InÌýthe broad sense
MachaerantheraÌýNees in the broad sense is a North American generic complex of about 30 species with chromosome base numbers ofÌýxÌý= 5, 4, 3 and 2. Plants are tap rooted, have spinulose tipped leaves and fruits with a multiseriate pappus. The evolution of the various groups within the genus is complex. Hartman (1990) provided a conspectus of the complex, following the break up of the oldÌýHaplopappusÌýsensu Hall (1928). Morgan (1997) examined the cpDNA of the complex and related genera; he showed thatÌýPsilactisÌýwas a distinct group of species, which are part of theÌýSymphyotrichumÌýs.l. aster clade. Morgan and Hartman (2003; Sida 20: 1387-1416.) divided the complex into a number of monophyletic genera leavingÌýMachaerantheraÌýsensu strictu limited to just a few species. The single-species genusÌýXanthismaÌýwas expanded to include sect.ÌýBlepharodon, sect.ÌýHavardiiÌýand, sect.Sideranthus. In doing so, both North American species of Astereae with well-studied B-chromosomes were placed in a single genus:ÌýX. texanumÌýandÌýX. gracileÌý(syn:Haplopappus gracilis; 2n=4, the plant with the lowest known chromosome number). Other species of the broadly definedÌýMacherantheraÌýwere placed in the segregate generaÌýAridaÌýandÌýDieteria.Ìý
InÌýthe narrow sense
Morgan and Hartman (2006 Flora North America) recognized just two violet-rayed, aster-like species:
- Machaeranthera tanacetifolia
- Machaeranthera tagetina
