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Page number:701 
Remarks (internal):Arrhenia acerosa var. acerosa is a variable taxon in some macro and microscopic features and in habitat. Different taxonomic levels (species, forms and varieties) have been used to accommodate every set of the morphological features observed. Specimens with reduced lamellae to vein-like wrinkles have been found and described mainly from Nordic countries (Blytt 1905, Egeland 1911, Høiland 1976, 1992).
The spore width of Arrhenia acerosa var. acerosa is of 3-6 µm (Favre 1955, Kühner and Lamoure 1972, Høiland 1976, Redhead 1984, Watling and Gregory 1989, Kuyper 1995b) but collections with somewhat broader spores (6.5-7.5 µm) have been described from alpine zones and a new form, Pleurotellus acerosus f. latisporus, was proposed based on this criterion (Favre 1955). The separation of these taxa was followed by some authors (Kühner and Lamoure 1972, Høiland 1976), although the recognition of Arrhenia latispora as a different species also has been considered (Bon and Courtecuisse 1987, Watling and Gregory 1989). In contrast, and based on the extremely variable spore size of the specimens examined, Redhead (1984) and Kuyper (1995b) accepted the unique taxon A. acerosa and did not recognize forms or varieties of this species. Pleurotellus acerosus f. latisporus differs only from the typical form of A. acerosa in the spore width (Favre 1955). However, we observed a large range in spore width in the type material (5-7.5 µm) with some overlap with the spore size of A. acerosa var. acerosa. Variation in spore size also was observed in the Iberian collections; AH 20885 and AH 26853 possessed spores 4.5-6.5 µm wide, and no differences were observed among the spores of A. acerosa var. acerosa and A. latispora. Arrhenia acerosa is known from the oromediterranean and supramediterranean belts of that region, in southern (Granada, Ortega et al 1997) and central Spain (Segovia), respectively, and from the coline to Eurosiberian alpine belts of Andorra and northern Spain (Guipúzcoa, Navarra).
 
Description type:Non-original description 
Description:Arrhenia acerosa (Fr.) Kühner var. acerosa, Bull. Mens. Soc. Linn. Soc. Bot. Lyon, Numéro Spéciale 49: 893, 992. 1980. Figs. 1-6
Agaricus acerosus Fr., Syst. mycol. 1: 191. 1821 (basionym).
= Agaricus glaucus Batsch, Elench. fung. Continuatio prima: col. 169 and 171, pl. 24, fig. 123a-c. 1786 (basionym); cf. Redhead 1984: 869-870.
Arrhenia glauca (Batsch) Bon & Courtec., Doc. Mycol. 18(69): 37. 1987.
Arrhenia glauca (Batsch) HÀ¸iland in L. Hansen & Knudsen eds., Nordic macromycetes 2: 99. 1992; superfl. comb. and basionym or reference not indicated.
= Pleurotellus acerosus f. latisporus J. Favre, Ergebn. Wiss. Untersuch. Schweiz. Nationalparks, Neue Folge 5(33): 38. 1955.
Arrhenia latispora (J. Favre) Bon & Courtec., Doc. Mycol. 18(69): 37. 1987.
Basidiomata pleurotoid, 1-2.7 x 1-3 cm. Pileus 1-3 cm wide, flabellate to spathulate or reniform, flattened, margin lobed and undulate, glabrous, longitudinally slightly zonate, hygrophanous, gray-brown to gray (10YR 5/1, 10YR 6/1 to 6/2) or light gray to whiteish (10YR 7/1 to 8/1) when moist, gray dark to fuscous or very pale brown on drying (10YR 4/1 to 4/2, 10YR 3/1 to 3/2, 10YR 8/3 to 8/4). Pileipellis a cutis of parallel hyphae, 4-10.5 µm diam, usually with parietal and ± fine encrusting pigment, irregularly distributed on walls. Hymenophore concolorous with pileus, lamellate. Lamellae thin (L = 7-25), moderately spaced, somewhat decurrent to adnate, with irregular lamellulae (l = 1-2). Stipe absent or very short, 0.3-0.6 x 0.2-0.4 cm, lateral to eccentric, whitish, tomentose, solid. Cystidia absent. Basidia 33-42 x 6-8 µm, cylindrical to clavate, 4-spored, sterigmata usually straight and up to 3 µm long. Spores 7.5-10(-11) x (4-)5-6.5 µm, Q = 1.5-1.7, ellipsoidal to broadly ellipsoidal, oval or lacrimiform, smooth, hyaline, nonamyloid. Clamps present in all tissues.
Specimens examined. Switzerland. Grissons: val Sesvenna, bord de la Sesvenna, à l'amont de Marangun, sur brindilles mortes de mousse, 2400 m, 24-VIII-1943, J. Favre Z.A.21 (holotypus G K13896 of Pleurotellus acerosus f. latisporus J. Favre: two portions, probably belonging to a single basidioma).
 
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