BFNA Title: Platydictya
AMBLYSTEGIACEAE - PLATYDICTYA
XX. PLATYDICTYA Berkeley, Handb. Brit. Mosses 145. 1863 * [Greek, platy, broad; dictyon, net, alluding to pattern of leaf cells]
Howard A Crum
Serpoleskea (Hampe) Loeske
Plants very small and slender, in light- to dark-green or brownish mats. Stems creeping, freely and irregularly branched, with a narrow central strand or central strand absent, without a hyalodermis; paraphyllia absent; rhizoids or rhizoid initials below or at abaxial leaf insertion, or axillary, smooth or granular-papillose, slightly branched; axillary hairs with 1--2 distal hyaline cells. Stem leaves erect-appressed to loosely spreading, sometimes subsecund, lanceolate, acuminate, not plicate, slightly concave, very small, 0.15--0.3(--0.5) mm; marginal lamina cells 1-stratose, margins plane, entire to denticulate; costa none or very short, double, and indistinct; median lamina cells subquadrate to oblong-rhombic or short-rhomboidal, smooth; alar cells scarcely differentiated to subquadrate, in small, inconspicuous groups, not decurrent. Sexual condition autoicous or dioicous. Inner perichaetial leaves from sheathing base acuminate, smooth; margin entire to dentate; lamina cells smooth. Capsule oblong-cylindric to oblong-obovoid, erect and symmetric, or variously inclined and asymmetric; peristome perfect or somewhat specialized; exostome margin entire or dentate distally. Spores 8--13 \um, finely papillose.
Species 10 (4
in the flora): Europe, Asia, North America,
Plants of Platydictya resemble Amblystegium but are even smaller and have leaves virtually ecostate. The capsule is variously inclined and asymmetric but not slenderly cylindric, curved-asymmetric, or greatly contracted below the mouth and at the neck when dry. The genus, except P. jungermannioides (see below) belongs to the Amblystegiaceae despite its ecostate, often secund leaves which suggest an alliance with the Hypnaceae. L. S. Cheney's (1897) treatment of the genus Amblystegium, in a broad sense, is a useful reference for the species of Platydictya. Illustrations are presented in that work, as well as by H. A. Crum and L. E. Anderson (1981). Platydictya confervoides and, P. subtilis, and P. minutissima should preferably be treated as a separate genus, Serpoleskea in the Amblystegiaceae. Platycictya jungermannioides differs from the other three species in having purple (rather than red-brown), granular-papillose (versus smooth), axillary rhizoids (versus inserted below leaves), a narrow branching angle (versus wide), and easily detached branches (versus firmly attached). These features in fact fit better the Plagiotheciaceae than in the Amblystegiaceae.
REFERENCES Crum, H. A. and L. E. Anderson. 1981. Mosses of
1. Rhizoids axillary, at least when young purplish and granular-papillose; branching angle narrow and branches easily detached . . . 1. Platydictya jungermannioides
1. Rhizoids inserted at or just below costa, red-brown, smooth; branching angle wide and branches more firmly attached.
2. Leaf cells short throughout, 2--3:1 . . . . . 2. Platydictya confervoides
2. Leaf cells elongate, 3--6:1.
3. Leaves entire; perichaetial leaves entire; capsule erect. . . . . 3. Platydictya subtilis
Leaves serrulate; perichaetial leaves coarsely dentate; capsule suberect to
1. Platydictya jungermannioides (Bridel) H. A. Crum, Mich. Bot. 3: 60. 1864
Hypnum jungermannioides Bridel, Musc. Recent. Suppl. 2: 255.1812; Amblystegiella sprucei (Spruce) Loeske
Plants in dense, soft, silky, green to yellow-brown mats; pseudoparaphyllia none. Branching angle narrow, branches easily detached; rhizoids axillary, at least when young purplish and granular-papillose. Leaves erect-spreading or sometimes + secund, 1.17--1.3(--0.5) mm, not or somewhat narrowed to the insertion, serrulate all around, especially at the insertion; distal cells 3--4:1, shortly oblong-rhomboidal; alar cells subquadrate in small and inconspicuous groups of 3--7 cells along the margins. Specialized asexual reproduction by axillary obcuneate propagula. Sexual condition dioicous. Perichaetial leaves ciliate-dentate above. Seta 6--11 mm. Capsule erect and symmetric or nearly so, 0.5--1 mm, oblong-cylindric, contracted below the mouth and at the short neck when dry; annulus of 2 layers of cells; operculum convex-conic, stoutly mammillate to + rostellate; stomata in the neck; peristome teeth yellow; cilia of endostome 1--2, rudimentary to well developed. Spores 11--13 µm.
Rocks in damp, sheltered places, crevices of cliffs or under rock ledges, peaty soil or humus under overhanging turf, hollows under roots of trees, lower sides of logs, perhaps limited to calcareous habitats; low to high elevations; Greenland; Alta., B. C., N. B., Nfld. and Labrador, N.W.T., N.S., Nun., Ont., Que., Sask., Yukon; Alaska, Ariz., Calif., Colo., Idaho, Iowa, Mich., Mont., N.Mex., N.Y., N.Dak., Vt., Wash., Wyo.; Europe; Asia.
Platydictya jungermannioides is easily recognized by its small size, absent or almost absent leaf costa, easily detached branches, and its axillary rhizoids that are purplish and granular papillose at least when young. The branching and rhizoid characters differentiate this species from the other three Platydictya species.
2. Platydictya confervoides (Bridel) H. A. Crum, Mich. Bot. 3: 60. 1964
Bridel, Musc. Recent. Suppl. 2: 153. 1812; Amblystegiella confervoides
(Bridel) Loeske; A. conferva
Plants in dull, dark green to yellow-brown, rather rigid mats; pseudoparaphyllia none. Branching angle wide, branches firmly attached; rhizoids below or at abaxial leaf insertion, red-brown and smooth. Leaves erect-appressed or rarely subsecund when dry, erect-spreading when moist, 0.15--0.3 mm, not much narrowed to the insertion, entire or nearly so; distal cells 2--3(--4):1, oblong-rhombic; alar cells subquadrate in small, inconspicuous groups of 3--6 cells, rarely more, at the basal margins. Specialized asexual reproduction absent. Sexual condition autoicous. Perichaetial leaves entire or obscurely serrulate distally. Seta 6--8 mm. Capsule inclined to horizontal and asymmetric or rarely suberect, 0.8--1.2 mm, shortly oblong-cylindric, contracted below the mouth when dry; annulus of 2 layers of cells; operculum convex-conic, stoutly apiculate; stomata at base of the capsule; peristome teeth yellow-brown; cilia of endostome in 2's and 3's. Spores 11--13 \um.
Moist, shaded limestone rocks, especially in stream valleys; low to moderate elevations; Nfld., N.S., Labrador, Ont., Que., Yukon; Alaska, Ala., Conn., Ill., Ind., Iowa, Kans., Md., Mich., Minn., Mo., N.J., N.Y., Ohio, Pa., S.Dak., Tenn., Vt., Va., Wis.; Europe; Asia (Caucasus).
Diagnostic features of Platydictya confervoides include the entire or obscurely serrulate leaf margins and short cells, as well as its rupestral habitat.
3. Platydictya subtilis (Hedwig) H. A. Crum, Mich. Bot. 3: 60. 1964
Leskea subtilis Hedwig, Sp. Musc. 221. 1801; Amblystegiella subtilis (Hedwig) Loeske; Amblystegium subtile (Hedwig) Schimper; Serpoleskea subtilis (Hedwig) Loeske
Plants dull, in green to brownish mats; pseudoparaphyllia foliose. Branching angle wide, branches firmly attached; rhizoids below or at abaxial leaf insertion, red-brown and smooth. Leaves erect-spreading and often subsecund, 0.25--0.5 mm, somewhat narrowed to the insertion, entire or nearly so; distal cells typically elongate, (2--)3--5:1, oblong-rhomboidal; subquadrate alar cells rather numerous but inconspicuous, 6--8 or more along the basal margins. Specialized asexual reproduction absent. Sexual condition autoicous. Perichaetial leaves entire. Seta 7--12 mm. Capsule erect or nearly so, symmetric, 1--1.5 mm, oblong-cylindric, somewhat contracted at the short neck and sometimes also below the mouth when dry; annulus of 1 layer of cells; operculum conic, obliquely short-pointed to rostellate; stomata in the neck; peristome teeth yellow- brown; cilia of endostome 1--2 and rudimentary or lacking. Spores 9--13 \um.
Bark at base of hardwoods, exposed roots, rarely on logs; low to moderate elevations; N.B., N.S., Ont., P.E.I., Que., Yukon; Ariz., Ind., Maine., Mass., Mich., Mo., N.H., N.J., N.Y., N.C., Okla., Va., Vt., Wis.; Europe; Asia; South America.
Particularly distinctive features of Platydictya subtilis include the entire leaf margins, elongate distal cells, and fairly numerous subquadrate alar cells. The capsule is erect or nearly so. This is the only species of the genus typically found on bark.
4. Platydictya minutissima (Sullivant & Lesquereux) H. A. Crum, Bryologist 72: 244. 1969
Hypnum minutissimum Sullivant & Lesquereux, Musci Bor.-Amer. 74. 1856; Amblystegiella minutissima (Sullivant) Nichols; Platydictya jungermannioides var. minutissima (Sullivant) H. A. Crum
Plants very slender, in thin, + silky, green to brownish mats. Branching angle wide, branches firmly attached; rhizoids below or at abaxial leaf insertion, red-brown and smooth. Leaves rather remote, loosely spreading, 0.18--0.3 mm, narrowly lanceolate, gradually acuminate, not narrowed to the insertion, serrulate nearly all around but entire at the insertion; distal cells 3--6:1; alar cells scarcely differentiated. Specialized asexual reproduction absent. Sexual condition autoicous. Perichaetial leaves coarsely dentate distally. Seta 4--9 mm. Capsule small, 0.4--0.45 mm, suberect or, more commonly, inclined or horizontal, broadly oblong-ovoid, asymmetric, without a neck, not contracted below the mouth when dry; annulus of 1 row of cells; operculum convex-conic, stoutly apiculate; stomata apparently none; peristome teeth yellowish; cilia of endostome well developed, single or paired. Spores 8--13 \um.
shaded rock (limestone and sandstone); low to moderate elevations; Que.;
Platydictya minutissima is rare, and occurs in thin, rather silky mats. The leaves are somewhat remote, loosely spreading when dry, and serrulate except near the insertion; the leaf cells are elongate, and those in the alar regions are scarcely differentiated.