Palynology of the Genus Passerina (Thymelaeaceae): Relationships Form and Function

Pollen of the genus Passerina L. differs markedly from that of other southern African members of the Thymelaeaceae. Grains of most members of the Thymelaeaceae are characterised by a typical croton pattern, comprising rings of more or less trihedral sexine units mounted on an underlying reticulum of circular muri. In Passerina, however, the supratectal subunits are fused completely to form a continuous reticulum, which replaces the underlying reticulum. The reticulum in Passerina is therefore secondary in origin and not homologous with the basal reticulum of typical crotonoid grains in the family. The croton pattern has often been used as indication of a possible relationship between the Euphorbiaceae and Thymelaeaceae. Pollen of Passerina is adapted to anemophily. Grain sculpturing clearly demonstrates secondary derivation of a reticulate pattern from the typical croton pattern, through reduction, aggregation and fusion. Pollen of Passerina represents a climax of a continuum of variation in the exine of pollen in the Thymelaeaceae. As Passerina is considered phylogenetically advanced in the subfamily Thymelaeoideae, the subtribe Passerininae is raised to tribal rank, namely tribe Passerineae.