C h a r a c t e r s. Fruit bodies stipitate-pileate, small or medium-sized.
Pileus surface dry, matt, neither viscid nor sticky when moist, at first glabrous, velutinous or pruinose, usually without a distinct fibrillose aspect when young, only in later stages sometimes becoming subtomentose, in some species gradually cracking with age in a typical manner and becoming areolate-rimose along the margin or overall, exposing the pallid context in the cracks. Pileipellis composed of a palisadoderm consisting of anticlinally arranged, parallel or subparallel hyphae whose terminal elements reach the same or almost the same level. The hyphae consist of chains of cells which are short to moderately long, usually more or less broadened and often incrusted in a typical way. Thickness of the palisadodermal layer varies according to species, age of fruit bodies and location on the pileus in the range of (80–)120–350(–500) μm (for the meaning of the term ‘palisadoderm’, see Clémençon 1997: 685). The palisadoderm, which is one of the most typical characters of this genus and which does not occur in this form in the other European boletes, is usually most typically developed and arranged in an early stage (see Fig. 9, and Šutara 2007: Fig. 5). In further stages the palisadoderm sometimes changes secondarily into a more or less disarranged layer. In most Xerocomellus species, however, the palisadoderm usually retains its characteris-tic appearance and arrangement for a longer time (see Fig. 10b), sometimes up to maturity.
Tubes at most 10(–14) mm long, nearly adnate or somewhat depressed around the stipe apex and shortly decurrent with a small tooth, light yellow to deep yel- low when young, later olive-yellow to greenish olivaceous. Pores concolorous, at maturity angular and – particularly in comparison with the smaller size of fruit bodies – relatively large (ca 1–2.5 mm). Tubes and pores more or less bluing or greening when bruised, rarely almost unchanging. The structure of the hymenophoral trama in a fully developed state is not phylloporoid but intermedi- ate between the phylloporoid and boletoid types, with lateral strata weakly but distinctly gelatinous. (Note: the hymenophoral trama of Xerocomellus species, like tramas of almost all other boletes, is developed best in younger developmen- tal stages; therefore the arrangement of this trama should be studied above all on younger fruit bodies). In a fully developed stage the hyphae of the lateral strata are 2–4(–8) μm distant from each other. In microscopic sections stained with Congo Red there is a distinct difference in coloration of layers of the trama. The mediostratum is stained much redder than the lateral strata. This colour contrast is substantially greater than in species of Xerocomus s. str. (for further informa- tion on the tramal structure intermediate between the boletoid and phylloporoid types, see Šutara 2005). Pleurocystidia scattered, mostly fusoid to lageniform, with walls smooth and thin, rarely slightly thickened (up to 0.6 μm).
Spores of boletoid shape, subfusoid or fusoid-ellipsoid, sometimes truncate at the top, with a more or less distinct suprahilar depression in profile. Spore surface smooth or longitudinally striate, never bacillate as in Xerocomus s. str. (for SEM- microphotographs of the spores of some Xerocomellus species, see Heinemann et al. 1988, Oolbekkink 1991, Klofac and Krisai-Greilhuber 1992, Engel et al. 1996, and others). Spore print brownish or brown, usually with a more or less discern- ible olivaceous shade when fresh.
Stipe less fleshy and more slender than in Boletus. With the exception of the basal part, the stipe is covered by a caulohymenium with scattered fertile caulobasidia. The caulohymenium, which is at first formed by a continuous, un- broken hymenial layer, soon fragments into small islands of caulohymenial ele- ments as the stipe gradually lengthens and expands. Macroscopically the islands of the caulohymenium look like very minute floccose granules on the stipe sur- face. Stipe surface sometimes longitudinally striate, usually non-reticulate, rarely with reticulation (e. g. in Xerocomus armeniacus var. venosipes Redeuilh). Lat- eral stipe stratum mostly absent, rarely present in a very reduced form, not thicker than 30(–40) μm. Stipe trama composed of densely longitudinally ar- ranged hyphae. Surface of the stipe base sterile, covered with a tangled tomentum of filamentous hyphae. Basal tomentum and mycelium whitish, yellow-whitish, grey-yellowish or dirty yellow. Partial veil and annulus absent.
Context yellow, light yellowish or whitish, sometimes partly reddish or wine- red in the middle or lower part of the stipe and brownish, dirty brown, orange- coloured or carrot-red in the stipe base, when cut or bruised more or less bluing, rarely almost unchanging. Hyphal system monomitic. Clamp connections not found in the fruit body.
Ontogenetic development of fruit bodies gymnocarpous (according to Watling 1985 and the author’s own observations).
Ectomycorrhizal with a range of trees, both frondose and coniferous.
Delimitation. The macroscopic appearance of Xerocomellus species is mainly characterised by the following features: small or at most medium-sized and often vividly coloured fruit bodies, a dry, at first velvety and later often rimose-areolate pileus surface, and a minutely granulose, sometimes longitudi- nally striate but mostly non-reticulate stipe which is usually slender and not very firm. The general appearance of Xerocomellus species, which is called ‘xerocomelloid’ in this paper, is so typical that the members of this genus are usu- ally distinguishable from the other boletes (including the macroscopically some- what similar species of Xerocomus s. str.) without great difficulties.
In 1989 the present author came to the conclusion that the Boletus chrysenteron group is well characterised by its anatomical characters, which moreover correlate with macromorphological features and therefore this group should be separated from the other xerocomoid boletes as a new genus. The au- thor hesitated for a very long time and finally decided to realize this intention in this contribution.
The separation of the B. chrysenteron group as an independent genus, Xeroco- mellus, is supported by results of recent molecular analyses, which show that in the phylogenetic tree this group forms a separate monophyletic branch which is sufficiently distant both from the Xerocomus subtomentosus group (Xerocomus s. str.) and from the other xerocomoid species (see Binder and Hibbett 2004, Eberhardt and Taylor 2005, Bakker and Noordeloos 2005, Binder and Hibbett 2007, etc.). Binder (1999), in his dissertation concerning molecular systematics of the Boletales, used the term „Paraxerocomus” for the group in question. This pro- visional name was, however, never validly published (as far as the present author knows).