The genus Hypoxylon is delimited by four main characters: one, anamorphs Nodulisporium-like—the primary criterion for recognizing a xylariaceous fungus as a member of Hypoxyloideae; two, stromata unipartite; three, stromatal tissue below the perithecial layer solid and homogeneous; and four, stromata not upright, i.e., with the height The genus Hypoxylon is delimited by four main characters: one, anamorphs Nodulisporium-like—the primary criterion for recognizing a xylariaceous fungus as a member of Hypoxyloideae; two, stromata unipartite; three, stromatal tissue below the perithecial layer solid and homogeneous; and four, stromata not upright, i.e., with the height less than the length and the breadth.less than the length and the breadth.
Some other characters are useful in suggesting Hypoxylon. Unfortunately, these characters are not universally possessed by taxa of Hypoxylon, and therefore their usefulness in defining Hypoxylon is limited. These characters include the dehiscent perispore, the flat ascal apical ring, the germ slit on the convex side of the ascospore, the waxy stromatal tissue, and the KOH-extractable stromatal pigments. For example, there are more than thirty taxa of Hypoxylon in which the perispore is not easily dehiscent, and many taxa have nearly equilateral ascospores which make the position of the germ slit-whether it is on the convex or the flattened side-impossible to assess.
