Petrusia Baillon.

Shrubs. Young branches and leaves with T-shaped trichomes. Stipules dry membranous. Leaves opposite or fascicled. Flowers axillary, solitary. Sepals 4. Petals 4. Stamens 8, in 2 whorls; filaments with white membranous appendages at base. Ovary 4-carpellate; stigma persistent. Fruit 4-valved. Seeds without endosperm.

B.-A. Beier et al. (Pl. Syst. Evol. 240: 11-39. 2003) proposed a new phylogenetic system in the subfamily of Zygophylloideae based on molecular and morphological data. This treatment recognized 40 species under the genus Tetraena, most of which were new combinations transferred from the genus Zygophyllum. According to this concept, Tetraena is distributed from Africa to Asia in a very wide range, which contrasts with the traditional concept that recognizes Tetraena as a unispecific genus endemic to Nei Mongol.

● One species: China.