At that time the flora of India and Western Himalayas was not well known. Stewart embarked on plant collection trips, often on his bicycle, as far as Kashmir and into Western Tibet. He continued to collect plants every summer (1912–1959) without any financial support. In 1960, when Dr. Stewart retired at the age of 70, he gave his collection of over 50,000 plant specimens, now called the Stewart Collection, to Professor E. Nasir at Gordon College (Rawalpindi). The Stewart Collection has been deposited in the National Herbarium of the government of Pakistan at Islamabad, leaving a very rich heritage for the students of plant sciences.