82. Iva L. (marsh elder)

About 9 species, North America.

Until recently, Iva was considered by most botanists to be a nearly cosmopolitan genus of about 15 species (Jackson, 1960). Bolick (1983) performed a cladistic analysis of Heliantheae subtribe Ambrosiinae Less. based on morphological, palynological, and phytochemical characters and suggested that some of the traditional generic circumscriptions did not describe natural phylogenetic lineages. In particular, she noted that some species of Iva apparently were more closely related to species of Ambrosia than to the remainder of Iva. Karis (1995) expanded on this study by sampling additional species and characters, and also concluded that the traditional circumscription of Iva included members of more than one evolutionary lineage. Molecular phylogenetic research by Miao et al. (1995a, b) provided further evidence that Iva in the broad sense is not a natural group. Strother (2000) reviewed the data from these earlier studies and concluded that a number of generic segregations were necessary in order to reflect the phylogeny of the group, resulting in the recognition of six total genera for the North American species formerly included in Iva. For Missouri, the practical consequence has been that I. xanthiifolia is now treated as the monotypic segregate, Cyclachaena. Cyclachaena differs from Iva in a number of gross morphological features, including its more strongly paniculate inflorescences with small, bractless clusters of heads at the nodes and its pistillate florets lacking a corolla, as well as differences in secondary compounds and pollen ultrastructure.

Species of Iva shed abundant wind-borne pollen and are important causes of hay fever in areas where plants are abundant. Some species also can cause contact dermatitis in susceptible individuals.