Amblystegium Bruch & W. P. Schimper, 1853.

Amblystegium is a genus of small to medium-sized pleurocarpous mosses of moist to aquatic sites. It is the type genus of the traditional family Amblystegiaceae that has been substantially divided by modern workers. Three new families concern us in California. Two of these differ from the narrowed Amblystegiaceae in the presence of paraphyllia: the Cratoneuraceae and the Helodiaceae. The fourth family, Campyliaceae, includes a number of generally larger plants with falcate or squarrose or reflexed, often plicate leaves, often with hyaloderm layers on the stems, and often with blunt apices. None of these latter features characterizes the plants in the present key. The Amblystegiaceae in the older sense included the majority of the pleurocarpous and costate mosses of aquatic to semiaquatic sites. These were contrasted with the similar mosses of mesic sites in the Brachytheciaceae. More recent work by Ignatov and Huttunen (2002) allows close morphological definition of the Brachytheciaceae relative to the Amblystegiaceae sensu lato. Both these families have foliose pseudoparaphyllia but the outermost bract is inserted laterally or proximally in the Amblystegiaceae while the Brachytheciaceae has it inserted on the distal side of the pseudoparaphyllial cluster.

Key to Ablystegium Etc.

The plants that we include in the following key are small (leaves mostly less than 1.5 mm long), and most have short median laminal cells and quadrate to short rectangular, never inflated alar cells. The lanceolate leaves are acuminate to acute, never plicate, and the stems either lack paraphyllia or the paraphyllia are so scarce as to be seldom noted. With the exception of Leptodictyum riparium, the mosses in this key have cells shorter than 5:1.

Species included in this key are in the Amblystegiaceae except Brachythecium (Brachytheciaceae), Conardia (Campyliaceae), and Platydictya (Hypnaceae):
Amblystegium juratzkanum W. P. Schimper
Amblystegium radicale Schimp., recognized from CA since 2004
Amblystegium serpens (Hedwig) Bruch & W. P. Schimper
Amblystegium varium (Hedwig) Lindberg
Brachythecium bolanderi (Lesquereux) A. Jaeger
Conardia compacta (C. Müller Hal.) H. Robinson
Hygroamblystegium tenax (Hedwig) Jennings
Leptodictyum humile (Palisot de Beauvois) Ochyra
Leptodictyum riparium (Hedwig) Warnstorf
Limbella fryei (R. S. Williams) Ochyra, not known from CA
Platydictya jungermannioides (Bridel) H. Crum

A. Costa very strong, ending at least in the base of the acumen .....B
A. Costa absent or shorter .....F

B. Leaves with a prominent border of elongate and thick-walled cells arranged as multistratose limbidium that resembles an additional two costae .....Limbella: L. fryei not known from CA
B. Leaf border undifferentiated or consisting only of slightly more elongate cells .....C

C. Leaf serrate to serrulate at base, often with the corners of two adjacent cells contributing to a single tooth; rhizoids typically papillose .....Conardia: C. compacta
C. Leaf entire or distally minutely serrulate; rhizoids smooth .....D

D. Median leaf cells mostly more than 15:1; leaves 2-4 mm long, inserted somewhat obliquely on the stem .....Leptodictyum: L. riparium
D. Median leaf cells 2-6:1; leaves mostly less than 1.5 mm long, inserted transversely on the stem .....E

E. Costae of most leaves broadened at the base and tapering above, filling about 1/4-1/5 of leaf base, percurrent to short-excurrent; leaves with 2-3 rows of enlarged, short rectangular to elliptical cells at their insertion and below the numerous quadrate alar cells .....Hygroamblystegium: H. tenax
E. Costa more narrow, typically ending before the leaf apex or percurrent; leaves with only a single row of such enlarged cells at insertion .....Amblystegium varium

F. Alar cells gradually shorter than median laminal cells but remaining rectangular and not at all well-defined .....G
F. Alar cells quadrate to short-rectangular or inflated, forming a rather abruptly defined group at cell angles .....H

G. Leaves on at least some stems obliquely attached and asymmetric; median leaf cells mostly more than 10:1; plant usually in slow-flowing or stagnant water .....Leptodictyum: L. riparium
G. Leaves all transversely attached and symmetric; median leaf cells less than 10:1; plant usually on soil or logs on stream sides .....Leptodictyum: L. humile

H. Leaves seldom exceeding l mm in length; costa essentially absent .....Platydictya: P. jungermannioides
H. Leaves larger; costa mostly extending to mid-leaf .....I

I. Leaf margins strongly serrate nearly to base; some of the upper leaf cells with distal prorations .....Brachythecium: B. bolanderi
I. Leaf margins entire or weakly serrate above the middle; none of the distal leaf cells with prorations .....J

J. Leaves erect to erect-spreading with the angle between leaf and stem seldom exceeding 30 degrees; alar cells quadrate .....Amblystegium serpens
J. Leaves widely spreading (leaf/stem angle exceeding 45 degrees); alar and basal cells rectangular and wider than upper cells .....Amblystegium juratzkanum