Xerocomellus zelleri, commonly known as Zeller's bolete, is a species of fungus in the family Boletaceae. First described scientifically by American mycologist William Alphonso Murrill in 1912, it has been juggled by various authors to several genera, including Boletus, Boletellus, and Xerocomus. The fruit bodies are distinguished by their dark reddish brown to nearly black caps with uneven surfaces, the yellow pores on the underside of the caps, and the red-streaked yellow stems. The development of the fruit bodies is gymnocarpic, meaning that the hymenium appears and develops to maturity in an exposed state, not enclosed by any protective membrane. Found solely in western North America from British Columbia south to Mexico, the mushroom grows in summer and autumn on the ground, often in Douglas-fir forests or on their margins. It is edible but not choice.
Taxonomy Xerocomellus zelleri was first described by American mycologist William Alphonso Murrill in 1912, based on specimens he found on the campus of the University of Washington. Murrill named it Ceriomyces zelleri before switching the genus later that year to Boletus. In 1944, Walter Henry Snell thought the taxon would be more appropriate in the genus Xerocomus. In 1959, mycologists Rolf Singer, Snell and Esther A. Dick transferred the species to Boletellus, explaining that the microstructure of the trama and the faint ornamentation of the spores were inconsistent with placement in Xerocomus. American mycologist Harry D. Thiers, in his 1976 monograph on North American boletes, claimed that he failed to consistently find ornamentation on the spores of material he collected, and preferred to retain the species in Boletus. In 2011, it was moved to the genus Xerocomellus. The specific epithet zelleri was chosen by Murrill to honor Professor Sanford Myron Zeller, mycologist at Oregon State University. Zeller accompanied Murrill in his Seattle expedition, and discovered the first specimens of the mushroom.
Description The cap is typically between 3–16 cm (1+1⁄8–6+1⁄4 in) in diameter, initially convex but flattening somewhat in maturity. It is fleshy, with an uneven velvety surface, and dark brown to nearly black; the margin of the cap is a pale cream color. Young specimens are covered by a grayish bloom.
The tubes that comprise the undersurface of the cap (the hymenium) are up to 1.5 cm (5⁄8 in) long and angular, yellow, becoming dirty yellow and finally greenish-yellow; there are 1–2 pores per millimeter on the hymenium surface. They may turn slightly brownish when exposed to the air for a time. The flesh is yellow to dirty yellow, up to 1.5 cm (5⁄8 in) thick, and inconsistently bruises blue when cut or broken. The stem is up to 12 cm (4+3⁄4 in) tall, 1–3 cm (3⁄8–1+1⁄8 in) thick, and swollen toward the base. The stem surface is red or yellowish with red lines, often white or yellow at the base, and solid (that is, not hollow), with fibrous flesh; in maturity the stem ages to yellowish-red to dark red. The spore print is olive-brown, with one source noting that it may be juicy. The spores are ellipsoid in shape, smooth, and have dimensions of 12–16 by 4–6 μm, although occasionally there will be some "giant spores" with lengths of up to 24 μm. The basidia, the spore-bearing cells, are 26–35 by 9.5–12 μm, and four-spored. The cystidia are roughly cylindrical and thin-walled, with dimensions of 38–77 by 5.5–14.8 μm. There are no clamp connections present in the hyphae. The fruit body tissue stains a greenish color when a drop of ammonia solution is applied.
Fruit body development
In 1914, Zeller published a study of the development of the mushroom, made possible by the prolific fruiting of the fungus in Seattle in the fall of 1912. Development was studied by examining thin sections of tissues in different stages of development, and the differentiation of tissues and structures followed by using histological stains. The growth form of Xerocomellus zelleri is called gymnocarpic, meaning that the hymenium appears and develops to maturity in an exposed state, not enclosed by any protective membrane. In this type of development, the cap is formed from hyphae at the top of the stem and subsequently expands by growth along the margins; the hymenium forms later beneath the cap in a direction away from the center. The mushrooms originate as minute fruit bodies (called "pins" due to their shape) from a yellow mycelium that forms a mat and tends to engulf pine needles. The pins, typically 1–2 mm in diameter, lengthen vertically until they are roughly three or four times longer than they are thick. Until this point, the fruit body is a homogenous mass of tissue. It differentiates simultaneously into cap and stem along a cleavage plane (an axis along which any cell division occurs) from the outside inward, which gives rise to deep furrow encircling the fruit body. The hymenium is formed in the roof of this furrow, growing inward and upward from the outside edge. The cap develops from the upper section of this division, the stem from the lower.
Similar species The red-cracked bolete (Xerocomellus chrysenteron) has an olive-brown cap that cracks, exposing flesh that ages to pinkish red. Boletellus chrysenteroides, found only in eastern North America, has a velvety to smooth, dark reddish brown, cracked cap with pale exposed flesh. Also similar is Boletus mirabilis.
Habitat and distribution
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