The single most important characteristic of the Physarales is the presence of lime (calcium carbonate) deposits in the peridium, capillitium, or stalk of the fruiting body. The presence of lime is usually an obvious feature, but under certain environmental conditions fruiting bodies are sometimes produced that have very little lime. Such fruitings are often difficult to identify. The fruiting body produced by a member of this order is most often a sporangium, but some species produce a plasmodiocarp or (more rarely) an aethalium. A columella may or may not be present. The capillitium consists of threadlike or tubular filaments throughout. Spore colour in mass ranges from deep violaceous brown to purple-brown or black.
Three families are recognised in the Physarales, but one (the Elaeomyxaceae) is not known from New Zealand. The other families,the Physaraceae and the Didymiaceae,are well represented. Both are large and diverse, but they differ in one important respect. In the Physaraceae the capillitium is, at least in part, calcareous, but it is limeless in the Didymiaceae. Physarum, with more than 150 described species, is the largest genus of the myxomycetes (Lado 2001).