Prairie Verbena. Picture by Jo Roberts.

Latin names: Glandularia bipinnatifida
                          Verbena ambrosifolia

Spanish name (much prettier!): moradilla (little purple one)

Other common names: Dakota mock vervain

There is not a lot of information on the internet about this plant.  No legends or associations with famous historical characters.  But if you want more details, you can go to these websites:
Lady Bird Johnson page
Uvalde office of Texas AgriLife Extension page

This plant does not appear in Toxic Plants of Texas.