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Page number:889 
Remarks (internal):Systematic position. Russula aucarum is the type-species of section Delicoarchaeae Singer (Singer et al 1983). The distinction, however, between Delicoarchaeae on the one hand, and subsect. Lactariodeae Mre. (= sect. Plorantes Bataille ex Singer) and even sect. Metachromaticae on the other hand, is not clear and is difficult to resolve at present without a better appreciation of the species in the field. The taxonomic importance of the major feature of Metachromaticae, i.e., hymenial cystidia with a thick metachromatic wall in cresyl blue, is overestimated in our opinion. On the other hand, Metachromaticae was considered synonymous with subsection Cyanoxanthinae Singer where strong metachromatic reactions in cresyl blue exist (Buyck 1989, 1992, 1994). This synonymy was partly based on a misinterpretation of the features of R. metachromatica ssp. notoleuca (see below), but re-examination of the microscopic features confirms that Cyanoxanthinae is a different group.
Commentary. The above description, the first report of R. aucarum since the original publication, is based entirely on Panama material and is in agreement with the essential features of the type. Russula aucarum was described from a single collection from a lowland tropical rain forest in Ecuador. The species seems quite well defined by the poor development of the pileipellis, robust sterigmata, decurrent amyloid suprahilar spots on the spores, as well as by the rather dispersed, inconspicuous, narrow, and relatively small cystidia. Both the hymenial macrocystidia and dermatocystidia of R. aucarum are inconspicuous and sometimes very dispersed. Ovrebo 3510 has so few pileocystidia and macrocystidia that rapid examination would easily lead to the conclusion of an acystidiate species.
Because it is the type-species of section Delicoarchaeae Singer (Singer et al 1983), one would suspect that R. aucarum is well characterized and easy to distinguish from other species-groups in the genus. This is not the case. There exist, for example, very strong similarities with Russula metachromatica Singer, described by Singer in 1952 from a collection made by Dennis in a Venezuelan cloud forest. The latter is the type species of the monotypic section Metachromaticae Singer (Pegler and Singer 1980). Metachromaticae is based principally on the presence of thick-walled hymenial cystidia that are metachromatic in cresyl blue. Russula metachromatica is also entirely white but has dispersed, rather rare lamellulae, a shortly sulcate-striate cap margin and taste that is first mild, but quickly becomes subacrid. It also possesses basidiospores with a decurrent, amyloid, suprahilar spot but has a slightly different spore ornamentation. It was later reported by Singer et al (1983) from shady campina and campinarana vegetation with sapotaceous and leguminaceous trees in Brazil, mostly on sandy earth and litter, but also on rotting wood. It was also reported by Pegler (1983) from Martinique and Guadeloupe near polygonaceous and nyctaginaceous trees. Yet, Pegler's photograph of R. metachromatica (1983, Plate 21B) shows a specimen that corresponds much better to the type description for R. aucarum because of the smooth margin and numerous lamellulae.
Singer (Singer et al 1983) later described two subspecies of R. metachromatica from Brazil, each based on a single specimen. Russula metachromatica ssp. notoleuca Singer nom. inval. (no Latin diagnosis) is based on smaller spores and narrower hymenial elements than the type subspecies. It is said to have an acrid taste and different habitat. In an earlier review of type specimens of tropical Russulas (Buyck 1992), the illustrations for extremities of the pileipellis of R. metachromatica ssp. notoleuca (Buyck 1992, fig. 27) were erroneously replaced by those representing similar elements in an African species of Cyanoxanthinae. Although the micromorphological resemblance between spp. notoleuca and the type and Panama collections of R. aucarum is very strong, the former stands out from both R. aucarum and R. metachromatica ssp. metachromatica by the very crowded, narrow lamellae. Together with the acrid taste, numerous lamellulae, and much better differentiated pileipellis, it is a good candidate for a new taxon close to R. aucarum, but better documented (and illustrated) collections are needed for an accurate species interpretation. The second subspecies is Russula metachromatica ssp. tarumaensis Singer 1992 (note the date (!) as the original publication was invalid because of repeated confusion of infraspecific ranks, but all requirements for validation were presented in Buyck 1992, p 60). It was described from Igapo vegetation with Swartzia in Brazil and lacks the thick-walled macrocystidia typical of Metachromaticae, but is otherwise very similar.
Having re-examined the type collections for R. aucarum, R. metachromatica, and its ssp. notoleuca, it is clear that the microscopic differences do not justify being placed in different sections. We, therefore, consider section Delicoarchaeae to be synonymous with sect. Metachromaticae.
At first, Singer et al (1983) believed R. aucarum to be very close to sect. Archaeinae R. Heim ex Buyck & Sarnari (Sarnari 1998), differing from R. archaea R. Heim only by the presence of hymenial cystidia. The latter was described from coastal forest in Madagascar. Singer (1986) consequently transferred to Delicoarchaeae several other cystidiate species thought to be close to R. archaea. Most of these species were recently discussed by Buyck (1998) and do not belong in Delicoarchaeae. Russula archaea itself is not only very different in the field (see Buyck et al 1998 for color photographs), it is also very different in almost every other character from R. aucarum, although both possess abundant hymenial cystidia. Russula aucarum and R. archaea clearly belong in different sections (Shaffer 1990, Buyck 1992). Russula pusilla Murrill was placed in Delicoarchaeae by Gomez and Alfaro (1996) without explanation. It is, however, very unlikely that this species belongs here because of the pink cap and the equal, white to yellow lamellae.
Description type:Non-original description 
Description:Russula aucarum Singer, Beihefte Nova Hedw. 51: 243 (1975). Figs. 1, 7-12
Basidiomata solitary or scattered, on soil. Pileus 27-50 mm wide, depressed at center with margin plane or concave overall, glabrous, appearing cottony under hand lens (10x), not striate or sulcate, slightly uneven-bumpy over surface, white, occasionally light tan in areas or browning where damaged by insects. Lamellae 3-4 mm wide, adnate to subdecurrent, off-white, not discoloring, entire, subdistant (lamellae and lamellulae 1-2 mm apart at mid-pileus), rugulose between lamellae at pileus edge; lamellulae present in varying lengths. Stipe 20-30 mm long, 7-9 mm thick, equal, base rounded, glabrous or faintly longitudinally ridged in places, white; context solid, concolorous with surface. Context 3-5 mm thick in the pileus, white, odor and taste absent. Basidiomes not reacting with 2.5% KOH. Spore print not observed.
Basidiospores subglobose to broadly ellipsoidal, (6.7-)7.1-7.36-7.90-8.3(-8.9) x (6.1-)6.3-6.56-6.80-7.1(-7.3) µm, Q = 1.06-1.12-1.16-1.23, n = 30, ornamentation of blunt, cylindrical to acute, conical elements of variable dimensions, many quite large and stout, measuring 1-1.5(-2) µm high, strongly but often partially amyloid, interconnected by a few fine lines, never distinctly reticulate except for young spores; suprahilar plage large and verruculose, decurrent on the apiculus, variably amyloid. Basidia (41-)46-54 x 10-12 µm, clavate, 4-spored; sterigmata stout, 5-7(-8.5) x 1.5-2 µm. Cystidia dispersed to moderately numerous (600-900/mm2) but inconspicuous to very inconspicuous, immerged to slightly emergent, dispersed on lamella edge, quite small, 44-63 x 7-8 µm, narrowly subfusiform, apically tapering to mucronate, thin-walled and fragile, with finely granular to crystalline, refringent contents at least in their central part, scarcely reacting to SV. Subhymenium composed of relatively small, nearly isodiametrical to more globose cells. Lamellar trama with many large sphaerocytes. Pileipellis scarcely delimited from the underlying trama, poorly developed, a loose cutis of narrow, 2-4 µm wide, thin-walled and very long hyphae running often in straight lines on top of the pileus surface (resembling intersecting highways), mixed with the upper sphaerocytes of the trama, some with a distinctly encrusted-glutinous sheath; hyphal extremities not forming an individual layer, differentiated as slightly larger cells that either taper gradually and are hair-like, 1 µm diam, or that are constricted abruptly into a narrow, long appendage; pileocystidia dispersed, lying on the surface or ascending from the underlying trama (but these also very rare), 33-81(-124) x 2-5 µm diam, with granular-refringent contents. Stipitipellis similar to the pileipellis, but hyphal extremities and caulocystidia never appendaged, simply obtuse-rounded, near the base with trichoids composed of long, thin-walled and narrow hyphae 2-3 µm diam; caulocystidia (3-)4-5(-6) µm diam with distinct refringent contents.
ECUADOR. Napo, Shushufindi, 300 m alt, 15 May 1973, Singer B 7440 (HOLOTYPE!, F) (B 7740 cited in Singer 1975 and B 7400 cited in Singer et al 1983 are typographical errors).
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