Genus Hypoxylon
Key to Taxa of Hypoxylon
Accepted Taxa
List of Names

Yu-Ming Ju
Michael J. Adams

Hypoxylon Bull., Hist. Champ. France I, p. 168. 1791; non Mentzel ex Adans., 1763; [nom. cons.].
LECTOTYPE [selected by Shear (1928)]: H. coccineum Bull. [= H. fragiforme (Pers.: Fr.) J. Kickx fil.]

º Euhypoxylon Füisting, Bot. Zeitung (Berlin) 25: 309. 1867.
TYPE: Hypoxylon coccineum Bull. [= Hypoxylon fragiforme (Pers.: Fr.) J. Kickx fil.]

= Perisphaeria Roussel, Fl. Calvados, ed. 2, p. 42. 1806.
LECTOTYPE [selected by Laessøe (1994)]: Sphaeria fragiformis Pers.: Fr. [ Hypoxylon fragiforme (Pers.: Fr.) J. Kickx fil.]

= Sphaeria Haller, Hist. Stirp. Helv. III, p. 120. 1768; [nom. rejic., ICBN Art. 14].
LECTOTYPE [selected by Donk (1964)]: S. rubra, fragi similis Haller [= Hypoxylon fragiforme (Pers.: Fr.) J. Kickx fil.]

= Discosphaera Dumort., Commentat. Bot., p. 91. 1822.
LECTOTYPE [selected by Donk (1964)]: D. radians (Tode) Dumort. [º Sphaeria radians Tode = Hypoxylon fragiforme (Pers.: Fr.) J. Kickx fil.]

= Pyrenodermium Bonord., Handb. Mykol., p. 272. 1851.
LECTOTYPE [selected by Laessøe (1994)]: Sphaeria rubiginosa Pers.: Fr. [º Hypoxylon rubiginosum (Pers.: Fr.) Fr.]

= Epixylon Füisting, Bot. Zeitung (Berlin) 25: 309. 1867.
TYPE: Hypoxylon rubiginosum (Pers.: Fr.) Fr.

= Hypoxylina Starb., Ark. Bot. 5: 29. 1905.
TYPE: H. umbilicata Starb. [= Hypoxylon subrutilum Starb.]

= Pyrenopolyporus C. G. Lloyd, Mycol. Writings 5: 706. 1917.
TYPE: P. hunteri C. G. Lloyd [= Hypoxylon polyporum (Starb.) Y.-M. Ju & J. D. Rogers]

Laessøe (1994) argued that the type species of Hypoxylon, H. coccineum, is not "a straightforward synonym of H. fragiforme as universally claimed" because "Bulliard stated his fungus to be common mainly on Juglans and Aesculus". He considered that this host range would indicate H. howeianum rather than H. fragiforme. Despite the logic of Laessøe's argument, it is almost certain that it will never be established whether or not H. coccineum is synonymous with H. fragiforme or H. howeianum because the Bulliard material of H. coccineum is not known to be extant, and because ascospore measurements-the principal required information to tell H. fragiforme from H. howeianum-were not cited in the protologue of H. coccineum. Since accepting H. coccineum as a Hypoxylon is beyond question and the uncertainty of the identity of H. coccineum is not causing any nomenclatural problem, we are not inclined to disturb the status quo but accept H. coccineum as a synonym of H. fragiforme, as has been done by numerous mycologists. Synonyms of Hypoxylon have been documented and discussed by Laessøe (1994). However, we do not accept Daldinia and Versiomyces Whalley & Watling as synonyms of Hypoxylon. The presence of concentric zone lines in Daldinia is the major character used to separate the genus from Hypoxylon. The zonation of stromata, according to Bayliss-Elliott (1920), is due to attempts to form successive layers of perithecia. The tissue between zone lines is fibrous, being composed of strands of hyphae which are vertically oriented to the zone lines. It is possible that such an architecture in stromata acts to efficiently channel water up to the peripheral layer of perithecia during brief moist periods in dry environments, and thus might have an important ecological significance. It is known that the rings gelatinize to some extent and probably hold water. Versiomyces seems related to Daldinia. It is zonate, and has tissue between zone lines that is very dense and leathery when dry but very gelatinous when wet. It is possibly congeneric with Xylocrea A. Möller, a genus without an extant type. Daldinia and Versiomyces are not the only genera that are separated from Hypoxylon on the basis of one major character. Entonaema differs from Hypoxylon in having a gelatinous matrix lining a liquid-filled cavity; Rhopalostroma differs in having erect stromata; and Biscogniauxia differs in having a dehiscent outer stromatal layer. Lumping Daldinia and Versiomyces with Hypoxylon seems to suggest lumping these other genera with Hypoxylon as well. The so-called Hypoxylon sensu stricto could end up in having a definition even broader than that of Hypoxylon sensu lato!

On the other hand, while these allied groups of Hypoxylon are kept as distinct genera, it is also arguable if Hypoxylon really represents one single homogeneous genus. The two sections of Hypoxylon can be differentiated by three major characters as shown in SUBGENERIC TAXA OF HYPOXYLON. It is reasoned in the EVOLUTION part that these two sections may have diverged before Daldinia, Entonaema, and Rhopalostroma diverged from the section Hypoxylon. To reflect the morphological differences and phylogenetic aspects, section Annulata might merit generic status.