Herbs, perennial, sometimes annual, not fleshy, autotrophic. Stems prostrate to ascending. Leaves cauline, opposite, simple; stipules reduced to ridges; petiole absent; blade not fleshy, not leathery, margins entire or finely serrate. Inflorescences terminal or axillary, dichasial or monochasial cymes, sometimes flowers solitary in axils. Flowers bisexual, perianth and androecium partly hypogynous [hypogynous]; sepals 4 or 5 (or 6), proximally connate, calyx radially symmetric; petals 4 (or 5), proximally connate, corolla radially symmetric, regular, short-funnelform [subrotate]; stamens 4 (or 5), adnate to corolla, equal, staminode 0; pistil 1, 2-carpellate, ovary partly inferior [superior], 2 [–4] -locular, placentation axile [basal]; ovules anatropous or amphitropous, unitegmic, tenuinucellate; style 1; stigma 1. Fruits capsules [nutlets], dehiscence septicidal. Seeds [4] 70–120+, yellow, angled to angled-globular; embryo straight, endosperm abundant.
Distribution
c, se United States, Mexico, West Indies, Central America, South America, Pacific Islands (New Zealand), elsewhere in Pacific Islands, Australia
Discussion
Genera 2, species 3 (1 in the flora).
Works by B. Oxelman et al. (1999) and S. J. Wagstaff (2004) are followed in circumscribing Tetrachondraceae as comprising two formerly disparate genera, Polypremum and Tetrachondra Petrie ex Oliver. Oxelman et al. found evidence from rbcL and ndhF analyses supporting their membership in a clade within the Lamiales. This was confirmed by B. Schäferhoff et al. (2010) from analyses using trnK/matK, trnL-F, and rps16 sequence data.
Polypremum and Tetrachondra have generally been placed in different families. Tetrachondra, a genus of two species with sessile leaves and flowers with a superior ovary and gynobasic style, known only from Argentina, Chile, and New Zealand, was first placed in Crassulaceae (where its two-carpellate ovary was anomalous) by W. W. Hamilton in 1885 (S. J. Wagstaff 2004). C. Skottsberg (1912) thought it might be best treated as a unigeneric family near the Lamiaceae; A. Cronquist (1981) placed it within the Lamiaceae. Polypremum has often been placed in the Buddlejaceae, Loganiaceae, or Rubiaceae (see G. K. Rogers 1986).
A. L. Takhtajan (2009) followed L. Watson and M. J. Dallwitz (http://delta-intkey.com) in recognizing Watson’s unigeneric, unpublished Polypremaceae, which Reveal published in 2011.
