Epithet = middle (marrow) - bread or loaf. Genus = perennial pored.
Annual to perennial. Widely effused, resupinate, tough-corky. Pore color variable, pale gray, cream color to yellow-buff or bright yellow. Pores very small, 5–7 per mm, with thick walls. Context cream color to yellow; showing layers, each layer up to 1 mm thick.
Microscopic examination is required to identify the species.
The spores are small, 5–6.5 × 3–4 µm, thick-walled, somewhat truncate (flattened on top at apical pore), and dextrinoid (orange in Melzer's reagent). The trama is dextrinoid with a trimitic hyphal system, the binding hyphae are narrow and much branched, contrasting with the thicker and unbranched skeletal hyphae.
The combination of very small pores and the small, thick-walled, dextrinoid spores, separate this species from the other resupinate Perenniporia, particularly those that may be found in the Upper Midwest.
P. ellipsospora has similar spores but the pores are 3–4 per mm, and the fruitbody is thin with angular and slightly dentate (toothy) pores.
Both P. subacida and P. tenuis have thin-walled, non-dextrinoid spores (negative in Melzer's reagent).
P. subacida has pores 5–6 per mm, and wider skeletal hyphae.
P. tenuis has pores 2–3 per mm.
P. variegata has pores 2–3 per mm, with spores that are small, dextrinoid, slightly thick-walled.
White rot of dead hardwoods; rarely on conifers.
Perennial polypores can be found year-round.
Worldwide species complex. Widespread in North America.
Chicago Region status
Uncommon or easily overlooked.
The historic collections by Harper and Moffatt from 1903 to 1918 were split and spread among 7 herbaria (F, FH, MIN, NY, TENN, UC, USDA). There is also a collection by V. O. Graham (1940). These collections need to be examined and verified; they don't look identical, particularly having variation in pore size.
P. R. Leacock 12353 (31 May 2015) and 12422 (11 July 2015) from same location (same fruiting).
Epitype: Norway, Vestfold NL 77, Guldkronen ved Jarlsberg hovedgård, on wood of Quercus, 01 Aug. 1971, L. Ryvarden 7587 (O);
This species has also been placed in Physisporus (1826), Trametes (1900) and Fomitopsis (1941). Some previous and current mycologists suspect this species name represents a group of similar species. Slava Spirin (2015 personal communication) says: There are at least 3 'medulla-panis' in US, and none of them is identical with the real P. medulla-panis from Europe.
J.L. Lowe 1966: This species was selected as the type for the genus Poria by Persoon in 1794 and made the lectotype by Donk in 1949, as type for genus Perenniporia by Murrill , and type for the genus Poria as emended by M. C. Cooke. But the genus Poria Persoon 1794 was an illegitimate homonym of the earlier genus Poria P. Browne 1756, whose type species, Polyporus michelii Fr. 1821, is a synonym of Polyporus squamosus. That is a bit of tangled taxonomy and nomenclature that explains why we no longer use the name Poria.
There are illustrations and 5 description excerpts on MycoBank
Lowe, J.L. 1966. Polyporaceae of North America. The genus Poria.
Technical Publication of the State University College of Forestry at Syracuse University. 90: 1-183.
Description on Mycobank