Silveus' Texas Grasses

[Mary Barkworth]

Does anyone know who holds the copyright on Silveus' Texas Grasses?  The
copy here has a copyright date of 1933, with Silveus himself being the
copyright holder.  I would like to obtain permission to use some of its
illustrations for the Manual of North American Grasses (of which
information is gradually (very gradually) becoming available at
http://www.biology.usu.edu/herbarium/manual/)