Vessel grouping patterns in subfamilies Apocynoideae and Periplocoideae confirm phylogenetic value of wood structure within Apocynaceae.

This study contributes to our understanding of the phylogenetic significance and major evolutionary trends in the wood of the dogbane family (Apocynaceae), one of the largest and economically most important angiosperm families. Based on LM and SEM observations of 56 Apocynoideae species-representing all currently recognized tribes-and eight Periplocoideae, we found striking differences in vessel grouping patterns (radial multiples vs. large clusters) between the mainly nonclimbing apocynoid tribes (Wrightieae, Malouetieae, Nerieae) and the climbing lineages (remaining Apocynoideae and Periplocoideae). The presence of large vessel clusters in combination with fibers in the ground tissue characterizing the climbing Apocynoideae and Periplocoideae clearly contrasts with the climbing anatomy of the rauvolfioids (solitary vessels plus tracheids in ground tissue), supporting the view that (1) the climbing habit has evolved more than once in Apocynaceae, (2) the three nonclimbing apocynoid tribes are basal compared to the climbing apocynoids, and (3) Periplocoideae belong to the crown clade. The wood anatomy within the nonclimbing and climbing lineages is rather homogeneous, although a combination of specific characters (e.g. presence of septate fibers, axial parenchyma distribution, abundance of uniseriate compared to multiseriate rays, and presence and location of prismatic crystals) may be used to identify several tribes.