Fructification closely gregarious; sporangia dark grey, ovate or cylindric, sessile, or narrowed below, but not strictly stipitate ; about 1 mm high, with apical dehiscence; the capillitium dense, the tubules very short, the nodes large, irregular, white, physaroid, more or less aggregate at the center, otherwise no columella; spores dark violaceous, coarsely spinulose, thick-walled save for a limited area apparently thinner, but bearing a few scattered spinules, 12-14 µm.

Resemblance is with B. papaveracea (Berk.) Rost.; the spores are adherent in fours, apparently; but there are no stems and the sporangia are cylindric or irregularly prolate.

PDD 16175, 16183, 22109

Fruiting body a sessile sporangium (or often forming a short plasmodiocarp), closely gregarious to crowded or heaped, subglobose to pulvinate, 0.3–0.5 mm in diameter. Hypothallus membranous, colourless to white, often inconspicuous. Peridium consisting of a single layer, membranous, more or less densely covered or impregnated with lime, white to cinereous (or iridescent to dark brown when limeless). Columella absent. Capillitium consisting of numerous, variously shaped, mostly angular or branching white lime nodes (these occasionally massed in the centre) connected by hyaline filaments. Spores purplish brown in mass, light violaceous by transmitted light, asperulate to minutely warted, 9–11 µm in diameter. Plasmodium watery white or colourless, reported to become at times bright yellow before fruiting.

Reported as cosmopolitan (Martin & Alexopoulos 1969) but probably most common in temperate regions of the world. First reported (as Didymium cinereum) from New Zealand by Berkeley (1855), based on a specimen collected by W. Colenso in Northland. Also known from Auckland, Coromandel, Waikato, Bay of Plenty, Taranaki, Taupo, Wanganui, Wellingon, Hawkes Bay, Wairarapa, Nelson, Marlborough, North Canterbury, Mid Canterbury, South Canterbury (Lister & Lister 1905), Dunedin (Rawson 1937), Southland (Rawson 1937), and Campbell Island.

Dead leaves; also fruiting on living plants.

Martin & Alexopoulos (1969), Nannenga-Bremekamp (1991), Stephenson & Stempen (1994), Neubert et al. (1995), Ing (1999).

Several other species of Physarum produce sessile sporangia and are found in similar situations, but none of these is known to produces the extensive fruitings sometimes characteristic of P. cinereum. One example of an exceptionally large fruiting in a grassy area in Auckland had a diameter of approximately 8 meters (Peter Buchanan, personal communication).