Stewart was the father of systematic botany in Pakistan, having spent more than 50 years in the Indo-Pakistan subcontinent. He wrote comprehensive reports on the flora of N. W. Himalayas, Western Tibet, Kashmir, Kurram Valley, flora of Balochistan and grasses of West Pakistan. Apart from collecting flowering plants, Dr. Stewart made a scientific contribution in the collection of mosses, plant disease specimens like rusts, smuts and fleshy fungi which have since been published in "Mycologia" by Dr. Arthur and Dr. Cummins of Purdue University. One of his most important contributions "An Annotated Catalogue of Vascular Plants of Pakistan and Kashmir (1972)" serves as the basis of writing the "Flora of Pakistan" edited by E. Nasir and S.I. Ali (1970–1988), S. I. Ali and Y. J. Nasir (1989–1991) and S. I. Ali and M. Qaiser (1992–).