Flora of North America species comparison

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Cirsium clavatum var. osterhoutii
Photo is of parent taxon

Osterhout's thistle
Involucres

ovoid to campanulate, 2–2.8 × 1.5–3 cm, villous-ciliate with long septate trichomes connecting adjacent phyllaries and/or thinly arachnoid-tomentose.
Phyllaries

in 5–6 series, imbricate or subequal, green or with maroon to dark brown subapical patch or appendage, linear to linear-lanceolate, abaxial faces with narrow glutinous ridge that is often concealed by trichomes;

outer and middle bases appressed, apical appendages erect or stiffly ascending, linear-lanceolate to acicular, entire or spinulose-dentate, spines erect or ascending, 2–5 mm;

apices of inner sometimes flexuous or reflexed, narrow, flat, and entire or expanded, scarious, and lacerate-dentate.
Abaxial

leaf faces ± tomentose.

Cirsium clavatum var. osterhoutii
Phenology Flowering summer (Jul–Aug).
Habitat Openings in montane coniferous forests, subalpine, alpine
Elevation 3000–3600 m (9800–11800 ft)
Distribution
from FNA
CO
[BONAP county map]
Discussion

The densely pubescent involucre of var. osterhoutii is a feature that links those plants with Cirsium eatonii var. eriocephalum. Some specimens tentatively assigned to C. clavatum var. osterhoutiii may be derivatives of past introgression with C. eatonii var. eriocephalum. Congested heads and strongly undulate leaves with numerous closely spaced lobes are features that suggest such a relationship. Further studies at both population and regional levels are needed to establish the nature of the variation patterns. Where I have observed var. osterhoutii in the field, populations appeared to be morpologically stable, and there was no evidence of current introgression with C. eatonii.

(Discussion copyrighted by Flora of North America; reprinted with permission.)
Source FNA vol. 19, p. 128.
Parent taxa Asteraceae > tribe Cardueae > Cirsium > Cirsium clavatum
Sibling taxa
C. clavatum var. americanum, C. clavatum var. clavatum
Synonyms Carduus osterhoutii, C. osterhoutii, C. araneans
Name authority (Rydberg) D. J. Keil: Sida 21: 212. (2004)
Web links 
Unless otherwise noted, all data comes from the online version of the Flora of North America, who have made most of their content available via a Creative Commons license (CC BY 4.0).



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