Progression en im Synfloreszenzbau und in der Verbreitung bei den Betulaceae

Abstract

Summary The synflorescence structure of almost all genera and sections of Betulaceae s.l. has been analysed. The different arrangement of the male and female catkins is traced back to a common hypothetical ancestral type with terminal male and basal female synflorescence branches and subfloral sympodial shoot innovation (similar to Alnus sect. Alnus ). From there, the following progressions are realized in separate lines: the translocation of the shoot innovation into the synflorescence; the tendency to monopodial ( Alnus subgen. Alnaster ) or sympodial ( Corylus avellana -group, Betula sect. Nanae ) proliferation of the synflorescence; the reduction of the number and size of the catkins; the protection of the female and male catkins during wintertime by bud scales. These progressions correspond to the formation of the shrubby growth form, to the reduction of the leaf size and of the number of lateral veins, and to an increasing extension of the distribution of Betula , Alnus and Corylus into cold and continental regions. In the light of fossil evidence, these progressive areas are explained by a successive southern shift and reduction of formerly large distribution ranges due to the cooling of the climate during the tertiary, and by a development of new taxa into the free cold regions. This explanation is contrary to the usual interpretation of similar distribution patterns by a successive development from a common southern evolution centre. The synflorescence proliferation is shown to be a principle of morphological progression also in other anemogamous trees, comparing them with their entomogamous relatives ( Fraxinus sect. Ornus — sect. Fraxinus , Acer sect. Platanoidea — sect. Negundo ). Hence, the synflorescence of Fraxinus excelsior and of Acer negundo are interpreted as to be not lateral, but proliferating terminal panicles.