Thanks, Little Ethanol,
You correctly indentified my adoptee as a member of the Wandering Jew family. After I previously posted my question about the plant I adopted Sunday, I sent the picture to the information deak at the Cylburn Arboretum. This reply came back from staff botanist Scott Ritchie:
"That little gem is Trandescantia sillamontana. White Velvet Plant, sometimes put under the name Setcreasea.
We use it as a bedding plant in full sun."
Which as a geek,, I find very poetic. I moved it into a south facing window.