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Mem. N. Y. Bot. Gard.2: 37. 1971. -- Polytrichum alpinum Hedw., Sp. Musc. 92. 1801. -- Pogonatum alpinum (Hedw.) Rohl., Ann. Wett. Gesell. 3: 226. 1814.

Plants in loose to rather dense turfs, to 12 cm high, unbranched to rather closely branched by innovations which arise mostly below and lateral to the gametoecia. Plants green to yellow green with rather inconspicuous sheathing leaf bases which may be glossy, red-brown to stramineous. Leaves to 15 mm long, about 8: 1, spirally arranged along the stem, erect spreading to widely spreading when moist, patent to appressed and not at all crispate when dry, narrowly lanceolate from an oblong to obovate clasping base which occupies up to 1/6 of leaf length. Elamellate lamina very narrow at mid leaf, often only two cells wide, with those cells quadrate to transversely elongate, thick walled, to 10 µm wide. Abaxial costal cells to 12 µm wide, quadrate to short rectangular, 1–2: 1 with the walls thick walled and somewhat collenchymatous. Marginal cells above shoulder very thick walled, transversely elongate with rounded lumens, smooth, in a well defined group. Cells of margin well demarcated in a limbidium which extends along most of leaf sheath, thin walled and often collapsed, rectangular, to 6: 1, to 15 µm wide. Cells of sheathing base thin walled, rectangular, 1.5–4: 1, to 10 µm wide. Margins ascending but not involute, very sharply toothed throughout most of limb, often appearing limbidiate because of the close approximation of the enlarged and elongate cells of the teeth. Costa excurrent in a sharp cusp, filling nearly all of the leaf limb flattened but well-defined in the sheathing base. Photosynthetic lamellae 30–50, in lateral view, with low crenations on the distal margins; in cross section, with distal cells flat-topped and notched, sometimes appearing almost geminate. Costa cross-section flattened, mostly 6–10 times as broad as thick, with a single layer of guide cells and with a very broad abaxial stereid bands and a much more narrow adaxial one. Axillary hairs to 1000 µm long, inserted on each of the basal cells of leaf, many celled with no basal brown cells. Stem cross-section rounded pentagonal, with strongly pachydermous red brown cortical cells filling nearly all of the portion exterior to the very well defined central strand. Rhizoids to 15 µm wide at insertion, hyaline to pale brown, mostly restricted to near the base of the plant, sparse to very dense, often closely monopodially branched.
    Dioicous with male plants somewhat smaller than females. Perichaetia and perigonia terminal, with the latter forming prominent splash-cups. Monosetous with setae red brown, smooth, to 8 cm high, straight. Urn green to brownish green to red brown, oblong with 4–5 sharp longitudinal ridges, about 1.5: 1, to 4 mm long, with an abruptly demarcated hypophysis at base. Operculum apiculate to short rostrate, flattened except for the apiculus. Exothecial cells not arranged in regular rows, very irregular in size and shape, to 30 µm broad, quadrate to very short rectangular, with the outer cell without a thin spot centrally placed over each lumen. Stomates dense but nearly restricted to the constriction between the hypophysis and the urn. Basal membrane low, constituting less than 1/5 of the peristome length. Peristome teeth 64, to 250 µm long, cellular, not paired, hyaline throughout or occasionally with some red brown near base. Epiphragm hyaline and chambered, attached at about the mid point of the peristome teeth by means of a fringe of tooth-like appendages. Calyptra cucullate, so densely hairy as to obscure the body of that calyptra. Spores green, smooth finely verruculose, to 20 µm in diameter.

Plants matching several of the subspecies of this widely distributed and highly variable species are found in California. These subspecies are, however, largely recognized on size differentials of the sporophyte and gametophyte -- characters surely subject to considerable ecologically induced variation. present below a key to the several subspecies, but I prefer to present my description without reference to such infra-specific categories.

Elevation by latitude plot for Polytrichastrum alpinum
   in California

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