The genus Atrichum in the Pacific Northwest
Atrichum is another genus that is easy to recognize in the field on account of its large, upright stature, toothy leaves, lines of cells along the middle of the leaf and the long, hairless cap that covers the capsule, making it the only hairless "haircap" moss genus in the region. There are four species of Atrichum known to occur in the British Columbia and adjacent regions. Only two of them seem to be common, though there is cause to be on the lookout for the other two.
I, among others, have found it challenging to distinguish between Atrichum selwynii and A. undulatum. They both have undulating, toothy leaves and they both grow at low elevations on variations of soil bank habitat. A. selwynii is considered a native species and tends to be in more "natural" environments, while A. undulatum is considered introduced and lover of "weedy" environments.
Ireland (1969) elaborated on the distinction:
Atrichum selwynii has often been confused with A. undulatum and its varieties but the former differs by its dioicous condition, smooth calyptra, large, thin-walled, noncollenchymatous, usually irregularly angled leaf cells (often more than 30 X 24 um), numerous clustered sporophytes (sometimes over three per perichaetium), and straight or slightly arcuate capsule. In contrast, A. undulatum, including its varieties, is distinguished by its polyoicous or monoicous condition, hispid-tipped calyptra, smaller, thickwalled, collenchyinatous, rounded leaf cells (rarely reaching 30 X 24 um), few clustered sporophytes (never over three per perichaetium), and often strongly arcuate capsule.
This Schofield comment in his Some Common Mosses of British Columbia is unfortunately hard to parse:
From A. undulatum it (A. selwynii) is most readily distinguished by the sharper marginal teeth and more sharply acute apices compared to the blunter points of A. selwynii.
The other two species have very few records, but both A. tenellum and A. flavisetum are reported from low elevation habitats. Below are two tables to help me (and hopefully you) distinguish between these 4 taxa based on (1)sporophyte characters visible with a hand lens and (2) Vegetative characters.
Sporophytic Characters of Atrichum Species in the PNW
| Species | Habitat | Arrangement of Antheridia and Archegonia | Capsule Orientation | Sporophytes per gametophyte | Sporophyte Position |
| A. selwynii | Soil & Sediment Banks, Tree Roots | On seperate plants | straight to slightly arched, erect to slightly inclined | variable, often 3+ | Terminal |
| A. undulatum | "weedy" soil habitats | Typically on same plant, different locations | Arched to horizontal | Typically solitary | Terminal |
| A. tenellum | Soil banks along ditches and trails, especially in clearings | On seperate plants | Erect to inclined, sometimes slightly arched | 1 | Terminal |
| A. flavisetum | Soil banks | Same location on same plant | straight, erect or inclined | 2-6 | Appears lateral |
Vegetative Characters of Atrichum Species in the PNW
| Species | Leaf arrangement | Leaf teeth | Leaf Apices | Lamellae Number | Lamellae Height | Unudulations | Costa |
| A. selwynii | "distinctly rosulate" | Crowded, large | Acute to obtuse | 2-6 | 8-12 | Prominent | Percurrent |
| A. undulatum | Crowded towards apex | Large, crowded | Acute | 2-6 | 2-4 | Prominent | Percurrent |
| A. tenellum | Evenly distributed | Small and distant | Acute | 2-5 | 4-7 | Absent or barely present | Percurrent |
| A. flavisetum | appx. 10 / growth increment | Small from base to tip | Acute | 4-6 | 2-4 | Prominent in distal half | Percurrent to excurrent |
Relevant links
Flora of North America Treatment of the Genus Atrichum
California Moss Eflora Treatment of the Genus Atrichum
