1. Betula lutea from Minnesota is hexaploid as is the New England plant. 2. B. papyrifera and its varieties have interesting chromosome numbers. B. papyrifera has the haploid number of thirty-five; B. papyrifera var. cordifolia has twenty-eight; B. papyrifera var. subcordata has twenty-eight; B. papyrifera var. kenaica has thirty-five; B. papyrifera var. occidentalis has forty-two. It is hoped that this will help to explain the polymorphism in the group. 3. B. pumila var. glandulifera has twenty-eight pairs of chromosomes. 4. x B. purpusii (B. lutea x pumila var. glandulifera) presents another complete document, taxonomically and cytologically, of natural hybridization. 5. Carpinus, Ostrya, and Ostryopsis species have eight as the fundamental number of chromosomes. 6. Carpinus betulus var. fastigiata is octoploid with thirty-two chromosomes as the reduced number. 7. Carpinus cordata shows all the cytological characteristics of a hybrid and is accordingly suspected of having a heterozygotic ancestry. It may be a bigeneric hybrid. 8. The existence of polyploidy and irregular meioses in polymorphic plants, as here reported, furnishes more evidence for the theory that multiplication of species has come about to a considerable extent by hybridization.