Euphorbia sect. Tithymalopsis (Euphorbiaceae) comprises seven species of herbaceous perennials restricted to the south-eastern USA. The species of sect. Tithymalopsis are united and distinguished from other sections in Euphorbia by having a thick and cylindrical root system, a glabrous cyathium, five glands with petaloid appendages, one to three stems arising from the root, absence of vestigial sepals in pistillate flowers, rounded leaf apices, and pitted or smooth seeds without a caruncle. Cladistic analyses indicate that sect. Tithymalopsis consists of two subgroups: (1) E. corollata, E. discoidalis, E. pubentissima and E. polyphylla; and (2) E. curtisii, E. gracilior and E. mercurialina. Comparative data from vegetative morphology, pollen morphology, seed morphology, cytology, crossability, reproductive biology, habitat and distribution are presented. Keys and descriptions to the species are provided.
