Cirsium mohavense
Cirsium discolor
Mohave thistle, Mojave thistle, Rusby's thistle, virgin thistle
chardon discolore, field thistle
1–several, erect, proximally simple, distally branched, ± densely gray-tomentose;
branches 0–many, ascending to spreading.
single, erect, villous with septate trichomes, sometimes ± glabrate, distally ± tomentose;
branches few–many, ascending.
blades oblong-elliptic to oblanceolate, 10–60 × 2–15 cm, unlobed and merely spinulose or spiny-dentate or shallowly to deeply pinnatifid, lobes linear-lanceolate to ovate-triangular, spreading, entire to coarsely dentate, main spines slender to stout, 3–30 mm, faces ± gray-tomentose, sometimes ± glabrate;
basal often present at flowering, winged-petiolate;
principal cauline decreasing distally, proximal winged-petiolate, distal sessile, bases decurrent as spiny wings 1–5 cm;
distalmost well separated, bractlike.
blades oblanceolate to elliptic or ovate, 10–25(–50) × 1–13 (–25) cm, usually deeply divided more than halfway to midveins, proximal sometimes undivided, lobes linear-lanceolate, margins revolute, ascending, entire or spinulose to remotely few toothed or sharply lobed, main spines 1–5 mm, abaxial faces white-tomentose, adaxial faces green, villous with septate trichomes or glabrate;
basal usually absent at flowering, winged-petiolate, bases tapered;
principal cauline well distributed, gradually reduced, bases narrowed, sometimes weakly clasping, not decurrent;
distal cauline well developed.
0–15 cm.
0–5 cm (not overtopped by crowded distal cauline leaves).
ovoid to hemispheric, 1.5–2.5 × 1.5–2 cm, loosely arachnoid on phyllary margins or glabrate.
ovoid to broadly cylindric or campanulate, 2–3.5(–4) × 1.5–3 cm, thinly arachnoid.
white to pink or lavender, 16–25 mm, tubes 7–12 mm, throats 4–7 mm, lobes 4–8 mm, style tips 3–4 mm.
pink to purple (white), 25–32 mm, tubes 12–16 mm, throats 7–10 mm, (noticeably wider than tubes), lobes 6–9 mm;
style tips 4–6 mm.
in 5–8 series, imbricate, (inner greenish to brown or stramineous), lanceolate or ovate (outer) to linear-lanceolate (inner), entire, abaxial faces with narrow glutinous ridge;
outer and middle appressed, spines spreading, 3–7 mm;
apices of inner often flexuous, flattened, spineless, scabrid.
in 10–12 series, strongly imbricate, greenish with subapical darker central zone, ovate (outer) to lanceolate (inner), abaxial faces with narrow glutinous ridge, outer and middle bodies appressed, margins entire, spines abruptly spreading to deflexed, slender, 3–9 mm;
spines slender, 3–9 mm;
apices of inner phyllaries spreading, narrow, flattened, finely serrulate.
1–many, in corymbiform or paniculiform arrays.
1–many in corymbiform or paniculiform arrays.
stramineous to dark brown, 3–6 mm, apical collars 0.2–0.3 mm, yellowish;
pappi 14–16 mm.
tan to brownish, 4–5 mm, apical collars straw-colored, 0.5–75 mm;
pappi 18–25 mm.
= 30, 32.
= 20, 21, 22.
Cirsium mohavense
Cirsium discolor
Of conservation concern.
Cirsium mohavense ranges from scattered sites in eastern California east in the Basin and Range Province of southern Nevada to southwestern Utah and nortwestern Arizona, mostly in Mojave Desert region. When Welsh proposed Cirsium virginense for a geographically limited group of plants from southwestern Utah and northwestern Arizona (and subsequently discovered in extreme southeastern Nevada), he indicated that its relationship to other western thistles was unknown. Subsequently, he indicated (S. L. Welsh 1983; Welsh et al. 1993) that the affinities of the taxon apparently lie with C. mohavense, but he did not attempt to distinguish C. virginense from C. mohavense (in the strict sense) because the latter was not known to occur in Utah. A. Cronquist (1994) attempted the distinction. The only character he used in his key was life span of the plants: biennial (C. mohavense) versus perennial, spreading by creeping roots (C. virginense). In the descriptions of the two taxa he elaborated on this character, indicating that C. mohavense is single-stemmed and C. virginense often multistemmed. In the remaining features the plants are very similar or overlap extensively.
Distinction of two taxa on the basis of duration is impractical and probably inaccurate. Specimens commonly lack roots, and in those specimens in which bases are present, I have seldom been able to make any distinction between biennial taproots and perennial taproots. In particular I have seen no evidence of creeping roots. I am not aware of any study of either taxon that documents the life history of the plants. Some specimens of C. mohavense (in the strict sense) appear to have perennial bases like those attributed to C. virginense by Cronquist. For instance, a specimen of C. mohavense from Death Valley (Thorne & Ratcliff 2287, BRY) is indistinguishable from specimens of C. virginense (e.g., Atwood 13374, BRY) from Nevada and Utah. Both have a branched root crown with multiple rosettes and nearly identical leaves and heads.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Cirsium discolor is widespread in eastern North America from the prairies of southeastern Saskatchewan, western Minnesota, and Iowa south to northern Louisiana and east across southern Canada to the New England states and the southern Appalachians. It hybridizes with both C. altissimum (discussed thereunder) and C. muticum (G. B. Ownbey 1951b, 1964; W. L. Bloom 1977). Meiosis in first-generation hybrids between C. discolor and C. muticum is usually irregular (Bloom) and most pollen grains are infertile (Ownbey 1951b; Bloom). The presence of a small number of viable cypselae in heads of putative F1 hybrids (Ownbey 1951b) indicates that some F2 hybrids or backcrosses are formed.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
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