In February 1883 he accompanied Carl Humann on an expedition to Mount Nemrut in historic Commagene which was initiated by the Prussian Academy of Sciences. At Zincirli he discovered the ruins of Sam'al, capital of a small principality of the late Hittite period, which he later excavated between 1888 and 1902 together with Robert Koldewey. Descriptions of the plants he collected in the Ottoman Empire were later published by Otto Stapf.