Cytological Studies in the Betulaceae. I. Betula

Abstract

Polymorphic groups of plants are proving to contain species which hybridize readily. A wealth of evidence indicates hybridism to be one of the causes of polymorphism. Betula is a highly polymorphic genus. Such groups usually show polyploidy. Betula is a polyploid genus containing dipioid, triploid, tetra-ploid, pentaploid, hexaploid, and dysploid species and hybrids. Species of Betula hybridize very readily. The correlating taxonomic and cytological investigations of X B. jackii and X B. sandbergi present complete documents of natural hybridization. All of the meiotic abnormalities which are characteristic of hybrids occur in these 2 species crosses. Hybridization leads to the production of polyploid gametes by the very interesting semiheterotypic division and by non-reduction; consequently heterozygosis is to be considered one of the methods of the origin of polyploidy. Plants, commonly recognized as typical of the following species, exhibit meiotic irregularities which suggest heterozygotic origin: B. japonica var. mandshurica, B. davurica, B. pendula and B. schmidtii. Polymorphism in Betula is due, in part, to the readiness with which the species cross in nature. Betula is another genus wherein the multiplication of species has come about by hybridization.