Thallus: fruticose, ± erect and caespitose, or (v. tomentosoides) subprostrate and dorsiventral, sometimes becoming congested; primary thallus: evanescent surface: white to gray-white, dull, dying below but not ferruginous nor blackened towards the base, not of ligneous appearance; tomentum: whitish or cream-colored, thin and felty (to thick and floccose or spongy in v. tomentosoides); soredia: absent cephalodia: usually aeruginose-glaucescent, irregularly pulvinate, becoming subglobose, indistinctly tuberculate, not scabrid, formed as in S. myriocarpum Th. Fr. but often few or concealed in the tomentum, c. 0.2-0.3 mm across, containing Nostoc pseudopodetia: 20-24 mm long, in lower part c. 1 mm wide, rather loosely to somewhat narrowly and firmly attached, at the base simple to weakly branched, in the upper part infrequently short, erect-curved branched, or more often well-branched phyllocladia: usually numerous and crowded, lateral, granular (0.1-0.3 mm across, sometimes even less towards the base) to more often verrucose to flattened and crenate-squamulose, elongate-squamulose or digitate-squamulose [to 0.6 mm across according to Lamb, 1977], fully exposed, whitish or whitish gray, sometimes darker toward their bases Apothecia: usually frequent in upper part, mostly lateral, at the tips of short secondary branches, 0.3-0.5 (-1.0) mm diam., distinctly constricted at the base, rounded, not dividing into secondary discs; disc: plane to slightly convex, ± dark brown [black according to the protologue], when wet brown; margin: narrow, distinct at first, whitish to brown, entire, persistent; cortex: subchondroid, yellowish, 14-16 µm thick, plectenchymatous; exciple: dimidiate, in upper part around hymenium dark gray, in lower part hyaline and containing infrequent algae; hymenium: hyaline below to orangish brown or red-black and granular above, I+ blue, 40-45 µm high; paraphyses: filiform, dense, straight, loose to coherent, unbranched, non-septate (below apex), tips moderately clavate, thickened (2.5-3.5 µm wide in Sonoran material), brown; hypothecium: pale brown asci: oblong-clavate, in Sonoran material c. 42 x 8-10 µm ascospores: acicular-filiform with rounded ends, in Sonoran material rare and probably immature, 3-septate, c. 20 x 2.5 um (otherwise 23-25 x 2-2.5 µm) Pycnidia: not seen Spot tests: phyllocladia K+ yellow, C-, KC+ violet, P- or + pale yellow Secondary metabolites: atranorin and (usually) lobaric acid. Substrate and ecology: on soil, humus, or among mosses, or (v. tomentosoides) on rock World distribution: eastern (to south-central) Asia; North America (Alaska to the southwestern USA) Sonoran distribution: known from a few specimens from a single locality at 3,300 m in eastern Arizona. Notes: This taxon is similar to S. tomentosum Fr., in the cephalodia (small, bluish, indistinctly corticate), phyllocladia (granular to squamulose, non-sorediate, without darker centers), pseudopodetia (± fragile, whitish, and tomentose), and apothecia (under 1 mm diam., mostly lateral, with narrow spores). It differs from S. tomentosum mainly in chemistry (lobaric instead of stictic acid) and distributional pattern (restricted to E. Asia and W. North America, rather than occurring in circumboreal-Arctic areas and South America). Another similar species, S. myriocarpum Th. Fr. (known from Asia and North, Central, and South Americas), likewise contains stictic rather than lobaric acid, and further differs from S. sasakii in having an unpigmented hypothecium. While some authors (e.g., Goward, 1999) treat S. sasakii as a whole (along with S. myriocarpum Th. Fr., under a broad concept of S. tomentosum, a thorough study that includes material from areas outside the Sonoran Region is necessary to resolve the taxonomy of this complex. Although a few other species of Stereocaulon are known from northern California or from southern parts of Mexico, this is the only complex known to occur in areas with climates even approaching that of the Sonoran region.
The Arizona material reported by Nash et al. (1998) as var simplex (Riddle) Lamb [Journ. Hattori Bot. Lab. 43: 230 (1977)] actually seems to fit the protologue of S. sasaki (Zahlbruckner 1933) fairly well, in that the pseudopodetia are somewhat branched in the upper parts and ± richly covered with mostly granular phyllocladia; the various apparent differences in some apothecial characters in the description of our material are probably not significant. The var. tomentosoides Lamb [Journ. Hattori Bot. Lab. 43: 230 (1977)] differs from the typical variety in having prostrate-decumbent and crowded pseudopodetia forming dorsiventral mats, a thicker tomentum, and a different main distributional area (W. North America rather than E. Asia), and according to Lamb (1977) may eventually deserve species rank. The material reported by Nash et al. (1998) as v. tomentosoides seems to fit this variety, but the pseudopodetia are rather short and narrow (up to 15 mm long; often < 0.5 mm thick), the tomentum is not as thick as in S. tomentosum, and apothecia are lacking. This material also contrasts with the other Arizona collection (but fits within Lambs concept of S. sasakii as a whole) in that the phyllocladia are mostly distinctly squamulose.