In 1903 Pearson became the first Harry Bolus Professor of Botany at the South African College. In 1904 he set off for South West Africa on the first of several expeditions with the object of studying the monotypic "Welwitschia". This expedition was cut short by the first of the so-called Herero Wars. In 1907 he made a second attempt in the company of E. E. Galpin who had previously accompanied him on cycad-hunting trips to the Eastern Cape. His papers on the ecology, morphology and embryology of "Welwitschia", led to a Cantabrigian DSc in 1907, which in turn led to a study of the closely related "Gnetum", to which end he went on a collecting expedition to Angola in 1909. During this time he wrote an account of the Thymelaeaceae for the "Flora of Tropical Africa".