Abstract
B. lutea from Minnesota is hexaploid, as is the New England plant (reduced number, 42 chromosomes). B. papyri/era has the haploid number 35; B. papyrifera var. cordifolia has 28; B. papyri/era var. subcordata, 28; B. papyrifera var. kenaica, 35; B. papyrifera var. occidentalis, 42. B. pumila var. glandulifera, 28 pairs of chromosomes. B. purpusii (B. lutea X pumila var. glandulifera) presents another complete document, taxonomically and cytologically, of natural hybridization. Carpinus, Ostrya, and Ostryopsis spp. have 8 (reduced number). Carpinus betulus var. fasligiata is octoploid with 32, haploid C. cordata, shows all the cytological characteristics of a hybrid. It may be a bigeneric hybrid with a species of Ostrya.

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