The herbarium occupies one wing of the Warnock Science Building. There are approximately 100,000 specimens housed in 112 cases. The major collections include plants of Trans-Pecos Texas, which forms the largest part of the northern part of the Chihuahuan Desert Region in the United States. It is a prominent regional research collection that is available to botanists around the world and is also a valuable teaching facility for both graduate and undergraduate students interested in various botanical disciplines or in wildlife biology. The first significant collections are those made by O.E. Sperry in 1935-1946. The physical herbarium was probably started by L.C. Hinkley but B.H. Warnock was responsible for the largest early collection of plants and major development of the herbarium as a research facility. Another major contributor was M.C Johnston. More recently, the herbarium has grown through the efforts of A.M. Powell, in whose honor the university renamed the herbarium in 2003.