Xanthium

Xanthium is a difficult genus in that it requires careful examination of micromorphological details of the fruiting carpellate involucres (often referred to as “burs”). Accurate determination of many collections is complicated by the fact that they possess flowering or immature fruiting involucres. Tremendous disagreement exists as to the number of species to recognize. A moderate treatment, that of Cronquist (1945), is here followed with reservation (I suspect that earlier works that recognized more species are, in fact, closer to the truth). Though Cronquist’s ideas regarding Xanthium appear to be supported by correlated characters, some distinctive forms with ecological differences are not recognized (e.g., X. echinatum, a species of Atlantic coast beaches, dune hollows, and marsh borders). Reference: Fernald (1946).