While walking along an ephemeral pool created by recent flooding, I came across a handful of the purple-flowered plants shown here. Known as limestone bittercress or purple cress (Cardamine douglassii, Brassicaceae), this plant is usually found in wet wooded areas in eastern North America.
Although several plant families feature flowers with four petals, this trait usually screams “Brassicaceae!” to me. Sure enough, bittercress is indeed in that family. This taxon was formerly known as the Cruciferae because the four-petaled flowers were suggestive of crosses.
The most interesting thing to me about the Brassicaceae is that the family includes everything from crops like cabbage, broccoli, and turnip to invasive weeds like garlic mustard. Native woodland plants like this bittercress fall in the middle of that spectrum.