This pretty flower grows from a shamrock type plant, thus it is named sorrel. The stem is hairy and the leaves are deeply notched. The leaves also fold up in the distinctive way you see here. It is interesting to compare yellow wood sorrel with white wood sorrel (Oxalis montana) that you can see in my post here. The yellow bloom is much larger than the white. I photographed this yellow wood sorrel in the woods behind the house in early July. Yellow wood sorrel is also called sour grass.