The Anatomy of Secondary Xylem and the Classification of Ancient Dicotyledons

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Abstract

The stem wood of about 700 species belonging to 32 families of the order Magnoliales s.l., plus further taxa exhibiting primitive wood anatomical features, was investigated. On this basis, six structural groups can be established each of which show a marked gradation from primitive to advanced stages. Wood structure of Magnoliales sensu Takhtajan is only partially primitive, partially moderately derived, while the most primitively structured heteroxylous taxa belong to the “Dillenial-Hamamelidal” and “Theal” group, respectively. Accordingly, there is no compelling evidence to support phylogenetic schemes in which the Magnoliales is placed as the only common base for all recent dicotyledons. For this reason, and since there is no structural mechanism which justifies the derivation of the taxa, belonging to the “Dillenial-Hamamelidal” and “Theal” groups which are very primitive in their wood anatomy, from the types embracing Magnoliales sensu Takhtajan, a new phylogenetic model assuming an early separation of angiosperms into at least two branches is discussed. Furthermore, possible affinities between primitive and advanced taxa within the different structural groups are shown.