The Species of Cnidoscolus and Jatropha (Euphorbiaceae) in Far Western Texas

Keys, descriptions, and notes on nomenclature, ecology, and distribution are given for Cnidoscolus texanus, Jatropha dioica (with two varieties), and J. macrorhiza of the trans-Pecos and adjacent counties. No new names or taxa are published. This is one more of a series on the Euphorbiaceae of extreme western Texas. An index map of the area was published previously (Johnston and Warnock, 1962). Our two species of Jatropha L. are so dissimilar as to suggest their generic distinctness, and one of them, J. macrorhiza, has a striking habital resemblance to our Cnidoscolus species. The two genera are closely related, and together are marked among our other genera of the family by the cymose inflorescences (but these so reduced in J. dioica that the form is obliterated). Cnidoscolus Pohl, distinguished from Jatropha by the absence of petals, the petaloid calyx of the staminate flowers, and by the needlelike stinging hairs, has by some been considered a subgenus or section of Jatropha, or by others a separate genus. In taking the latter course we are simply following the last workers to reappraise these taxa and to revise their infrageneric systems, namely Pax and Hoffmann (1931), McVaugh (1944, 1945a, 1945b), and Miller and Webster (1962). The need for modern revisions at the species level becomes obvious, considering that since the last comprehensive revision, by Pax (1910), dozens of new species have been proposed.