Many thanks to the lovely folks who wrote me off-list, pointing out that
my mystery plant is Vicia faba, fava bean! It certainly bears no
resemblance at all to any species of Vicia I have ever seen in the wild.
I think the folks who place it in a separate genus might have
> This plant has come up in a compost heap. This heap receives the
> leftover props from a college Economic Botany course, so it might be
> anything. The foliage looks very much like regular Pisum sativum
> (English pea) foliage, except that so far, there are no tendrils--and
> the plant is over a foot tall. The leaflets are thin but a little
> rubbery and glaucous. The flowers are about an inch long, white with
> maroon blotches on the wing petals. No fruit yet.
> M. Reed