Wood Anatomy of Crossosomatales: Patterns of Wood Evolution with Relation to Phylogeny and Ecology

The seven families hypothesized by Sosa and Chase to comprise Crossosomatales possess relatively long vessel elements with scalariform perforation plates and scalariform to opposite vessel-ray pitting; tracheids; diffuse axial parenchyma; and heterogeneous rays. These and other primitive character states do not indicate relationships, but they do not offer any evidence against the idea that Crossosomatales are a natural order. Departures from the primitive character states are related to ecological adaptations. Crossosomataceae have simple perforation plates (scalariform briefly at the beginning of the secondary xylem), a feature correlated with the seasonal aridity of habitats occupied by the family, the sole family of the order to exhibit such an ecological shift. Presence of tracheids (which confer embolism resistance to a wood) in ancestors of Crossosomataceae probably pre-adapted the family for entry into highly seasonal habitats. Minimal vessel grouping in all other genera shows that tracheid presence deters vessel grouping; tracheid presence also deters shortening of vessel elements. Autapomorphies are shown by Aphloiaceae (tracheid dimorphism, rays of two distinct widths); Crossosomataceae (perforation plates predominantly simple, lateral wall pitting of vessels alternate); Geissolomataceae (wide rays); Ixerbaceae (fiber-tracheid tendency); Staphyleaceae (adjacence of axial parenchyma to vessels); Stachyuraceae (simplification of perforation plates); and Strasburgeriaceae (large cell size). Although tracheid presence seems plesiomorphic in Crossosomatales, a degree of lability in density and size of bordered pits on imperforate tracheary elements probably occurs within this order and in other dicotyledon groups.