resulted in a better understanding of the history of the flora of the United States and Canada. This study of the chromosomes, spores, and geographic relationships of Thelypteris nevadensis (D.C. Eaton) Morton illustrates how such diverse data can be synthesized and are applicable to broader phytogeographic concepts. This fern of moist, open meadows and wooded slopes ranges from southern British Columbia to central California and geographically is a critical species. Its closest relative is T. noveboracensis (L.) Nieuwl. of eastern North America; also it is related to other species of eastern Asia. Geographic analyses of these American species and other taxa having similar Cordilleran-Appalachian ranges show predominantly circumboreal affinities. There is a lesser element among the North American mesic ferns that has geographic relations with tropical regions. The cytological and palynological data illustrated here document basic information useful in establishing evolutionary lines and supplement other evidence pertaining to the geography and migration of species. This study has developed out of previous work on the alliance of three species of Thelypteris in north-eastern America (Tryon & Tryon, 1973). CYTOLOGY