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Page number:420 
Remarks (internal):The lectotype has virtually no hymenium or spores left (a few collapsed and bloated were seen), but is otherwise in a tolerable condition. The other contemporary specimens attached to the same herbarium sheet have been collected from the same region by the same collector, J.R. Weir. They clearly represent the same species as the lectotype and have been utilised in composing the present description and for spore measurements. As the type, they are not in the best condition either, lacking proper hymenium.
Oxyporus similis is characterised by a resupinate habit, heavily encrusted tramal cystidia and thin-walled, acyanophilous, ellipsoid spores. Several other species of Oxyporus share similar characters.
Vampola (1992) considered O. similis as a synonym of O. obducens, described from Germany. Donk (1967) placed O. obducens in synonymy with Oxyporus populinus (Schumach.) Donk. The current concepts clearly treat Oxyporus populinus and O. obducens as two distinct species: the pileate, perennial O. populinus with subglobose spores versus resupinate, annual, thin O. obducens with ellipsoid spores. Here we discuss O. obducens as regarded by European authors (e.g. Ryvarden & Gilbertson 1994).
Although O. similis and O. obducens are rather similar, we prefer to keep them apart for the moment. They share similarly shaped spores, hyphal structure and deep-rooting, heavily encrusted tramal cystidia. There are nevertheless minor differences. Spores in the specimens of O. obducens studied here were in average slightly shorter (4.1—3.0 ?m, Tab. 1) than those of O. similis (4.7—3.1 ?m). The spores of O. obducens are also slightly thick-walled and weakly cyanophilous, similar to those of Hyphodontia sambuci (Pers.) J. Erikss. but unlike O. similis. The hyphae of O. obducens are more or less homogenous throughout the basidiocarp and clearly thick-walled in trama, (1.8-)2.6-3.2(-4) ?m in diameter. In O. similis the tramal hyphae are thin- to slightly thick-walled, and they contrast to the thick-walled and broader subicular hyphae. We have not studied the North American material of O. similis extensively and can not state if it is homogenous or if O. obducens is also present in North America. Comprehensive research on these taxa, based on fresh material, would be needed to answer such questions, and also to confirm the status of O. similis.
Oxyporus corticola (Fr.) Ryvarden and Oxyporus latemarginatus (Durieu & Mont.) Donk are fairly similar to O. similis macroscopically. Microscopically, their larger, cyanophilous spores and cystidia, which are hymenial and not arising in the trama as in O. similis, separate them from O. similis. Oxyporus pellicula was described from Java and has pellicular basidiocarps with irregular, often dentate shallow pores quite unlike those of O. similis. Other resupinate Oxyporus and Rigidoporus species differ in the nature of their cystidia, shape or size of the spores, or by having thick-walled and cyanophilous spores.
 
Description type:Non-original description 
Description:Oxyporus similis (Bres.) Ryvarden, Persoonia 7:20, 1972.
Basionym: Poria similis Bres., Mycologia 17:76, 1925. Lectotype: USA. Idaho: Bonner County, Coolin, on Populus trichocarpa, 18.IX.1919 Weir US242516 (BPI, studied). Selected as type by Lowe (1966:21).
Basidiocarp resupinate, pore surface straw-coloured, contrasting with the white margin, patches from 10 cm2 to at least the size of a palm, 0.2-2(-3) mm thick. Consistency rather tough when dry. Pores regular, thin-walled, 4-6 per mm, tube layer up to 2 mm thick. Subiculum cream-coloured, but towards substratum turning into a brownish, woolly basal layer, usually just a thin line. Margin sharply delimited, thinning out, sterile part 0.5-0.2 mm wide. Hyphal system monomitic, hyphae rather uniform throughout the basidiocarp, yellowish in mass, CB(+), IKI-, KOH-, commonly septate, clamps absent. Subicular hyphae thick-walled, (2.2-)3.2-4(-5) µm, with a wide lumen and walls up to 1 µm thick. Tramal hyphae thin- to slightly thick-walled, parallel to subparallel, tissue rather dense, (2-)2.5-3.2(-4.2) µm. Cystidia abundant in trama, clavate, projecting through hymenium, heavily encrusted, 20-40(-70) x (5.5-)7.5-10(-13) µm, including crystal cover. Young cystidia thin-walled, apically encrusted hyphal ends, older ones eventually thick-walled. They are born in trama or subhymenium but bend through hymenium, occasionally projecting a little. Hymenium. Basidia few in the studied material, cylindrical or clavate, with 4 sterigmata, 8-15 x 4.5-5.6 µm. Basidiospores ellipsoid to broad cylindrical, thin-walled, smooth, CB-, IKI-, (3.8-)4.2-5.5(-6.3) x (2.4-)2.7-3.5(-4.2) ?m, L=4.72 ?m, W=3.10 ?m, Q'=(1.3-)1.4-1.8(-1.9), Q=1.52, n=122/4.
Distribution. The type material and other specimens studied here come from the Pacific Northwest of the United States.
Ecology. All the specimens studied have been collected from Populus trichocarpa.

 
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