Sphagnum Linnaeus, Sp. Pl. 1106. 1753. * [From the Greek for an unknown plant].
Authors: Richard E. Andrus
Plants typically with upright stems, young branches arranged spirally around stem at growing apex into a capitulum, branches clustered into fascicles along stem, stem and branch leaves of alternating inflated, S-shaped to rhomboidal hyaline cells and narrow linear chlorophyllous cells, hyaline cells typically fibrillose and porose on branch leaves. Stems differentiated into a central cylinder of thin-walled parenchymatous cells, merging into a cylinder of thick-walled cortical cells surrounded by 0--4 layers of thin-walled inflated cells, superficial layer of cells usually aporose, but may be porose. Stem leaves may be less fibrillose or efibrillose and less porose or aporose compared to branch leaves, often septate, a distinct border of narrow linear chlorophyllous cells often along margins and at base, and with a greater width:length ratio than branch leaves in anisophyllous forms, partly differentiated in hemiisophyllous forms, and identical in isophyllous forms. Branches typically dimorphic as spreading and pendent branches, but some species lack branches or branches are not clearly differentiated, pendent branches typically more slender than spreading branches and with a tendency to adhere to and cover the stem. Branch fascicles typically with 2 spreading and 1--2 pendent branches, but there may be to 12(14) per fascicle. Branch stems typically green, with a superficial layer of inflated retort cells; these grouped or solitary, usually porose at the distal end with a conspicuous or inconspicuous neck. Branch leaves with 2/5 phyllotaxy, of a 1-stratose network of alternating chlorophyllous and hyaline cells; hyaline cells usually S-shaped, rarely rhomboidal, nearly always strengthened with conspicuous spiral fibrils, small to large round to elliptic and sometimes ringed pores occur along commissures or rarely on cell lumen, convex surface typically with more pores per cell than concave surface; chlorophyllous cells may be enclosed on both surfaces, more broadly exposed on one surface or equally exposed on both surfaces as viewed in transverse section, adjacent cell walls typically smooth, but various types of cell wall projections may be clearly visible in transverse section. Sexual condition dioicous or monoicous, stalked globose antheridia borne at the tips of branches usually with swollen colored tips of branches near capitulum, long-necked archegonia borne on short branches singly surrounded by perichaetial leaves that are typically longer than branch leaves. Sporophyte a spherical capsule with pseudostomata on capsule surface, a very short seta, and a foot, exserted on a pseudopodium of gametophyte tissue. Capsule spherical, brown to black, lacking an annulus or peristome with a operculum convex; calyptra membranous; spore sac amphithecial in origin, over arching columella. Spores tetrahedral, with prominent trilete mark, fine to coarse superficial surface, distal surface may have raised Y-mark, bifurcated Y-mark sculpture, or none. Protonema thallose, typically 1-stratose, gametophyte developing from lateral margin.
Species 250 (88 in the flora); primarily in boreal regions$ but also in cool, moist montane and oceanic habitats such as nutrient poor and acidic wetlands and mires; worldwide in distribution, except for Antarctica.
The concept of species in the genus is controversial. We have followed the lead of P. Isoviita (1966) and K. I. Flatberg (1994) in the recognition of species. H. A. Crum (1984) and others (R. E. Daniels and A. Eddy 1985; A. L. Andrews 1958, 1959) have adopted more conservative taxonomic species concepts for species in the Northern Hemisphere. Discussion of the spores above is from T. Cao and D. H. Vitt (1986); for additional discussion of the protonema see C. McQueen (1988).
Microscopic features can be observed by using a concentrated aqueous or alcohol solution of Crystal Violet. A 50% solution of alcohol and Methylene Blue or Safranin Red can be used, but these usually do not stain features such as minute pores, fibrils, wall thinnings, and surface sculpture on the chlorophyllous cells. The number and kinds of branches should be determined, individual stem and branch leaves (from the middle of a spreading branch) should be examined from the distal 2 cm of the plant, and the superficial surface of stem cortical cells may need examination as well as cross sections of branch leaves and stems.
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