Cirsium arvense
 
Canada Thistle

Male Flower Head Bottom

Moose Mountain Provincial Park
21-August-2009

Note the outer flowers are reflexed and hide a larger fraction of the involucre than the outer flowers of the female heads do.

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Arvense: Answers to key questions in Budd's Flora and Flora of Alberta leading to this species. 
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perennial plants

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plants from deep creeping rootstocks; NOT [plants without creeping rootstocks]

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male and female flowers on separate plants; NOT [male and female florets on the same plant or flower head]

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bracts of involucre only slightly, if at all, cobwebby, and if so the inner bracts twisted and not spine-tipped; NOT [bracts of involucre covered with cobwebby hairs, and all bracts spine-tipped]

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heads small, less than 2.5 cm high; NOT [heads larger, more than 2.5 cm high]

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involucres 1-2 cm high; NOT [involucres more than 2 cm high]

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involucres rarely over 12 mm wide; NOT [involucres usually more than 12 mm wide]

 

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Cirsium: Answers to key questions in Budd's Flora  and Flora of Alberta leading to this genus. 
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juice watery; NOT [juice milky]

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leaves with spiny margins

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bracts of involucre not dry or parchmenty, usually green; NOT [bracts of involucre dry, parchmenty or membranous, not green]

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some involucral bracts spiny-tipped

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involucral bracts without hooked bristles

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receptacle densely bristly; NOT [receptacle naked]

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flower heads with all florets tubular

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stamens united to form a tube around the pistil

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pappus present, distinct; NOT [pappus none or inconspicuous]

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pappus of capillary bristles; NOT [pappus of scales, or of awns, or a crown or none]

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pappus bristles much longer than achene; NOT [pappus hairs not longer than achene]

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pappus bristles plumose; NOT [pappus bristles naked or merely minutely barbellate]