The September Mystery Plant photo shows the unopened involucre of a Swamp Thistle, Cirsium muticum. They grow up to nine feet tall in wet areas. The one shown was photographed on a flood plain of Sin and Flesh Brook in Tiverton. A distinguishing feature of this species is the fine white hairs on the involucre visible in the photo. Above at right is a photo of the plant in flower, not unlike other thistle species.