The chromosome numbers of two diploid species—Cary a aquatica (n = 16), andC. illinoensis (n = 16)—and one tetraploid—C. texana (2n = 64)—are recorded for the first time. Counts ofn = 16 and 2n = 32 forC. cordiformis and2n = 64 forC. tomentosa confirm earlier reports. Positive correlations were validated between ploidal level and stomatal size. All species with small stomata were diploids, and all species with large stomata proved to be tetraploids. Sect.Apocarya C.DC. contains only known or suspected diploids, whereas Sect.Eucarya includes both diploids and tetraploids. The diploids (Caryalaciniosa, C. ovata, andC. carolinaeseptentrionalis) in Sect.Eucarya have several features in common with species in both sections of the genusCarya. Ultimately, it may be shown that these diploids have contributed genomes to the tetraploid species and thus furthered the evolutionary advancement of the genus.