Wood anatomy of Australian mirbelioids and allies (Fabaceae, Papilionoideae)

Detailed wood anatomical data for 22 species from 11 genera belonging to the tribes Bossiaeeae, Mirbelieae and Hypocalypteae (Fabaceae, subfamily Papilionoideae) are presented. No wood traits to distinguish clearly between the three tribes were found. On the contrary, they share a common character, namely, short vessel elements (distinctly shorter than in the tribe Baphieae, their sister group). This may be interpreted as a synapomorphy for the three tribes. The presence of numerous strongly branched protuberances in chambers of the vestured intervessel pits is seemingly an ancestral condition for this group. The occurrence of tanniniferous tubes in some Daviesia and Gastrolobium species is coherent with a close relationship between the mirbelioid clade and Hypocalypus, the only legume genus where these structures have previously been reported. The accumulation of tannins in ray cells can start at an early stage of their differentiation. The formation of tanniniferous tubes is seemingly a result of uneven elongation of ray cells with and without tannin deposits. In general, wood anatomical characters support the hypothesis (originally proposed in the pre-DNA era, based on morphological, cytological and chemical data) that the monotypic South African tribe Hypocalyptieae has the Australian tribes Bossiaeeae and Mirbelieae as its closest relatives, rather than Cape genistoid legumes.