Cirsium altissimum
Cirsium arizonicum
roadside thistle, tall thistle
Arizona thistle
single, erect, villous with septate trichomes, sometimes ± glabrate, sometimes distally thinly tomentose;
branches few–many, ascending.
1–several, erect or ascending, glabrous to thinly arachnoid-tomentose with fine non-septate trichomes and/or villous with septate trichomes, sometimes ± glabrate;
branches 0–many, ascending.
blades oblanceolate to elliptic or ovate, 10–40 × 1–13 cm, margins flat, finely spiny-toothed and otherwise undivided to coarsely toothed or shallowly pinnatifid, lobes broadly triangular, main spines 1–5 mm, abaxial faces white-tomentose, adaxial faces green, glabrate to villous with septate trichomes;
basal usually absent at flowering, winged-petiolate, bases tapered;
principal cauline well distributed, gradually reduced, bases narrowed, sometimes weakly clasping;
distal cauline well developed.
blades oblong-elliptic, 3–40 × 1–13 cm, unlobed and spinulose to shallowly lobed or divided nearly to midvein, lobes few–many, ovate to linear-acuminate, often again lobed or divided, main spines 2–30 mm, abaxial faces green, glabrous to densely gray tomentose, sometimes midveins villous with septate trichomes, adaxial green, glabrous to gray-tomentose, sometimes glabrate;
basal sometimes present at flowering, unlobed to deeply spiny-lobed, winged-petiolate or sessile;
principal cauline sessile, well distributed, gradually diminished distally, bases sometimes decurrent as spiny wings to 2.5 cm or clasping;
distalmost sometimes ± bractlike.
0–5 cm (leafy-bracted).
0–15 cm.
ovoid to broadly cylindric or campanulate, (2–)2.5–3.5(–4) × (1.5–)2–3(–4) cm, thinly arachnoid.
cylindric or ovoid to campanulate, 1.5–4 × 1–2.5 cm (body), loosely arachnoid or ± glabrous.
tips 4–6 mm.
pink to purple (white), 20–35 mm, tubes 10–16 mm, throats 5–12 mm, lobes 5–9 mm.
pink to red, lavender, or purple (white), 25–31 mm, tubes 7–12.5 mm, throats 1.5–8.5 mm, lobes 10–17 mm;
style tips 1–4 mm.
in 10–20 series, strongly imbricate, greenish with subapical darker central zone, ovate (outer) to lanceolate (inner), abaxial faces with a narrow glutinous ridge (milky when fresh, dark when dry), outer and middle entire, bodies appressed, spines slender, abruptly spreading, 3–4 mm;
apices of inner phyllaries spreading, narrow, flattened, entire, spines spreading, slender, 3–4 mm;
apices of inner phyllaries spreading, narrow, flattened, ± dilated, ± erose or finely serrulate.
in 7–9 series, imbricate, green or the inner reddish to rich reddish purple, ovate or lanceolate (outer) to linear (inner), margins of outer entire, abaxial faces often with narrow, inconspicuous glutinous ridge;
outer and mid bodies appressed, short, entire, apices spreading to ascending, inconspicuous to long, narrow, entire or minutely ciliolate, spines erect to reflexed (outer) to ascending (inner), slender to stout, cylindric or basally flattened, 1–30 mm;
apices of inner unarmed or with straight or flexuous spines, short, flat.
1–many, in corymbiform or paniculiform arrays, (± elevated above principal cauline leaves), not subtended by ring of spiny bracts.
1–100+, erect, in corymbiform or paniculiform arrays.
tan to dark brown, 4–5.5 mm, apical collars stramineous, 0.5–1 mm;
pappi 12–24 mm.
brown, 3.5–7 mm, apical collars stramineous, 0.2–0.3 mm;
pappi 17–28 mm.
= 18.
= 30, 32, 34.
Cirsium altissimum
Cirsium arizonicum
Plants of Cirsium altissimum ranging from southern Minnesota to Texas often have more deeply divided leaves than do populations in other portions of the species’ range. Some botanists (e.g., R. J. Moore and C. Frankton 1969; D. S. Correll and M. C. Johnston 1970) have treated those plants as C. iowense. Others (e.g., R. E. Brooks 1986; H. A. Gleason and A. Cronquist 1991; G. B. Ownbey and T. Morley 1991) have treated them as C. altissimum. Still others considered them to be derivatives of hybridization between C. altissimum and C. discolor (J. T. Kartesz and C. A. Meacham 1999) and treated them as C. ×iowense. Indeed the existence of these plants blurs the distinction between C. altissimum and C. discolor, and herbarium specimens are often difficult to assign.
Natural hybrids between Cirsium altissimum and C. discolor are well documented (R. A. Davidson 1963; G. B. Ownbey and Hsi Y.-T. 1963; Ownbey 1964; S. Dabydeen 1997). Ownbey and Dabydeen both reported that apparent F1 hybrids between the two species have low seed set in comparison with the parental taxa. W. L. Bloom (1977) reported that the chromosomes of the two species differ by several rearareaments. Dabydeen reported a count of 2n = 19 with multiple meiotic irregularities for an apparent F1 hybrid. However, the presence of numerous individuals and populations seemingly intermediate between C. altissimum and C. discolor indicates that although F1 hybrids have low fertility, long-term processes may have stabilized hybrid derivatives of higher fertility. Ownbey and Hsi reported mitotic counts of 2n = 18 and 20 from a population that they treated as C. altissimum. In their discussion they noted that their plants represented “the segregate called C. iowense” and had been collected a short distance from that taxon’s type locality. R. J. Moore and C. Frankton (1969) reported a chromosome number of 2n = 18 for a plant from Texas that they considered to be C. iowense. Further investigation of morphologic variation, chromosome number, meiotic behavior, and fertility is needed of populations named as C. iowense to determine how those plants should be treated.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
Varieties 5 (5 in the flora).
The Cirsium arizonicum complex is widely distributed from the Sierra Nevada, White Mountains, and New York Mountains of eastern California across the mountains of the southern Great Basin and Colorado Plateau to the mountains of eastern Colorado, Arizona, and New Mexico. This group of plants comprises a series of intergrading races with intricately overlapping patterns of variation. For plants that I am treating as C. arizonicum (in the broad sense), F. Petrak (1917) recognized three species, one with a variety and two subspecies plus his unstated type subspecies and variety. R. J. Moore and C. Frankton (1974b) revised the complex, recognizing six species, three of them newly described, for the plants I treat as C. arizonicum plus C. turneri, which I do not include in C. arizonicum. P. L. Barlow-Irick (2002), in a work focused on statistical analyses of variation patterns, recognized six species also, but circumscribed very differently from those of Moore and Frankton. Two of the species proposed by Barlow-Irick have not been formally described.
I have wrestled with how to treat these plants since beginning my research for this treatment. After careful consideration of the complex patterns of variation among members of the C. arizonicum complex, I acknowledged the futility of trying to distinguish more than one species. Any character combinations that I or others have attempted to use to distinguish species break down hopelessly when enough specimens are examined. Instead I have chosen to recognize that in this complex, as in several others, the plants in question are a work of evolution in progress. Cirsium arizonicum is a rapidly evolving, only partially differentiated assemblage of races that have not reached the level of stability that is usually associated with the concept of species. Certainly there is much variation within the group that deserves a level of taxonomic recognition, or at least should be mentioned, but I think it much more prudent to recognize varieties–entities that may be expected to freely intergrade–rather than species. The geographic area where these plants occur, the highlands of the American Southwest, has had a turbulent history in the Quaternary with major shifts in climate, vegetation, and elevational zonation accompanying the vicissitudes of glacial and interglacial episodes. The complicated patterns of variation in C. arizonicum reflect both that history and the geographic and topographic complexity of the region.
Heads of Cirsium arizonicum are visited by hummingbirds as well as a variety of insects (P. L. Barlow-Irick 2002). Hummingbirds are the most common visitors, but hummingbirds and bees are both apparently effective pollinators in C. arizonicum.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
1. Corollas bright red or reddish pink | → 2 |
1. Corollas lavender to reddish purple | → 3 |
2. Leaves glabrous on both faces | var. rothrockii |
2. Leaves ± tomentose, at least on the abaxial faces | var. arizonicum |
3. Stems and abaxial leaf midveins villous to tomentose with septate trichomes; leaves conspicuously decurrent; leaves deeply divided, lobes many, narrow, closely spaced, each tipped by a very slender spine 5–12 mm; northeastern Arizona and northwestern New Mexico | var. chellyense |
3. Stems and abaxial leaf midveins glabrous to tomentose with fine, non-septate trichomes; septate trichomes usually absent; leaf divisions various | → 4 |
4. Principal marginal leaf spines 3–10 mm; New Mexico, northeastern Arizona, southeastern Utah, and southwestern Colorado | var. bipinnatum |
4. Principal marginal leaf spines 5–30 mm; southeastern California and southwestern Nevada | var. tenuisectum |
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