Venus' Looking Glass (Triodanis perfoliata)

Also known as Clasping Bellwort, this annual wildflower has a very unique appearance.  The leaves clasp directly around the stem with no petiole. 

The unbranched stem grows 1-2 foot tall and deep violet to purple flowers appear from each leaf axil on the upper half of the plant.  The flowers have five petals, are about 3/4 inch across, and appear in groups of 1-3 at each leaf axil, but only open one at a time.  The flowers open in the morning and last one day.  They are pollinated by small butterflies and bees. 
The flowers on the lower half of the stem never open but are self-pollinating.  The tiny seeds are wind-dispersed and the plant can become weedy or invasive in some locales.  The main bloom period is spring to early summer.  Venus' Looking Glass grows in full sun, and poor soil that is well-drained.  They do best in areas with sparse or low vegetation.  They are native to most of North America.
This is the first year I've seen them on my property but there must have been at least one plant last year because this year I've got about a dozen.  By next year I'll know if it's going to become weedy.  I'm guessing it won't because I haven't seen large patches of this anywhere around here.

If you're wondering how on earth it got a name like Venus' Looking Glass, it's because there is a related species that has larger seeds that are very shiny (like a looking glass).