Sida20: 1410. 2003
Desert regions, sw United States, n Mexico.
Species 9 (6 in the flora).
Arida belongs to a group of taxa once included within a broadly circumscribed Machaeranthera containing more than 36 species. This genus was generally distinguished by its taproots, spiny-toothed pinnatifid leaves, bristly-tipped phyllaries, blue ray corollas, epappose ray florets, and chromosome numbers of n = 4, 5, or 6 (A. Cronquist and D. D. Keck 1957; R. L. Hartman 1990; L. H. Shinners 1950b; B. L. Turner 1987b; Turner and D. B. Horne 1964). Arida was recognized as a section of Machaeranthera with epappose ray florets, unequal phyllaries, and a chromosome number of n = 5. Studies incorporating data from cpDNA restriction site analysis (D. R. Morgan and B. B. Simpson 1992) indicated that section Psilactis was not closely related to the other taxa and it was removed (Morgan 1993). Further studies utilizing DNA sequence data (Morgan 1997) suggested that other genera, such as Oönopsis, Pyrrocoma, and Xanthisma, were closely related to various parts of Machaeranthera. The combined phylogenetic information from molecular, morphologic, cytologic, and flavonoid analyses did not support recent taxonomies, so a new one was proposed by Morgan and Hartman (2003). Machaeranthera was reduced to two species, and the remaining taxa were distributed in Xanthisma, Dieteria, and Arida.
Data from chloroplast DNA restriction sites and nuclear ETS sequences support a close relationship between Arida parviflora, A. turneri, both with pinnate or 2-pinnate leaves, A. riparia, and A. blepharophylla (D. R. Morgan 2003; Morgan and B. B. Simpson 1992). Conflicts between the two data sets were attributed to reticulate evolution resulting from occasional intergeneric crosses. Arida blepharophylla is somewhat unusual within the genus in being a short-lived perennial with rhizomes, basal rosettes of leaves, and receptacular scales (Morgan and R. L. Hartman 2003). These characters are similar to those found in Xanthisma and with these morphologic features as well as chloroplast versus nuclear DNA data, it may have evolved through intergeneric hybridization (Morgan 2003).
|1||Ray cypselae epappose (except in occasional plants)||> 2|
|1||Ray cypselae pappose or ray florets 0 (in A. carnosa)||> 4|
|2||Midstems densely hispiduloso-glandular; s Arizona, s California, s Nevada, n Sonora (Mexico)||Arida arizonica|
|2||Midstems glabrous or sparsely gland-dotted||> 3|
|3||Cauline leaf blades entire or toothed; w Texas||Arida mattturneri|
|3||Cauline leaf blades deeply pinnatifid; e Arizona, s Colorado, New Mexico, Utah, Chihuahua and Coahuila (Mexico)||Arida parviflora|
|4||Leaves (at least some) pinnatifid to 2-pinnatifid||Arida parviflora|
|4||Leaves entire or toothed||> 5|
|5||Ray florets 0||Arida carnosa|
|5||Ray florets present.||> 6|
|6||Perennials, rhizomatous, forming vegetative rosettes; leaf margins entire with 8–20 cilia per side (cilia 0.4–1.5 mm); involucres broadly turbinate, 5–8 mm wide (fresh); phyllaries oblong to oblanceolate or obovate, broadly acute to short-acuminate||Arida blepharophylla|
|6||Annuals; leaf margins entire, eciliate or with 1–8 cilia per side; involucres hemispheric, 10–16 mm wide (fresh); phyllaries linear-lanceolate, acute to acuminate||Arida riparia|
|Author||Ronald L. Hartman + and David J. Bogler +|
|Basionym||Machaeranthera Arida +|
|Common name||Desert tansy-aster +|
|Etymology||Latin aridus, dry, alluding to xeric habitat typical of members +|
|Illustrator||Bee F. Gunn +|
|Taxon name||Arida +|
|Taxon parent||Asteraceae tribe Astereae +|
|Taxon rank||genus +|
|Volume||Volume 20 +|