''Physarum pusillum'' is a

species A species () is often defined as the largest group of organisms in which any two individuals of the appropriate sexes or mating types can produce fertile offspring, typically by sexual reproduction. It is the basic unit of Taxonomy (biology), ...

of myxomycete (true slime mould) in the

polyphyletic A polyphyletic group is an assemblage that includes organisms with mixed evolutionary origin but does not include their most recent common ancestor. The term is often applied to groups that share similar features known as Homoplasy, homoplasies ...

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

Physarum ''Physarum'' is a genus of mycetozoan slime molds in the family Physaraceae. It contains the following species: *'' Physarum albescens'' *'' Physarum album'' *'' Physarum andinum'' *'' Physarum bivalve'' *'' Physarum bogoriense'' *'' Physarum c ...

''.

Taxonomy

''Physarum pusillum'' was first described by

Miles Berkeley Miles Joseph Berkeley (1 April 1803 – 30 July 1889) was an English cryptogamist and clergyman, and one of the founders of the science of plant pathology. Life Berkeley was born at Biggin Hall, Benefield, Northamptonshire, and educated at R ...

and M.A. Curtis in 1873 as ''Didymium pusillum''. The holotype (K-1492) was collected from South Carolina and kept in the

Kew herbarium The Kew Herbarium (herbarium code: K) is one of the world's largest and most historically significant herbaria, housed at the Royal Botanic Gardens, Kew in London, England. Established in the 1850s on the ground floor of Hunter House, it has gro ...

, however, a comprehensive search of the herbarium in 2018 was unable to locate it. In 1911, the species was reassigned to ''

'' by Gulielma Lister, with accompanying description and illustrations recognising two morphotypes: one with a subglobose sporotheca and another (rarer one) with a flatter, lenticular sporotheca. The latter "oblate" form was recognised as a distinct species in 2020 following morphological and molecular anyalyses, leading to the reinstatement of the name ''P. gravidum''. Further morphological differences that distinguish ''P. gravidum'' from ''P. pusillum'' include: the presence of an umbilicus, thin lime nodes in the capillitium, and spores largely ornamented with more-or-less evenly distributed warts and occasional large, poorly defined clusters of warts. Synonyms of ''Physarum pusillum'' include: ''Badhamia nodulosa'', ''Craterium nodulosum'', ''Physarum nodulosum'', ''Physarum calidris'', ''Lignydium calidris'', and ''Physarum mucoroides''.

Description

The fruiting bodies of ''Physarum pusillum'' are a stalked

sporangia A sporangium (from Late Latin, ; : sporangia) is an enclosure in which spores are formed. It can be composed of a unicellular organism, single cell or can be multicellular organism, multicellular. Virtually all plants, fungus, fungi, and many ot ...

, often gregarious, between 1–2mm tall. Each sporocarp has subglobose sporotheca, approximately 0.4–0.5mm in diameter, and a stalk that is membranous, limeless, longitudinally pleated, widened at the base, and standing either straight or with a slight curve at up to 1.3mm tall. The upper half of the stalk is translucent and orange or reddish-brown, while the lower half is opaque and dark brown. Its peridium is a single layer and encrusted with granular white to grey-white lime, although it is often orange-brown or reddish-brown and limeless at the base. Its

capillitium Capillitium (pl. capillitia) is a mass of sterile fibers within a fruit body interspersed among spores. It is found in Mycetozoa Mycetozoa is a polyphyletic grouping of slime molds. It was originally thought to be a monophyletic clade, but in ...

consists of enlarged, irregular lime nodes, filled with lime granules and connected by hyaline, branching threads. It does not have a

columella Lucius Junius Moderatus Columella (, Arabic: ) was a prominent Roman writer on agriculture in the Roman Empire. His in twelve volumes has been completely preserved and forms an important source on Roman agriculture and ancient Roman cuisin ...

or pseudocolumella.

Spores In biology, a spore is a unit of sexual (in fungi) or asexual reproduction that may be adapted for dispersal and for survival, often for extended periods of time, in unfavourable conditions. Spores form part of the life cycles of many plant ...

are globose, black in mass and brown by transmitted light, 8.5–12.5μm in diameter, and minutely warted. These warts are evenly distributed, with distinct groups of denser, larger warts. They are watery white as plasmodia.

Habitat and distribution

''Physarum pusillum'' is found on both herbaceous and woody plant debris, such as dead leaves and twigs, worldwide, including New Zealand, South Africa, Croatia, and South Africa. In general, ''P. pusillum'' favours warm temperate to tropical regions.

References

{{Taxonbar, from=Q70476743 Physaraceae Protists described in 1873