The following paper is an evaluation of the taxonomic and ecological status of the genus Xanthium L. A review of its systematics demonstrates that many so-called "species" described on material from Europe actually have their origin in America, except one, X. strumarium s. str., which seems to have a Mediterranean–European center of dispersal. Another conclusion drawn is that Xanthium consists of only two distinct species: X. spinosum L. and X. strumarium L. The former is a relatively stable species, the latter an enormously variable one readily subdivided into a number of minor taxonomic entities.Ecologically, in eastern North America at least, Xanthium is primarily a beach plant, which prefers open habitats and succumbs to crowding. The seeds are most often dispersed by water and wind. It enters easily into ruderal habitats, but only as long as these are open and unshaded.The generalized short-day flowering response in this genus supports our hypothesis that Xanthium has a tropical–subtropical origin, and we feel that it has its center in Central and/or South America, whence it has spread over the continents north and southward.There is no evidence for any sterility barriers separating the entities of X. strumarium, but we feel that an intense inbreeding with an occasional outbreeding is responsible for the enormous variation, often resulting in small, local, but unstable taxa.
A Circadian Rhythm Set by Dusk Determines the Expression of FT Homologs and the Short-Day Photoperiodic Flowering Response in Pharbitis