Pentaphragma is the sole genus in Pentaphragmataceae, a family in the order Asterales.

Pentaphragma is rayless, but eventually develops rays in at least one of the species studied.

This is interpreted as related to secondary woodiness or upright habit within a predominantly herbaceous phylad.

The vessel elements of Pentaphragma have features universally interpreted as primitive in dicotyledons: scalariform perforation plates with numerous bars; pit membrane remnants in perforations; scalariform lateral wall pitting; the genus also has fiber-tracheids with prominently bordered pits.

The presence of occasional scalariform perforation plates, often aberrant, in secondary xylem of families of Asterales sensu lato - Campanulaceae, Pentaphragmataceae, Valerianaceae, and even Asteraceae (e.g., certain Lactuceae) - can be attributed to paedomorphosis, extending these plates into secondary xylem from primary xylem.