Yellow Wood Sorrel
Oxalis stricta

f20 @ 1/640s, ISO:1600, Nikon D3S w 60mm micro

"Oxalis stricta," Wikipedia, the free encyclopedia. Oxalis stricta, called the common yellow woodsorrel (or simply yellow woodsorrel), common yellow oxalis, upright yellow-sorrel, lemon clover, or more ambiguously and informally "sourgrass" or "pickle plant", is an herbaceous plant native to North America, parts of Eurasia, and has a rare introduction in Britain. It tends to grow in woodlands, meadows, and in disturbed areas as both a perennial and annual. Erect when young, this plant later becomes decumbent as it lies down, and branches regularly. It is not to be confused with similar plants which are also often referred to as "yellow woodsorrel".
East Falmouth, Massachusetts