- Group 1Lycophytes, Monilophytes
- Group 2Gymnosperms
- Group 3Monocots
- Group 4Woody angiosperms with opposite or whorled leaves
- Group 5Woody angiosperms with alternate leaves
- Group 6Herbaceous angiosperms with inferior ovaries
- Group 7Herbaceous angiosperms with superior ovaries and zygomorphic flowers
- Group 8Herbaceous angiosperms with superior ovaries, actinomorphic flowers, and 2 or more distinct carpels
- Group 9Herbaceous angiosperms with superior ovaries, actinomorphic flowers, connate petals, and a solitary carpel or 2 or more connate carpels
- Group 10Herbaceous angiosperms with superior ovaries, actinomorphic flowers, distinct petals or the petals lacking, and 2 or more connate carpels
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).
Show photos of: Each photo represents one species in this genus.