Systematic Studies of Eschscholzia (Papaveraceae). I. The Origin and Affinities of E. mexicana

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Abstract
E. mexicana Greene differs from E. californica Chamisso only by its entire (vs. bifid) cotyledons and its consistent obligately annual growth form. Although the taxa are allopatric in the field, artificial hybrids are easily formed, and are fertile even in F2 and backcross generations. E. mexicana is best regarded as a subspecies of E. californica ssp. mexicana comb. et stat. nov. [E. mexicana Greene], which extended its range into the Sonoran Desert region during glacial times and subsequently was isolated by increasing warmth and dryness in the intervening regions.