Panarctic Flora


61 Betulaceae


Notes: The base chromosome number in the two genera, Alnus and Betula, is usually assumed to be x = 14 and the species with 2n = 28 and 56 to be, respectivelly, diploids and tetraploids. There are, however, a few species consistently counted with intermediate numbers (e.g., Betula divaricata with 2n = 42 = 3x; B. kenaica and B. papyrifera with 2n = 70 = 5x), suggesting that the base number may be 2n = 7. This could explain the high seed fertility found in assumedly triploid (i.e., hexaploid) hybrids (2n = 42), e.g., between B. nana (2n = 28) and B. pubescens (2n = 56). For simplicity, the traditional levels are included below.