Racomitrium obesum Frisvoll
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Synonym: Bucklandiella obesa (Frisvoll) Bednarek-Ochyra & Ochyra
Special status: NONE
Recognition: Perhaps the greatest difficulty will be separating R. obesum from R. occidentale. They both have bistratose leaf margins consistently present, at least in the distal one third of the leaf. The leaf section outline of bistratosity is often, but not always, almost circular like a lollipop in R. obesum as compared to R. occidentale. Racomitrium occidentale has not nearly as wide a costa as R. obesum near the leaf base. Leaf sections of R. obesum often have shallow indentations near the base of the leaf that mimic the grooves of R. occidentale. However, R. obesum has at most shallow grooves in the distal half of the leaf while in R. occidentale there is a deep groove in the middle with shallower grooves on either side. The costa in the distal half of the leaf tends to be wider in R. obesum than in R. occidentale, but not much thinner. It tends to be U-shaped instead of blocky. When the costa is observed in a leaf whole mount, it displays an unevenness unlike other similar species. It is of uneven width up the leaf, expanding and contracting and occasionally with a split or side spur.
The leaf margin is more tightly recurved (revolute) in R. occidentale but the recurvature does not extend to the tip of the leaf. In R. obesum the leaf margin is less strongly recurved but it extends up to the base of the hair point. Marginal alar cells are frequently elongate and thin walled in R. obesum. Pseudopapillae, seen as bulging protuberances above and below the lateral walls, are more prominent in R. obesum than in R. occidentale. The hairpoint tends to be much more slenderly tapering in R. obesum than in R. occidentale.
Distribution: Widespread in Oregon; found both at low and at higher elevations in the mountains, sometimes on calcareous rocks (unlike most of the rest of the genus). This species is endemic to the Pacific Northwest, from British Columbia to California. Jackson County: DHW 5943 (OSC).
Comments: Before identifying a specimen as R. obesum it is necessary to rule out R. occidentale as the latter is also common and, more likely to be encountered in moister habitats. In the past, I have misidentified many specimens of R. obesum as R. occidentale because I did not have a good grasp of the distinctions between these two. The other near look-alike is R. affine, which has mostly unistratose leaf margins. Its costa is also blocky but not as wide as R. obesum, more rectangular than U-shaped in the distal part of leaves.
Ridge above Carmen Reservoir, Linn County, Oregon. D. Norris 81788 (UC).
Oregon Caves, Josephine County, Oregon. T.C. Frye s.n. (WTU).
Oregon Caves, Josephine County, Oregon. T.C. Frye s.n. (WTU).
Racomitrium obesum; Oregon Caves, Josephine County, Oregon. T.C. Frye s.n. (WTU).
Racomitrium obesum; Oregon Caves, Josephine County, Oregon. T.C. Frye s.n. (WTU).
Racomitrium obesum; Oregon Caves, Josephine County, Oregon. T.C. Frye s.n. (WTU).
Racomitrium obesum; Oregon Caves, Josephine County, Oregon. T.C. Frye s.n. (WTU).
Racomitrium obesum; Oregon Caves, Josephine County, Oregon. T.C. Frye s.n. (WTU).
Racomitrium obesum; Oregon Caves, Josephine County, Oregon. T.C. Frye s.n. (WTU).
Racomitrium obesum; Oregon Caves, Josephine County, Oregon. T.C. Frye s.n. (WTU).
Racomitrium obesum; Oregon Caves, Josephine County, Oregon. T.C. Frye s.n. (WTU).
Racomitrium obesum; Oregon Caves, Josephine County, Oregon. T.C. Frye s.n. (WTU).
Racomitrium obesum - Cook and Green Pass, Siskiyou County, California. D. Toren 8221 (CAS).
Racomitrium obesum - Cook and Green Pass, Siskiyou County, California. D. Toren 8221 (CAS).
Racomitrium obesum; Slagle Creek, Jackson Co., Oregon. DHW 9488a.
Racomitrium obesum; Slagle Creek, Jackson Co., Oregon. DHW 9488a.
Racomitrium obesum; Slagle Creek, Jackson Co., Oregon. DHW 9488a.
Racomitrium obesum; Slagle Creek, Jackson Co., Oregon. DHW 9488a.
For more photomicrographs of this specimen, go to Racomitrium obesum Gallery
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Guide to Racomitrium of Oregon
Created 2007 Northwest Botanical Institute