New York City. During a break, botanist L.M. Underwood took Arthur to a pasture in the Bronx that was to become the future site of the New York Botanical Garden. Underwood confided in Arthur that the Garden was spearheading an effort to publish systematic descriptions of the entire North American flora and Arthur was most likely going to be in charge of the section on rust fungi. Arthur was elated by this news, as was the New York Agricultural Experiment Station where he was granted increased freedom to study rusts. For his part, Arthur joined a consortium of collaborators to write 12 parts of the North American Flora, totaling 884 pages of descriptions, measurements, and host and distribution records of rust fungi. The first part was published in 1907 and the last in 1931. The year after Arthur earned his Doctor of Science from Cornell in 1886, he was appointed professor of botany at Purdue University, where he remained until his retirement in 1915. He was also the first botanist at the Purdue University Agricultural Experiment Station. Purdue continued to support his contribution to the North American Flora. One of his first accomplishments while at Purdue was determining that formalin could be used as a fungicide to control potato scab. It was at Purdue where Arthur began over 19 years of culture experiments with American species of rusts. He was faced with the task of identifying a species concept appropriate for rust fungi, many of which exhibit up to five different spore types and can alternate between two hosts, known as heteroecism. The life cycle of rusts was not fully known for most species, so Arthur’s goal was to keep rusts under direct observation to note the succession of spore types. Heteroecism of North American rusts was unexplored at that point, so to find the alternate hosts for some species, he performed over 3750 inoculations with 2140 collections on potential hosts. Through those culture studies, Arthur realized the importance of host specialization to the delimitation of species. Arthur also introduced a greater emphasis on morphological, and especially microscopic characters, such as the number and arrangement of germpores on spores that had up to this point, not been considered in the taxonomy of rust fungi. Later, their taxonomic significance became shown. During his career, Arthur named 29 genera and 309 species in the North American Flora, and described an additional 50