DISCUSSION OF THE PLECTOMYCETES

The term "plectomycetes" is by now entirely superfluous and does not apply to any "natural" group of Ascomycota. However, the fact remains that many Ascomycota produce their asci and ascospores within cleistothecia and have no means of shooting their ascospores forcibly into the environment. Mycologists agree that these fungi represent specialized types of organisms, probably all with ancestors who possessed forcible ascospore discharge, and that they cannot be lumped together into a single group. So why do here what mycologists no longer condone? The reason is that when a collector finds a cleistothecial fungus he or she will not know immediately what group it belongs to and will file it as a "plectomycete" until there is time to study it in more detail.

Plectomycetes occur within several classes and orders of Ascomycota. The largest number belong to the Eurotiomycetes, a structurally enigmatic class containing large numbers of lichenized pyrenomycetes. Within the Eurotiomycetes the two orders Eurotiales and the Onygenales embrace the largest number of cleistothecial species.

Although the Eurotiales and Onygenales contain the greatest number of cleistothecial genera and species they are not alone in this. A number of common and significant (from a human point of view) species belong to other groups such as the Sordariomycetes, Pezizomycetes, Dothideomycetes and Leotiomycetes. Our approach here is to make a rough division of the plectomycetes into four groups that will be recognizable most of the time and then under each of these discuss some further divisions. The four major groups we wish to consider are:

• Eurotiales

• Plectomycetes having mostly brightly-coloured to almost colourless ascomata and ascospores. The majority have phialidic asexual stages belonging to the genera Aspergillus and Penicillium or, less commonly, to Paecilomyces or even simpler types. Rarely there is no anamorph at all. Most occur on materials of plant origin, including such domestic items as jam and bread. The spherical to egg-shaped asci are always uniformly packed in the centrum and the ascospores are always single-celled.

• Onygenales

• Similar to the Eurotiales in producing mostly colourless cleistothecia and ascospores. Asexual stages when present are nearly always of non-phialidic types such as Chrysosporium, Microsporum, Trichophyton or, commonly, there is no asexual stage at all. Many species in this group are associated with vertebrate animals and grow on dung, feathers, hair, or nesting materials, sometimes directly on living animals, including humans. The spherical to egg-shaped asci are always uniformly packed in the centrum and may be dispersed among hyphal elements. The ascospores are always single-celled.

• Plectomycetes related to pyrenomycetes

• Although by definition the pyrenomycetes should be confined to fungi producing their asci in perithecia many members of the Sordariomycetes and Dothideomycetes produce cleistothecia. As a group they are difficult to define, but should include most Ascomycota with dark cleistothecia or ascospores. Species having septate ascospores always go here. Some members having both colourless cleistothecia and colourless ascospores are recognized by their Acremonium asexual stages. Habitats include soil, dung, other fungi, decaying plant materials. None occur as parasites on living leaves.

• Plectomycetes related to discomycetes

• This is a relatively small group of species characterized by their consistently colourless and single-celled ascospores and their consistent lack of phialidic asexual stages. Some species have cylindrical asci arranged in a basal layer in the cleistothecia, a feature suggesting that they are not far removed from ancestors having forcible ascospore discharge. The ascospores are often large and highly symmetrical. Many grow on dung, soil and composting plant materials. The powdery mildews are a large group occurring as parasites on living leaves of higher plants.