There are few crops native to the United States, causing us to rely on introduced species for most of our food. The United States government recognized the importance of such introduced plants before the turn of this century, and began recording and consecutively numbering plant introductions (PI) in 1898.
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Clark, Raymond L.; Widrlechner, Mark P.; Reitsma, Kathleen R.; and Block, Charles C., "Cucurbit Germplasm at the North Central Regional Plant Introduction Station, Ames, Iowa" (1991). NCRPIS Publications and Papers. 10.