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Page number:613 
Remarks (internal):Microscopic variability is evident in this taxon with regard to spore ornamentation, shape of pileal terminal cells and dermatocystidia. For the latter, variation ranges from few and with poor contents (e.g. Buyck 04-282) to many and with abundant, refractive contents (e.g. Boyd 0724). In some collections the form of the terminal cells in the pileipellis is more frequently ampullaceous to rostrate-appendiculate (e.g. Boyd 0905, DEWV 1800), while in others obtusely convex (e.g. DEWV 4811).
Spores vary considerably in size-not only among collections, but also within a single individual-as well as in ornamentation height and distribution (especially the relative abundance of linear elements versus isolated warts). Some collections have spores with large, almost droplet-like warts. This phenomenon seems to occur when fruiting conditions at the time of spore development are not ideal. Atypical for section Heterophyllae, this new taxon can have spores with an amyloid, suprahilar plage.
As mentioned above it is easy to separate R. parvovirescens under the microscope from other palespored, mild russulas that have a similar appearance in the field (i.e. R. crustosa Peck and R. virescens Fr.). These species have much more slender, less voluminous extremities in the pileipellis ending in more narrow, subulate, terminal cells. Some collections of R. crustosa, however, may have equally short, but in this case much less voluminous, narrower, terminal cells. Because of the much smaller size of these cells in R. crustosa, confusion with this new species is excluded. On the other hand the virescens-crustosa group is much more complex than suspected and embraces at least a dozen taxa in the eastern US as suggested by a forthcoming molecular study (Buyck et al 2005). A provisional identification key to these taxa has been posted at http://www.mtsn.tn.it/russulales-news/id_virescentinae.asp.
Some other Russula species in sect. Heterophyllae Fr., such as R. aeruginea Fr., R. redolens Burl., R. variata Bann., R. heterophylla Fr. or R. subgraminicolor Murrill, also might be confused because of their green pilei. The aforementioned green Russulas, however, have cap surfaces that are never so strongly disrupted or fragmented as in R. parvovirescens.
 
Description type:Original description 
Description:Russula parvovirescens spec. nov. figs. 1-9
Cap 40-60(80) mm diam, quite regular in form, slightly depressed to plane in the center, toward the margin rather thin and slightly striate with age; surface feltyvelutinous, dull, never viscous; suprapellis separable for one-half to one-third of the radius, greenish brown to dark olive green or metallic bluish green, rapidly becoming coarsely disrupted forming typically large pellicles exposing the grayish to bluish green or pale brownish gray fibrous flesh of the subpellis, developing ferruginous discolorations upon exposure, often entirely discoloring pale brown in the center, at other times remaining much darker in the center; the appressed pellicles often turning paler than the surrounding context because of the presence of a pale reticulate pattern easily observed with a hand lens. Gills adnate to subfree, 4-8 mm high, slightly narrowing towards the cap margin, more or less 1/mm, brittle, locally anastomosing to forking around the stipe, some forking closer to the cap margin, not interveined, cream; edges concolorous, even. Stipe 30-62 x 8-13 mm, slender, not hard-fleshed, smooth, cylindrical, longer than the cap diameter, not pruinose, whitish, rounded at the base, the interior with a few cavities; veil absent. Flesh 4-5 mm thick above the gill attachment to the stipe, mild, inodorous, white, turning brownish orange with FeSO4 on stipe context, but only slightly discoloring on stipe surface, moderate to strong reaction with guaiac. Spore print pale cream (Romagnesi IIa, Dagron 3-4). Exsiccatum with dark green (4F4-8) to grayish green (3E3-5) to olive green (4DE3-5) or exceptionally reddish brown (6-7DE5-7), sometimes decolored and pale brown (5CD5-7) in the center, cap surface strongly fissured and exposed flesh much paler (3-4B3-4). Stipe and gills not colored.
Spores 6.7-7.5-8.0-9.1 x 5.7-6.4-6.5-7.2 µm, Q = 1.07-1.17-1.26-1.43 (n = 80); ornamentation a dense to moderately dense incomplete network of often interconnected obtuse warts and short crests, strongly amyloid, less than 1 µm high, suprahilar plage either nonamyloid or with a weakly amyloid spot or amyloid verruculae, some collections with a clear amyloid spot on most spores. Basidia 38-45 x 8-9 µm, inflated in upper half, 4-spored, rarely 2-spored near gill edge. Cystidia irregularly dispersed (600-800/mm2 in most dense areas), (50)60-70 x (6)8-9 µm, slender, subfusiform to clavulate, generally capitulate-appendiculate at tip, originating in subhymenium, thin-walled, with coarsely crystalline contents, reacting strongly and immediately black in sulfovanillin, yellow in KOH. Marginal cells distinct, rendering the gill edge sterile; end cells slender, distinctly or gradually tapering, (15)25-40 x 3-5(6) µm diam, optically empty, occasionally replaced by cheilocystidia. Subhymenium well developed, especially in the gill interspace with more voluminous cells resembling the pseudoparenchymatous structure of the pileipellis. Trama with dispersed oleiferous fragments, abundant sphaerocytes, with distinctly encrusted slender hyphae of variable diameter, especially close to the gill margin. Pileipellis orthochromatic in cresyl blue, two-layered; subpellis a slightly gelified layer of thin, clearly encrusted hyphae, 2-3 µm diam, underlying a strongly developed pseudoparenchymatous suprapellis; the latter with basal cells measuring 15-20(30) µm diam, narrowing to 6-10(15) µm diam for the terminal cell, which is sometimes yellowish-oily in KOH. Pileocystidia mainly situated in the subpellis, some ascending from subpellis in between the cells of the suprapellis, long and slender, cylindrical, often attenuated at the tip, with abundant, coarsely crystalline contents. Stipitipellis less well developed than pileipellis, composed of much smaller elements, measuring 4-5(6) µm diam. Caulocystidia numerous and evident because of the abundant, crystalline contents, 39-70 x 5-7 µm, subfusiform to subcylindrical, minutely capitulate to appendiculate. Clamps absent.
Habitat. - Terrestrial, solitary or scattered, among leaves, in deciduous or mixed broadleaf forests.
 
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