A summary of the known chromosome numbers for the North American species of Coreopsis (Compositae) is presented. New chromosome counts are presented for C. delphinifolia (2n = 52), C. major var. major (n = 13), C. major var. stellata (n = 13), and C. verticillata (n = 26). Including this paper, chromosome counts of 43 species of the approximately 50 species described for North America have been reported; counts from all sections are available. The major phyletic lines in the genus are suggested, based on correlation between 17 morphological trends and presumed chromosomal derivation. The following new combinations are suggested: C. wrightii (A. Gray) H. M. Parker (based on C. basalis var. wrightii), C. tinctoria Nutt. var. similis (F. Boynton) H. M. Parker (based on C. similis), and C. tinctoria Nutt. var. atkinsoniana (Dougl.) H. M. Parker (based on C. atkinsoniana). A pattern of descending aneuploidy is indicated for the North American species of Coreopsis, with a primitive base number of x = 14. The bearing of this base number on that for the tribe Heliantheae is considered.
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Edwin B. Smith (1975) studied this question.