Guide to Liverworts of Oregon: Blepharostoma arachnoideum M.A. Howe
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1b plants leafy > 3a leaves divided 1/2 or more > 4b leaves divided to base > 5b segments uniseriate to base > Blepharostoma 1b leaf segments mostly 2, rarely 3 > Blepharostoma arachnoideum
Synonym: none.
Special status: ORBIC List 2.
Recognition: Small, thread like plants with frequent lateral branches. Leaves are transversely inserted, remote, divided to the base into 2-3 (rarely 4), capillary, uniseriate segments. Leaf segments are occasionally forked a few cells above the base. The cells of the leaf segments are rectangular, with the cross walls not protruding when viewed in profile but profile smooth or even depressed slightly over cross walls. Underleaves are similar to the lateral leaves but with fewer lobes, sometimes one lobe reduced and not as long as the other lobes of the underleaf. Gemmae are frequent toward the stem apex, unicellular when dispersed, ellipsoidal, formed by budding of leaf segment tips, often forming moniliform chains of 12 -- 18 cells.
Separating B. arachnoideum from B. trichophyllum is based on the evenly thickened cell walls and frequent forking of leaf segments in B. arachnoideum. In particular, the junctions of the cell walls of B. arachnoideum are not thickened. Early suggestions that the cross walls of the leaf segments not protruding were diagnostic for B. arachnoideum are misleading because B. trichophyllum brevirete does not have protruding cross walls; this subspecies does, however, develop distinct thickenings around the periphery of the cross walls. Leaf segments are fewer on average in B. arachnoideum, only 2-3 per leaf in relatively robust plants instead of the 3-4 found in typical B. trichophyllum. Leaf segments are frequently forked in B. arachnoideum, the forking seen on every shoot in robust plants, as many as half the leaves with a forked segment in well developed material. Forked segments are uncommon in B. trichophyllum, seen occasionally but never on every shoot.
Distribution: Grows on rotting wood at or occasionally on rock in moist to wet habitats. May be aquatic, even found at 110 m (362 feet) deep in Lake Tahoe, Nevada (Frantz et al. 19, WTU). In Coastal Mountains and the Cascade-Sierra axis from Mendocino County, California to Vancouver Island, British Columbia.
Comments: This species has been a bit of a problem in the past, since some like Schofield (1968), doubted its existence. However, the original decription in Howe (1899), described from Mendocino County, California, is correct in all details. I had long doubted the validity of the taxon myself until encountering distinctive plants I thought were B. arachnoideum (Smith Ridge, Linn County, Oregon, DHW m1064a); an experience paralleled by Schofield (2003) and Martin Hutten (pers. comm.). Ironically, I have since concluded that the collection from Smith Ridge is not B. arachnoideum but a short celled form of B. trichophyllum better treated as B. trichophyllum ssp. brevirete. The Kofranek collection from Lane County, Oregon was the first specimen I examined that exactly fit the original description of B. arachnoideum. Recognizing the uniqueness of this specimen led me to do a comprehensive study of the species and publish an illustrated paper on its distinctness (Wagner 2011).
Shortly after a field bryology workshop in 2017, a number of Willamette National Forest botanists began finding this species in numerous localities in their ranger districts. They were able to recognize a particular likely microhabitat: wet logs at the edge of streams. This species is locally abundant, similar to the situation with Macrodiphyllum rubrum on the coast, where there is a concentration of sites in a coastal strip in two counties. These clusters of populations do not justify removing the species from ORBIC conservation status because other than these hot spots the species are not abundant.
Blepharostoma arachnoideum - Near Horton, Lane County, Oregon. Kofranek 4134 (OSC).
Blepharostoma arachnoideum - Near Horton, Lane County, Oregon. Kofranek 4134 (OSC).
Gemmae developing on segment tips. Near Horton, Lane County, Oregon. Kofranek 4134 (OSC).
Blepharostoma arachnoideum Leaf segment bases showing cell junction. Lake Tahoe, 110 m deep, Douglas County, California. Franz 19 (NY).
Big Creek, Jefferson County, Washington. Walton 2536 (OLYM).
Comparison of shoots at same scale. A: Blepharostoma arachnoideum - Big Creek, Jefferson County, Washington. Walton 2536 (OLYM). B: Blepharostoma trichophyllum subsp. trichophyllum - Arrow indicates forked segment. Myrtle Island, Douglas County, Oregon. Wagner 7642 (OSC).
Blepharostoma arachnoideum - Dry shoots showing crumpled appearance. A: Near Railroad Gap, Jackson County, Oregon. Helliwell 9682 (OSC). B: Vancouver Island, British Columbia. Schofield 112244 (UBC).
Comparison of perianths at same scale. A: Blepharostoma arachnoideum - Big Creek, Jefferson County, Washington. Walton 2536 (OLYM). B: Blepharostoma trichophyllum subsp. trichophyllum - Myrtle Island, Douglas County, Oregon. Wagner 7642 (OSC)