摘要：

The genus Betula belongs to the family Betulaceae and consists of about 65 species (Hegi 1981). Birches are deciduous trees or shrubs with a size of 0.5 m ( B. nana ) to 40 m ( B. papyrifera ) and a maximum stem diameter up to 1.2 m (Schenck 1939; Preston 1961; Krüssmann 1976). Typical characteristics are the horizontally elongated lenticels and often the peeling in thin papery layers (Preston 1961) of the white, gray, yellowish, reddish, dark red or brown-colored bark (Schenck 1939). Birches are comparatively short-lived, with a maximum age up to 90–120 years ( B. pendula ) (Hegi 1981) and 300 years ( B. lutea = B. alleghaniensis ) (Schenck 1939). Birches are monoecious and anemophilous (Hegi 1981). The basic chromosome number of the genus Betula is x = 14. Diploid species with 2x = 28 are B. pendula and B. nana . Other species are polyploid. Examples of tetraploidy (2n = 4x = 56) are B. pubescens and B. davurica , and for hexaploidy (2n = 6x = 84) B. grossa and B. lutea (= B. alleghaniensis ). The varieties of B. papyrifera have 56, 70, or 84 chromosomes. Hybrids between B. pendula and B. pubescens are triploid (2n = 3x = 42) (Rohmeder and Schönbach 1959).

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