Anamorphs of Hypoxylon
are referable to form-genus Nodulisporium. For convenience of
communication, Nodulisporium is subdivided into four types based on the
branching patterns of conidiogenous structures. These types are, listed in ascending order
of complexity: Sporothrix-like, Virgariella-like, Nodulisporium-like,
and Periconiella-like. These four major types are roughly in accordance with
the corresponding hyphomycetous genera defined in Jong and Rogers (1972). However, here
they primarily refer to the branching patterns of conidiogenous structures, regardless of
whether the conidial secession scars are poroid or denticulate.
The four types of conidiogenous structures based on branching patterns are described in the following links.
The hallmark of the Periconiella-like type is the property of "apical dominance" of the main axis, which differentiates it from the other three types. Nonetheless, the Periconiella-like type is like the Nodulisporium-like type in having the tendency to produce more than two conidiogenous cells at each branch terminus. Both the Nodulisporium-like and Virgariella-like types are similar in frequently producing additional branches, but differ in that the latter is simpler in its branching-unbranched or dichotomously branched. Conidiophores of the Sporothrix-like type are so reduced that the conidiogenous cells are, in fact, the entire conidiogenous structures. Additional branches, which proliferate from the conidiogenous regions of previous conidiophores, are present in all of these four types, although they are less frequently found in the Sporothrix-like and Periconiella-like types.
In the text, when the conidiogenous structure of a taxon corresponds to a particular type, the reader will be referred to one of those types. It should be emphasized that, even though a taxon has the conidiogenous structure typical of one of those four types, conidiophores deviating from that type are common in cultures. However, the dominant type found in cultures grown under the conditions set forth in this work is that given in the description of a given taxon. Conidiogenous structures of those taxa that do not correspond to any of those four types are briefly described and illustrated. Some of these conidiogenous structures represent intermediate forms of these four types, whereas others simply have branching habits that hardly resemble any of these four types.
We do not endeavor to key out the anamorphs of Hypoxylon in this work because we have cultures from only ca. one-third of Hypoxylon taxa. However, we have no doubt that anamorphs are extremely important toward erecting a natural classification of Hypoxylon. Some Hypoxylon that are difficult to differentiate on the basis of teleomorphs are greatly different in their anamorphs. Examples can be found in some pairs of taxa such as H. ticinense vs. H. subgilvum var. microsporum, H. lividicolor vs. H. lividipigmentum, H. subgilvum vs. H. jecorinum, etc. In H. rubiginosum, two distinct biotypes, which probably represent two different species, differ only slightly from each other in the stromatal surface color tones but have significantly different anamorphs: one produces a Nodulisporium-like conidiogenous structure, whereas the other produces a Virgariella-like one. The same situation may exist in other taxa as well. This will be manifested when more fresh collections become available for culturing. Culturing will become essential in order to achieve a more accurate identification of Hypoxylon, and, even now, should be done at every opportunity.