all mosses with complanate arrangement of the ecostate leaves. The rearrangement of familial and generic placements comes from studies of branching patterns and pseudoparaphyllial morphology as well as of outer cortical morphology of the stem. As now defined, the genus Plagiothecium has strongly decurrent leaves, no pseudoparaphyllia, and a differentiation of a stem hyaloderm. Our species of Plagiothecium sensu stricto are mostly plants of humid sites of the northwestern corner of the state.

Key to Plagiothecium Etc.

The mosses keyed here are ecostate pleurocarps mostly with a prostrate growth habit. Most have a clearly complanate leaf arrangement; those with falcate leaves or hamate stems are here keyed under Hypnaceae.

Species included in this key are in Hypnaceae, except Plagiothecium (Plagiotheciaceae):
Buckiella undulata (Hedwig) Ireland
Dacryophyllum falcifolium Ireland
Herzogiella seligeri (Bridel) Iwatsuki
Herzogiella striatella (Bridel) Iwatsuki
Isopterygiopsis pulchella (Hedwig) Iwatsuki
Isopterygium tenerum (Swartz) Mitten
Plagiothecium cavifolium (Bridel) Iwatsuki
Plagiothecium denticulatum (Hedwig) Bruch & W. P. Schimper
Plagiothecium laetum Bruch & W. P. Schimper
Plagiothecium piliferum (Swartz ex C. J. Hartman) Bruch & W. P. Schimper
Pseudotaxiphyllum elegans (Bridel) Iwatsuki
Vesicularia vesicularis (Schwägrichen) Brotherus

A. Leaves strongly decurrent .....B
A. Leaves not decurrent or with decurrency formed by a group of only 1-3 cells in total .....F

B. Leaves long-acuminate .....C
B. Leaves acute to obtuse or short-acuminate .....D

C. Leaves gradually long-acuminate, closely serrate to serrulate; central basal cells thick-walled and pitted; capsule sulcate .....Herzogiella: H. striatella
C. Leaves abruptly long-acuminate with the acumen variously twisted or contorted away from stem axis; acumen remotely serrulate to almost entire; median basal cells thin-walled without pitting; capsule smooth .....Plagiothecium piliferum

D. Decurrency broad and somewhat auriculate; cells in decurrency markedly inflated and usually 2-4:1 .....Plagiothecium denticulatum
D. Decurrency mostly only 1-3 cells broad, narrowing to a single cell, not at all auriculate; cells in decurrency not inflated, mostly 5-10:1 .....E

E. Median leaf cells mostly less than 10 µm wide; plant strongly complanate throughout .....Plagiothecium laetum
E. Median leaf cells mostly broader; plant julaceous or with some branches julaceous .....Plagiothecium cavifolium

F. Leaves mostly more than 3 mm long, lightly undulate; median leaf cells with cuticular papillosity (view under high magnification and at low light intensity .....Buckiella: B. undulata
F. Leaves markedly shorter, not undulate; median leaf cells completely smooth .....G

G. Basal cells strongly pitted; leaves serrate to serrulate; capsule sulcate .....H
G. Leaves entire to serrulate; basal cells not much pitted; capsule smooth .....I

H. Leaves long-acuminate, strongly decurrent with somewhat inflated rectangular, often reddened cells .....Herzogiella: H. striatella
H. Leaves acute to short acuminate, almost not at all decurrent .....Herzogiella: H. seligeri

I. Leaves broadly ovate with median cells more than 12 µm broad, mostly less than 3:1; alar cells not differentiated .....Vesicularia: V. vesicularis
I. Leaves lanceolate to ovate-lanceolate; median cells more narrow and relatively longer; alar cells various .....J

J. Leaves cultriform with distal one-half falcate; pseudoparaphyllia narrowly lanceolate .....Dacryophyllum: D. falcifolium
J. Leaves mostly symmetric or falcate; pseudoparaphyllia absent or filamentose .....K

K. Alar cells well demarcated, quadrate to short rectangular, thin-walled and vesicular in part; pseudoparaphyllia filamentose; gemmae filamentose and papillose .....Isopterygium: I. tenerum
K. Alar cells not well differentiated; pseudoparaphyllia absent; gemmae various, not papillose .....L

L. Gemmae usually present in the axils of upper leaves, these gemmae being clusters of microphyllous, caducous branchlets; margin serrulate or even slightly serrate near apex; costa often extending to 1/3 of leaf .....Pseudotaxiphyllum: P. elegans
L. Gemmae uncommon, fusiform, 4-6 celled; margin almost completely entire; costa very short and sometimes not at all visible .....Isopterygiopsis: P. pulchella