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Ditrichum Hampe, 1867, nomen conservandum.

Ditrichum gametophytes closely resemble those of Pleuridium in leaf shape and in having distal crassiserrulation of the leaf marginal cells. Pleuridium, seldom collected except with sporophytes, resembles D. montanum and D. schimperi in that the larger vegetative and perichaetial leaves have a rather well demarcated leaf base relative to the leaf limb. Furthermore, the two aforementioned Ditrichum differ from Pleuridium in having a fairly broad costa filling more than 1/4 of the leaf base.

Key to Ditrichum Etc.

In the key that appears below we treat acrocarpous mosses mostly with narrowly subulate serrulate to denticulate leaves and rectangular laminal cells. There is no alar differentiation but instead there are several cells of the basal margin that are slightly more narrow than the more interior basal cells.

Species included are in Ditrichaceae:
Ceratodon purpureus (Hedwig) Bridel
Ceratodon stenocarpus Bruch & W. P. Schimper
Ditrichum ambiguum Best
Ditrichum flexicaule (Schwägrichen) Hampe, not known from CA
Ditrichum heteromallum (Hedwig) E. Britton
Ditrichum montanum Leiberg
Ditrichum pusillum (Hedwig) Hampe
Ditrichum schimperi (Lesquereux) Kuntze
Trichodon cylindricus (Hedwig) W. P. Schimper

Comparable with this key is the key to Dicranella Etc., a genus with similarly subulate and denticulate leaves and rectangular laminal cells. Dicranella tends to have the more lateral marginal cells larger than the more interior ones. It should be noted here that all members of the Dicranaceae and Ditrichaceae have one row of extreme marginal cells somewhat more narrow than the adjacent interior cells. Among the subulate-leaved acrocarpous mosses, Ditrichum and its close relative Trichodon are most easily confused with Dicranella. Perhaps the best characteristic of Ditrichum is the rather constant pattern of thickening of the transverse cell walls of the lamina cells. Among our local Ditrichaceae, this thickening is best seen in Trichodon whose awn therefore appears crassiserrate on the margin. All our Ditrichum can, however, be seen to have a small amount of such thickening mostly seen as crassiserrulation on the distal margin. This pattern of wall thickening is not seen in any of our local Dicranella. The sporophytes of the two genera are very different: Ditrichum has erect and smooth sporophytes with filiform terete peristome teeth; most species of Dicranella have arcuate and often sulcate sporophytes with triangular flat peristome teeth.

A. Leaf margins recurved and usually bistratose (if unistratose, the vegetative leaves ovate to lanceolate, not at all subulate .....B
A. Leaf margins plane to incurved .....E

B. Median cells quadrate and isodiametric; leaves evenly contracted to the narrowly acute apex so that the line depicting the leaf margin is almost straight; peristome teeth 16, triangular .....C
B. Median cells rectangular, at least 2:1; leaves contracted above the base into a distinctive acumen .....D

C. Seta yellow; plant occasionally locally introduced, usually on roofs of buildings and on road pavement .....Ceratodon: C. stenocarpus
C. Seta red-brown; plant widespread, especially in disturbed sites .....Ceratodon: C. purpureus

D. Leaves of dry plant curved to crispate; peristome teeth spinose-papillose; seta red; operculum long-rostrate, usually over 1/3 as long as the urn .....Ditrichum ambiguum
D. Leaves of dry plant straight and erect; peristome teeth low- papillose to striate; seta brown to yellowish brown; operculum short-rostrate .....Ditrichum pusillum

E. Leaf squarrose from an abruptly expanded and strongly sheathing base; awn dorsally prorate .....Trichodon: T. cylindricus
E. Leaf base not abruptly expanded and sheathing .....F

F. Median leaf cells mostly about 2:1 .....G
F. Median leaf cells mostly more than 4:1 .....H

G. Plant of seepages in montane calcareous springs; stems mostly exceeding 5 cm long, densely matted below with rhizoids; leaf lamina mostly unistratose or with bistratosity near apex .....Ditrichum flexicaule not known from CA
G. Plant of moist soil, often on roadbanks; plant smaller not matted with rhizoids; leaf lamina and margins bistratose or with bistratose streaks in the upper one-half .....Ditrichum montanum

H. Costa very broad, filling about 1/3-1/4 of leaf base; plant autoicous; spores rough, 20-29 µm .....Ditrichum schimperi
H. Costa more narrow; plant dioicous; spores smooth and smaller .....Ditrichum heteromallum

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