The Field Guide to Edible Plants has nice drawings but it is modern. I lookedd an found Urticacaea and Umbelliferae. There are also lots of U's in a book called Wild Flowers by Homer House. It is out of print, my copy is copywrited 1934, it has beautiful color illustrations, hope this helps. Are you doing an alphabet book? Email me.

I don't know if there's anything starting with 'U', but this site has tons of botanical prints

Here is a link that might be useful: allposters.com