Ranunculus scleratus

Common Names: Blister Buttercup, Rouge Buttercup, Cursed Crow's Foot, Celery-leaf Buttercup

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solitary inflorescence flower typical of the Ranunculaceae - prominent, elongated receptacle - Can you see the stamens?
habit -- small, herbaceous -- scleratus = "cursed, growing in vile places" upper leaves entire
basal leaves lobed  
The acrid sap is reported to cause blisters on human skin, and the plant is reportedly poisonous to livestock. Sap was reportedly used by Native Americans as a poison for arrow points.