Polytrichastrum alpinum
Polytrichaceae
alpine haircap, alpine polytrichastrum moss
(1–)4–6(–14) cm, densely leafy above, often leafless and thread-like below, simple or sparingly to fasciculately branched.
erect, acrocarpous, from a ± developed underground rhizome, simple or rarely branched, bracteate proximally, grading gradually or abruptly to mature leaves.
(4–)5–8(–19) mm, loosely to densely imbricate, erect-spreading and subtubulose when dry, erect-spreading to widely spreading when moist;
sheath ± nitid, elliptic to obovate, with tapering shoulders (in var. fragile contracted above the sheath and the blade caducous), broadly hyaline-margined;
blade linear-lanceolate, the apex narrowly acute to finely acuminate;
marginal lamina 2–5 cells wide, erect, coarsely serrate with multicellular teeth, distantly serrulate to subentire;
costa excurrent, ending in a short, brownish, toothed awn;
lamellae 5–8 cells high, entire in profile, the marginal cells with the free wall appearing greatly thickened, the marginal cells in section enlarged, yellowish to dark brown, ovate to narrowly ovate, the lateral walls strongly thickened, the lumen narrowly pentagonal and pointed at the apex, coarsely papillose;
median cells of sheath 40–60(–80) × 6–12 µm, elongate-rectangular, thin-walled;
cells of the marginal lamina 10–15 µm, subquadrate, sometimes transversely elongate;
perichaetial leaves scarcely longer than the stem leaves.
various, with a chartaceous, sheathing base and a divergent, firm-textured blade (polytrichoid), or the whole leaf membranous and sheath not or weakly differentiated, the blade rarely transversely undulate, crisped and contorted when dry;
adaxial surface of blade with numerous closely packed longitudinal photosynthetic lamellae across most of the blade, the marginal lamina narrow, or the lamellae restricted to the costa, flanked by a broad, 1 (rarely 2)-stratose lamina, rarely with abaxial lamellae;
margins 1(–3)-stratose, entire, denticulate, serrate, or toothed (in Atrichum bordered by linear, thick-walled cells);
costa narrow in basal portion, in the blade abruptly broadened and diffuse, smooth or toothed adaxially, rarely with abaxial lamellae, in cross section with a prominent arc of large diameter guide cells and an abaxial stereid band;
lamellae entire, finely serrulate, crenulate, or coarsely serrate, the free margin smooth or cuticular-papillose, the marginal cells in cross-section undifferentiated or sharply distinct in size and/or shape from those beneath;
transition in areolation from sheath to blade gradual or abrupt, with “hinge-tissue” at the shoulders (except Atrichum and Psilopilum);
cells of back of costa (or cells of the membranous lamina) typically in longitudinal rows, ± isodiametric to transversely elongate-hexagonal.
(1–)3–5 cm, brownish.
solitary or rarely several from the same perichaetium.
dioicous or rarely monoicous;
male inflorescence indeterminate, innovating from the center and continuing the growth of the stem, often several successive perigonia per shoot;
female inflorescence terminal, perichaetial leaves long-sheathing or not much differentiated.
various, (1.5–)3–5(–8) mm, terete, narrowly cylindric to oblong-cylindric and curved, ovate-cylindric, or ovoid to almost spherical, suberect to inclined to almost horizontal;
hypophysis tapering, rugose, with numerous conspicuous stomata in a broad basal band;
exothecial cells irregularly rectangular, not bulging or mammillose, thin spots absent, rather thick-walled;
peristome 600 µm (teeth 150–250 µm), divided to 0.6–0.75, the teeth 45–50, with some teeth irregularly developed and unequal, pale to somewhat darker in the median line.
obtusely to sharply (2–)4(–6)-angled, with indistinct longitudinal angles or ridges, or terete;
hypophysis tapering and indistinct or delimited by a constriction at base of capsule;
exothecium smooth, mammillose, or scabrous;
stomata present (absent in Atrichum and Pogonatum);
peristome pale or strongly pigmented, nematodontous, with a single series of [16–]32–64 rigid, unjointed teeth composed of elongate, fiber-like, sinuate cells, the teeth simple or compound, attached by their tips to the epiphragm (tympanum) covering the capsule mouth.
cucullate, with a matted felt of hairs arising from its tip and covering all or part of the capsule, or the calyptra sparsely ciliate to smooth.
14–20 µm.
minute and echinulate, or larger and finely papillose.
reproduction none, or by proliferation of an underground rhizome.
Polytrichastrum alpinum
Polytrichaceae
Varieties 8 (4 in the flora).
Polytrichastrum alpinum is highly variable in habit and plant size, dentition of the leaves, and capsule shape. However, all forms of the species are easily recognized by the entire-margined, coarsely papillose lamellae and terete capsules with smooth, non-pitted exothecial cells. The marginal cells of the lamellae in section are distinctive in shape and wall thickening, elegantly described by A. J. E. Smith (2004) as “strawberry-shaped.” The wall thickenings extend down the lateral walls, so that in profile the free margin appears to be much thicker-walled and the lumen more restricted than is actually the case. The marginal cells of P. sexangulare are similar in shape and wall thickening, but smooth. The only North American taxa of Polytrichaceae likely to be confused with P. alpinum when sterile are Meiotrichum lyallii and Pogonatum urnigerum. In P. urnigerum the marginal cells of the lamellae are shorter and broader at the apex with a pentagonal lumen; in M. lyallii the marginal cells seen in profile are irregularly striate and pitted rather than papillose.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Genera 22, species ca. 260 (9 genera, 38 species in the flora).
The Polytrichaceae are widely distributed in all climatic zones except the lowland tropics and include many large, common, and familiar North American mosses. These “hair-cap mosses” have no close living relatives, and have a long (but disjunctive) fossil record (Eopolytrichum) from the Late Cretaceous of Georgia, United States (A. S. Konopka et al. 1997). The family is remarkable for the structural complexity of both gametophyte and sporophyte found in many of its members. The stems of robust taxa (Polytrichum) have a conspicuous central strand composed of hydrome and leptome, and traces extending into the leaves; in other genera the central strand is weak and indistinct.
The following descriptions use the shorthand term “polytrichoid” for taxa with a growth form and leaves like those of Polytrichum. The best analogy to the polytrichoid leaf is the grass leaf, with a clear distinction between a sheathing base and divergent blade. Typically a wedge-shaped group of transversely elongate, incrassate cells (“hinge tissue”) is present at the shoulders or just above. Most polytrichoid taxa have a hyaline-margined sheath, with a sharply defined border of thin-walled, decolorate cells. The leaves of Atrichum have a slender costa and a broad, membranous lamina. In Bartramiopsis and Lyellia, however, the lamellae are restricted to the “costa” and the broad “lamina” is 2-stratose. In the polytrichoid leaf the distinction between costa and lamina becomes blurred. The conducting strands are confined to a median band, flanked by an ambiguous zone bearing closely packed lamellae but only a few cells in thickness, and a narrow, usually 1-stratose marginal lamina. Gary L. Smith (1971) suggested that lamellae originally covered the adaxial surface of the leaf, and the broad membranous lamina developed as conducting strands and lamellae were restricted to the median portion.
As demonstrated by S. O. Lindberg (1868) the form of the marginal cells of the lamellae is often sufficient to distinguish between species of Polytrichaceae. Free-hand sections with a razor blade are adequate in most cases. Sections should be made from ordinary vegetative leaves, in the middle 1/3 (or in polytrichoid leaves, midway between the tip and the base of the blade). Lengths of lamellae should also be scraped from the leaves to be viewed in profile. Median cells of the lamina are best measured individually. The leaf cells are typically aligned in ± regular longitudinal rows, but it is difficult to measure a group of cells and divide to obtain an average cell width since the cells are not aligned transversely.
The peristome teeth are unique among mosses in being composed of compact bundles of whole, fiber-like cells. The teeth are not composed of remnants of wall thickenings as in arthrodontous mosses, but form by intrusive growth and elongation of living cells. Accordingly, the peristomes are not homologous. The teeth may be simple (with a single median line), or compound (the outlines of two teeth visible on the outer face of each tooth), although the exact nature of this difference is not well understood. The sinus between these “teeth” may be broad (Oligotrichum), or narrow and obscured (Pogonatum). The height of the peristome is measured from the tips of the teeth to the capsule rim, seen from without. The term “basal membrane” is not used. The generic classification used in this treatment follows that of Gary L. Smith (1971) with few modifications.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
1. Plants medium to robust; leaves sharply serrate; capsules short- to long-cylindric | → 2 |
1. Plants small (1-2 cm); leaves entire to finely and remotely serrate toward the tip; capsules ovoid to subglobose | → 3 |
2. Plants medium, (2-)4-6(-10) cm; leaves (4-)5-8 mm; capsule 3-5 × 0.8-1 mm, short-cylindric or slender and subarcuate. | var. alpinum |
2. Plants robust, to 14 cm; leaves 12-14(-19) mm; capsule 5-8 × 1.5-2.2 mm, often ± zygomorphic and widest below the middle. | var. sylvaticum |
3. Leaves not fragile, not constricted at junction of sheath and blade, the blade persistent | var. septentrionale |
3. Leaves fragile, constricted at the junction of sheath and blade, the blade caducous | var. fragile |
1. Protonema persistent; individual leafy plants small and scattered. | Pogonatum |
1. Protonema ephemeral; leafy plants gregarious, loosely tufted or caespitose | → 2 |
2. Lamellae 10 or fewer and confined to median portion of blade, often loosely spreading and wavy | → 3 |
2. Lamellae 20 or more, occupying most of the leaf width, compact and straight | → 7 |
3. Lamina 2-stratose, ± firm, the upper layer of laminar cells strongly mammillose | → 4 |
3. Lamina 1-stratose, broad and membranous, the cells not mammillose above (or if mammillose, then the leaf bordered by thick-walled elongate cells) | → 5 |
4. Margins of sheath entire; lamellae entire; leaf tip with a slender, caducous hair; plants polytrichoid. | Lyellia |
4. Margins of sheath ciliate; lamellae coarsely serrate; leaf tip not ciliate; plants slender and wire-like. | Bartramiopsis |
5. Leaf margins bordered by narrow, thick-walled cells, sharply toothed; lamina often transversely undulate, the back of lamina with diagonal rows of teeth; abaxial lamellae absent. | Atrichum |
5. Leaf margins serrulate to entire, not bordered by narrow, thick-walled cells; lamina not undulate and not toothed on back; abaxial lamellae often present | → 6 |
6. Leaves not hyaline-margined; well-developed abaxial lamellae often present on both costa and lamina | Oligotrichum |
6. Leaves narrowly bordered in part by thin-walled, hyaline cells; abaxial lamellae absent or restricted to costa near leaf apex. | Psilopilum |
7. Margins of lamellae papillose (or appearing so in cross-section) | → 8 |
7. Margins of lamellae smooth | → 11 |
8. Leaf sheath not hyaline-margined; marginal cells of lamellae rectangular and flat-topped | Pogonatum |
8. Leaf sheath hyaline-margined; marginal cells of lamellae rounded to elliptic to pyriform | → 9 |
9. Marginal cells in profile irregularly striolate and pitted. | Meiotrichum |
9. Marginal cells in profile distinctly papillose, with rounded papillae | → 10 |
10. Marginal cells pyriform, lateral walls strongly thickened, lumen narrowly pentagonal. | Polytrichastrum |
10. Marginal cells elliptic, thick-walled, the distal wall broadly convex. | Pogonatum |
11. Marginal lamina greatly expanded and sharply inflexed, entire, overlapping and enclosing the lamellae, the cells transversely elongate. | Polytrichum |
11. Marginal lamina narrow, serrate or toothed (if subentire then not broadly inflexed, the cells ± isodiametric) | → 12 |
12. Lamellae ± entire in profile, the apical cells rounded in cross-section, only slightly taller than those beneath, or transversely ovate | → 13 |
12. Lamellae crenulate in profile, the apical cells differentiated, retuse or flat-topped in cross-section | → 14 |
13. Leaves with a distinct sheathing base and a firm, divergent blade, the sheath entire, hyaline-margined. | Polytrichastrum |
13. Leaves lax, only weakly differentiated into sheath and blade, toothed nearly to the base, not hyaline-margined. | Pogonatum |
14. Costa rounded in cross-section, forming a low ridge on back of blade, the arc of guide cells narrow (8-12 cells wide); median sheath cells short-rectangular (to 5:1) to rhomboidal | Polytrichastrum |
14. Costa in cross-section broad and flat, the arc of guide cells 18-24 cells wide; median sheath cells linear (as 20:1). | Polytrichum |
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