The Reshaping of Phytogeographical Knowledge as the "Transition zone" Wallacea: An American Expansion Project in the Philippines, 1902-1928. This paper examines the development of a concept that to this day plays an important role in biogeography: the region Wallacea. Focussing on the work of the American tropical botanist Elmer D. Merrill in the Philippines, I argue that his research on the geographical movement and settlement of Philippine plants reflects a shift in the United States' scientific and cultural understanding of the Pacific area towards a notion of "American tropics". While inventorying plants for the American administration from 1901 to 1923, the composition of Philippine flora prompted Merrill to question biogeographical regionalization in Wallace's Malay Archipelago. Not being able to situate the Philippine flora within "Asian" or "Australasian" biota, Merrill described the islands as an area of trans-regional plant migration. Together with the paleontologist Roy E. Dickerson, he later conceptualized the region Wallacea as biogeographical "transition zone" between the Philippines and Australia. In addition to the scientific conceptualization, Wallacea constitutes a space between two biogeographical subregions, whose flora was considered as being part of both, but could not be unambiguously attributed to either of these regions. In this respect, the plants resembled the inhabitants of the United States' insular overseas territories, who were legally defined neither as aliens nor as citizens. In this sense, Wallacea embodies epistemic as well as geopolitical boundaries, while its case also illustrates some of the ways in which colonial interests in the usability of plants found expression in botanical research during the early period of American overseas expansion.