Since the appearance of a vascular system in plants, phloem has diversified tremendously but how it has evolved has been explored only across very broad scales. Pace et al. use the species-rich tribe Bignonieae (Bignoniaceae) as a model to study phloem anatomical evolution within a narrower scale by coding 19 characters in 56 species from all 21 genera. They find that the secondary phloem is extremely diverse, with sister lineages exhibiting distinct anatomies derived from contrasting patterns of evolution in fibre abundance. Fibre abundance in the tribe has diversified in correlation with sieve tube arrangement, sieve tube morphology, number of companion cells and parenchyma type.
Marcelo R. Pace, Suzana Alcantara, Lúcia G. Lohmann, Veronica Angyalossy, 2015, 'Secondary phloem diversity and evolution in Bignonieae (Bignoniaceae)', Annals of Botany, vol. 116, no. 3, pp. 333-358 http://dx.doi.org/10.1093/aob/mcv106