This is a genus of annual or perennial, mainly small, herbaceous plants of wide distribution in the Guayana area from the Guianas to Venezuela, and extending south in eastern Brazil to the states of Goyaz, Bahia, and Minas Geraes. The genus has not been treated since the account by Schumann in Martius's Flora Brasiliensis in 1889 (6°- 87-91). One of the species from Minas Geraes, Brazil, described as new below has been an herbarium name for the past three decades, while another from the same state is as yet inadequately known.
An interesting feature of the genus is the dimorphism of the corollas of some of the species, especially in P. galioides, in which flowers having long-exserted styles and included anthers on short filaments contrast with those having shorter included styles and stamens on filaments exserted from the orifice. Great variation is exhibited in the pubescence and foliage of the plants as well as habit, leading to many intergrading forms. In general, however, the species are well-characterized in a distinct, well-marked genus, which is notable in possessing a 2-parted calyx and a circumscissile dehiscent capsule.