A Systematic Study of the Genus Tetrachyron (Asteraceae: Heliantheae)

Abstract

Tetrachyron is a genus of five species restricted to eastern Mexico and northern Guatemala. Most previous treatments regard these species as members of various subgenera in the large polymorphic genus Calea. As a result of the various systematic techniques utilized in this study, the genus Tetrachyron is reestablished; it includes the subgenera Tephrocalea and Tetrachyron (in part). Diagnostic features for Tetrachyron are its shrubby habit, opposite, entire to serrulate margined leaves, sol- itary to corymbose capitulescences, and four-angled achenes which usually bear four pappus scales. Generally, these and other morphological, anatomical, and chemical features distinguish Tetrachyron from Calea and other allied genera. Tetrachyron Schlechtendal is a small genus of shrubs that occurs in eastern Mexico and Guatemala. Most species are unknown to the casual observer because of their uncommon occurrence and restricted distri- butions. Tetrachyron never has been studied intensively; the only taxo- nomic treatment produced for it is that by Robinson and Greenman (1896).