Amphistemon is distinguished by its stamens that are borne at two different levels in the flower, with 2 inserted in the corolla higher than the other two and thus with their 2 anthers situated higher than the other 2 anthers. The flowers of Amphistemon are distylous, with this arrangement of the anthers found in both flower forms; all 4 stamens are included in long-styled flowers, while 2 stamens are exserted and 2 are positioned in the corolla throat in short-styled flowers. This arrangement is unique in the tribe Spermacoceae (including Hedyotideae), and is otherwise found in very few genera of Rubiaceae (e.g., Paederia). The flowers are rather small and basically white, and the fruits are small capsules that open loculicidally across the top or beak portion. Amphistemon is similar to Oldenlandia, and these species were previously referred to that genus before the work of Groeninckx et al. (2010). Two species have been included in Amphistemon based on molecular and morphological characters, and both are from southwestern Madagascar but one is a slender herb while the other a short but strong shrub.
! CMT, V 2011