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Trochodendraceae is the botanical name for a family of flowering plants. The APG II system, of 2003 (unchanged from the APG system, of 1998), also recognizes this family: it is unplaced as to order and left among the basal lineages of the eudicots. As the default option, APG (in both versions of the system) accepts this as a family of two species, both trees, but it does allow the option of segregating family Tetracentraceae, as an optional segregate. This segregation would lead to two families of each one species, of trees: Tetracentraceae with Tetracentron sinense and Trochodendraceae with Trochodendron aralioides. These two species share the feature of secondary xylem without vessels, which is quite rare in angiosperms. As the vessel-free wood suggested primitiveness these two species have always attracted taxonomic attention. The Cronquist system, of 1981, accepted both families and placed these in the order Trochodendrales, in subclass Hamamelidae, in class Magnoliopsida. The Dahlgren system made the same choice at the rank of family and order, but placed this order in superorder Rosanae in subclass Magnoliidae. The Engler system, in its update of 1964, placed both families in a rather large order Magnoliales, in subclass Archichlamydeae in class Dicotyledoneae. The Wettstein system, last updated in 1935, united the two species in family Trochodendraceae which was placed in order Polycarpicae (the predecessor of order Magnoliales), in subclass Choripetalae, in class Dicotyledones. [edit] External links
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