Lindernia dubia var. dubia
Family: Linderniaceae
false pimpernel,  more...
[Gratiola attenuata Spreng.,  more]
Lindernia dubia var. dubia image
from USDA Plants website  
Etymology: Lindernia: for Franz Balthasar von Lindern (1682-1755)
Plants: erect, annual, 2"-8" tall forb, branching
Leaves: opposite, mostly entire or with tiny teeth, lower leaves narrower to the base
Flowers: white to violet, 5-parted, 1/4" long, irregular shape, upper lip slightly 2-lobed and much narrower than somewhat backward pointing, 3-lobed lower lip; solitary from the leaf axils; blooms July-Sept.
Habitat: wet; shores; in sandy soil
Conservation Status: Native