John C. Semple
WAT
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Machaeranthera Nees (in the broad sense) A North American generic complex of about 30 species with base numbers of x = 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 old Haplopappus sensu Hall (1928). Morgan (1997) examined the cpDNA of the complex and related genera; he showed that Psilactis was a distinct group of species, which are part of the Symphyotrichum s.l. aster clade. Morgan and Hartman (2003; Sida 20: 1387-1416.) divided the complex into a number of monophyletic genera leaving Machaeranthera sensu strictu limited to just a few species. The single-species genus Xanthisma was expanded to include sect. Blepharodon, sect. Havardii and, sect. Sideranthus. In doing so, both North American species of Astereae with well-studied B-chromosomes were placed in a single genus: Xanthisma texanum and X. gracile (syn: Haplopappus gracilis; 2n=4, the plant with the lowest known chromosome number). Other species of the broadly defined Macheranthera were placed in the segregate genera Arida and Dieteria. Some species have violet rays (Machaeranthera tanacetifolia), most have yellow rays (Xanthisma gracile), some have white rays (X. blephariphylla), and some lack rays (X. grindelioides). Species have been treated as members of Haplopappus and as members of Aster in the old sense.
Last update 26 October 2005 by J.C. Semple © 2005, J.C. Semple |
Macharanthera tanacetifolia; Texas
Xanthisma blephariphylla cult UMSL greenhouse
Xanthisma
grindelioides, Utah