Wood Anatomy and Evolution: A Case Study in the Bignoniaceae

Abstract

Wood (secondary xylem) is responsible for water transport and has been well-studied anatomically, ecologically, physiologically, and phylogenetically. Comparative methods can reveal patterns of evolution for xylem traits using knowledge from the phylogenetic history of the taxa and the branching pattern of phylogenies. Bignoniaceae (Lamiales) is a family of pantropical plants of various growth habits that includes trees, shrubs, and lianas, which display diverse wood anatomies and for which robust phylogenies are available. Here we review important aspects in classical wood anatomy and evolution and test hypotheses regarding patterns of wood evolution using the Bignoniaceae as a model. Altogether, 85% of the genera currently recognized in Bignoniaceae were sampled, and 30 characters were delimited and mapped onto a robust phylogeny of the family. Some patterns of wood evolution within the Bignoniaceae seem to have been shaped by ecophysiological and habit aspects in the family. For example, vessels increase in diameter in the lianoid lineages but decrease in trees and shrubs during evolution. Rays in trees have evolved from a mixture of homo- and heterocellular to exclusively homocellular and storied in some lineages, while in the lianas the opposite pattern was recorded. Other patterns are consistent with more general phylogenetic trends; for example, parenchyma increases in abundance from the most basal to the most derived nodes of the phylogeny. Other characters in the family that are delimited and discussed include growth rings, porosity, perforation plates, ray width, and height. This work provides evidence that wood evolution is rather labile and that the evolution of new habits and the occupation of new habitats greatly influence wood evolution.