Hypnum Hedwig, 1801, nomen conservandum.

NEEDS GENUS PARAGRAPH

Key to Hypnum Etc.

The mosses keyed in this section are ecostate pleurocarps mostly with a prostrate growth habit. Most have falcate leaves or their leafy stems are hamate when dry. There exists another group of ecostate pleurocarps with a prostrate growth habit, but these are largely complanate. Such mosses are keyed under Plagiothecium.

Species included in this key are all in Hypnaceae, except Sematophyllum (Sematophyllaceae):
Breidleria pratensis (Koch) Loeske, not known from CA
Calliergonella cuspidata (Hedwig) Loeske
Herzogiella seligeri (Bridel) Iwatsuki
Herzogiella striatella (Bridel) Iwatsuki
Homomallium mexicanum Cardot
Hypnum callichroum Bridel, not known from CA
Hypnum circinale W. J. Hooker
Hypnum cupressiforme Hedwig, not known from CA
Hypnum dieckii Renauld & Cardot
Hypnum fertile Sendtner, not known from CA
Hypnum lindbergii Mitten
Hypnum revolutum (Mitten) Lindberg
Hypnum subimponens Lesquereux
Hypnum vaucheri Lesquereux
Isopterygiopsis pulchella (Hedwig) Iwatsuki
Orthothecium chryseum (Schwägrichen in Schultes) W. P. Schimper in Bruch & W. P. Schimper, not known from CA
Platydictya jungermannioides (Bridel) H. Crum
Ptilium crista-castrensis (Hedwig) De Notaris, not known from CA
Sematophyllum adnatum (Michaux) E. Britton
Vesicularia vesicularis (Schwägrichen) Brotherus

The family Hypnaceae is easily recognized in the field by the combination of strongly falcate-secund leaves and very short costa. The presence or absence of an hyaloderm is especially important in this group. That feature is easily seen in a stem cross-section except that older stems have all the large and thin-walled cells of that hyaloderm collapsed. The resulting crenulate outer surface of the stem cross-section should be recognized as the product of such hyaloderm cell collapse. There is an unfortunate tendency for some of the hyaloderm cells to be removed with the leaves, and this may result in misinterpretations of those cells as alar or basal cells.

Inflated alar cells can be of special importance in Hypnum. Uncertainty about where the extreme base of the leaf joins the hyalodermous stem, can be resolved with an examination of the bases of leaves not detached from the stem. Another feature of importance in the Hypnum group is the difference between falcate leaves and circinate leaves. After removal of a leaf from the stem note the arc described by the mid-line of that leaf. Falcate leaves describe less than 180 degrees of arc; circinate leaves describe more than 180 degrees of arc.

A. Leaf margins recurved near the base or weakly reflexed nearly to apex, entire throughout or with a few weak teeth near the apex .....B
A. Leaf margins plane to ascending, clearly serrulate to serrate in distal 1/2 .....E

B. Leaves with many parallel plications reaching above mid-leaf; median basal cells porose .....Orthothecium: O. chryseum not known from CA
B. Leaves not plicate; median basal cells not or hardly porose .....C

C. Leaves only lightly concave with basal margins strongly recurved; alar cells quadrate in numerous rows, hardly inflated .....Hypnum revolutum
C. Leaves deeply concave with a weak reflexing along nearly the entire margin .....D

D. Alar cells quadrate but only gradually differerentiated from median laminal cells; leaves with a very strong contraction to the acuminate apex .....Homomallium: H. mexicanum
D. Alar cells strongly differentiated, inflated in one basal row of 2-4 cells; leaves rather gradually contracted to the acumen .....Sematophyllum: S. adnatum

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

F. The majority of the alar cells quadrate and not inflated, sometimes with a few inflated cells at extreme angles; quadrate alar cells mostly extending more than 10 cells up margin .....G
F. Quadrate alar cells few, mostly subtended by several inflated cells at basal angles .....I

G. Leaves 0.5-1.0 mm long, not falcate but sometimes secund or homomallous, spreading and distant or concave and julaceous; alar cells quadrate in a well demarcated patch; multicellular gemmae often present in leaf axils .....Platydictya: P. jungermannioides
G. Leaves larger, mostly falcate-secund; alar cells mostly few in a poorly demarcated patch; gemmae seldom present .....H

H. Leaves rather strongly concave, abruptly contracted to the acumen; median cells mostly less than 5:1; alar cells mostly more than 15 along margin .....Hypnum vaucheri
H. Leaves concave or not, gradually contracted to the acumen; median cells mostly more than 6:1; alar cells fewer than 12 along margin .....Hypnum cupressiforme not known from CA

I. Leaves strongly plicate and decurrent; plant so regularly pinnate and plumose as to have the branches parallel and contiguous; branches strongly hamate at apices .....Ptilium: P. crista-castrensis not known from CA
I. Leaves lightly, or not at all, plicate; plant irregularly to regularly pinnate, never plumose and ascending; branches homomallous but not hamate .....J

J. Outer cortical cells of stem hyaline, thin-walled and enlarged relative to the adjacent inner cells .....K
J. Outer cortical cells not enlarged .....S

K. Alar cells strongly hyaline and inflated, mostly >5 times as broad as the adjacent laminal cells .....L
K. Alar cells mostly not hyaline, less inflated, mostly with at least some wall thickening .....O

L. Area of inflated and hyaline alar cells extending 3 or more cells up the margin and most of the way to mid-leaf; leaves homomallous but not very falcate .....M
L. Area of inflated and hyaline alar cells inconspicuous and often left on stem with careless dissection, including only 1-2 rows of cells along the basal margin; plant mostly conspicuously falcate-secund .....N

M. Apices of stems and major branches with the leaves so tightly imbricate as to form an acicular tip; stem leaves typically nearly entire even near apex; leaves straight, never falcate .....Calliergonella: C. cuspidata
M. Stem and branch apices never acicular; stem leaves markedly serrulate near apex; leaves falcate-secund .....Hypnum lindbergii

N. Leaves rounded at the base in such a manner that the breadth of the insertion is only about 1/2 as great as the maximum leaf breadth immediately above that insertion; leaves straight to slightly falcate; leaf apices broadly acute to somewhat obtuse .....Breidleria: B. pratensis not known from CA
N. Leaf bases not so rounded; breadth of insertion mostly more than 3/4 as great as the maximum leaf breadth; leaves falcate to somewhat circinate; leaf apices so long and narrow as to be less than 8 cells broad within 3 cell lengths of apex .....O

O. Leaves rather strongly serrulate; median basal cells strongly pitted; capsule sulcate .....P
O. Leaves entire to serrulate; median basal cells not pitted; capsule smooth .....Q

P. Leaves strongly decurrent with inflated rectangular cells in a triangular patch 3-5 cells wide distally .....Herzogiella: H. striatella
P. Leaves not significantly decurrent (be careful that you do not interpret cells of the hyaloderm as decurrencies) .....Herzogiella: H. seligeri

Q. Stem leaves falcate from a somewhat erect base; basal marginal cells above the hyaloderm mostly quadrate to short rectangular, or with one or two somewhat inflated cells at extreme basal angles .....Hypnum subimponens
Q. Stem and branch leaves falcate to circinate from their bases; basal marginal cells above the hyaloderm cells of the stem enlarged relative to adjacent cells of the leaf .....R

R. Leaves serrulate at least at the apex; plants regularly pinnate with a red-brown wash on older stems and leaves; extreme leaf apex not filiform .....Hypnum dieckii
R. Leaves entire throughout; plants irregularly branched, green without a brownish wash; some leaves uniseriate for 2-3 cells at immediate apex .....Hypnum callichroum not known from CA

S. Quadrate alar cells numerous, mostly exceeding 15; plant olive-green and regularly pinnate, especially common on bark of coniferous trees .....Hypnum circinale
S. Quadrate alar cells fewer than 5; plant of soil and rocks mostly at high elevations .....Isopterygiopsis: I. pulchella