The Systematics of the Bidens ferulaefolia Complex (Compositae)

Abstract

Bidens ferulaefolia has been recognized as a species consisting of three varieties (var. ferulaefolia, var. foeniculifolia, and var. ludens) widely distributed from southwestern United States through Mexico and into Guatemala. Separation of these taxa was based on several highly variable morphological characters such as achene length, awn number, and degree of leaf dissection. These taxa were studied utilizing field and herbarium material, morphological data, cytotaxonomic studies, green- house and hybridization experiments, and comparative flavonoid chemistry. Results obtained from these investigations suggest that three species be recognized: B. fer- ulaefolia, B. carpodonta, and B. longistyla. Bidensferulaefolia occurs in disturbed habitats on the high plateau from Guanajuato in southcentral Mexico northwest into the foothills of the Sierra Madre Occidental in Chihuahua in northern Mexico. Bidens carpodonta inhabits disturbed areas in mountain valleys or on high plateaus, while B. longistyla is found in mountain valleys on gypsum and limestone outcrops, in the Sierra Madre Oriental of northeastern Mexico in the states of Nuevo Le6n and Coahuila. Morphological characteristics based on quantitative and qualitative fea- tures, such as style branch lengths and presence or absence of a red pigment in the flowering heads, are useful for distinguishing the three taxa. Cytotaxonomic data for these three species showed that B. ferulaefolia has a chromosome number of n = 10, while B. carpodonta and B. longistyla are both n = 12. Hybridization experiments at the intra- and interspecific levels confirmed that each species is reproductively isolated from the other two. Flavonoid comparisons of leaf and floral tissues from B. ferulaefolia, B. carpodonta and B. longistyla established the presence of species- specific compounds that would allow for the chemical separation of these three taxa. The two varieties previously recognized (var. foeniculifolia and var. ludens) are placed in synonymy with B. ferulaefolia, whereas B. carpodonta and B. longistyla are distin- guished as species. Finally, the geographical distribution formerly attributed to B. ferulaefolia was examined and was found to involve a number of related species. Throughout Mexico, particularly near the end of the rainy season in late September and October, one encounters population after population of grossly similar weedy Bidens annuals with large, yellow-rayed heads and pinnatisect leaves. In his classical studies of the genus Bidens in 1937 and 1955, Sherff treated all such plants with square stems and dimorphic achenes as a single species, B. ferulaefolia. Three varieties were recog- nized in both taxonomic treatments. Typical var. ferulaefolia was by far the most common, occurring in Arizona southeastward into Mexico to the states of Jalisco and Puebla, and rarely into Guatemala. This variety was characterized as having narrowly linear leaf segments and achenes which were 4-6 mm long with two aristae each. The latter were report- edly sometimes deciduous. In contrast, var. foeniculifolia had leaves