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Acaulon triquetrum (Spruce) C. Müller Hal. [Pottiaceae]
map of distribution

Plants glossy, yellow-green to golden, mostly scattered or somewhat gregarious, bulbiform, triquetrous. Leafy stems to 1.5 mm high with the stem essentially obsolete. Leaves appearing concave because of the very sharp keeling, erect and enclosing the capsules, not contorted when dry. Leaves ovate to broadly ovate-lanceolate, 1.5–2: 1, scale-like below but grading imperceptibly to the perichaetial leaves which may exceed 1.5 mm in length. Lamina unistratose throughout. Median and distal cells only gradually differentiated from the basal cells. Median laminal cells thin-walled and pellucid, rectangular to hexagonal-rectangular, to 15 µm broad, 1.5–2: 1, with angular lumens and without defined corner thickenings. Basal cells rectangular with straight lateral walls, to 14 µm broad, 2.5–3: 1, smooth, thin-walled, unistratose throughout. Costa narrow from its base to its short excurrent, reflexed apex. Costa cross-section at mid-leaf terete, about twice as thick as the adjacent lamina, with a prominent abaxial stereid band. Margin serrulate above the middle, not irregularly-dentate near apex, plane to inconspicuously recurved. Axillary hairs pale brown and rather thick-walled, to 120 µm long, 3-celled, with no basal brown cells. Rhizoids hyaline, smooth, sparingly branched, to 20 µm wide at insertion.
    Dioicous with the male plants so small as rarely to be seen. Perichaetial leaves larger than, but only gradually demarcated from the vegetative leaves. Seta about 0.5 mm long, cygneous. Capsule globose, to 0.5 mm in diameter, apiculate at apex but without a differentiated operculum. Calyptra mitrate, covering only the apiculus. Peristome absent. Exothecial cells quadrate or short rectangular to hexagonal, 1–1.8: 1, with straight lateral walls, to 40 µm wide. Stomates phaneroporous, restricted to base of urn. Spores 30–45 µm, short elliptic, 1.2–1.4: 1, finely but closely papillose.

Mail a correction to Paul Wilson ·
IllustrationsCrum and Anderson 1981; Ignatov and Ignatova 2003; Smith 1978; Zander 1993.
BioregionsCW, DSon, SW.
VouchersSan Diego Co.: canyon above Bow Willow Canyon Campground, Anza Borrego State Park, Norris 77711 & 77720 (confirmed by Zander); San Luis Obispo Co.: Tower Road near west edge of Camp Roberts Military Reservation, Norris 100879.
   

Elevation by latitude plot for Acaulon triquetrum
   in California

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