Sections through the petiole and blade midrib and sometimes the node of 20 genera of Luxemburgieae have revealed variations in vascularization which appear significant in determining the phylogeny of the tribe. The most primitive group are the pentacarpellate genera; they are characterized by multilacunar nodes and numerous leaf traces which mostly remain free to the leaf apex; some of these become medullary bundles in the petiole. The more advanced bicarpellate genera have multilacunar or trilacunar nodes, still with many leaf traces in the case of Wallacea. These traces fuse in the blade, resulting in a siphonostele. The most advanced genera are tricarpellate and have trilacunar nodes with only three traces; these traces fuse, forming a siphonostele. There are no medullary bundles present in this last group of genera.
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Jane M. Decker (1967) studied this question.
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