Résumé
More than 350 years after the first microscopic observation of fossil wood, the understanding of the processes underlying the evolution of secondary vascular tissues remains one of the major research areas in paleobotany. In this talk we will present recent and ongoing work that adresses two key questions: (1) How many times did secondary vascular growth evolve? The fossil record indicates that during the Paleozoic secondary vascular tissues were produced in a wide array of plants, including some groups that are extinct or that do not have such secondary growth today. Recent investigations of Devonian-Carboniferous representatives of the Stenokoleales, progymnospems, seed plants, Cladoxylopsidales, and Sphenophyllales are providing new information on their anatomy that will ultimately allow to test the presence of shared mechanisms of secondary growth accross vascular plants.(2) What were the functional properties of Paleozoic wood? While the wood of extant plants shows a trade-off between hydraulic capacities and mechanical strength, the first woody plants seem to have been more optimized for water transport than for support. By the Early Carboniferous, plants showed a diversity of wood anatomies that likely allowed them to cover a broad range of hydraulic properties. The development of hydraulic modelling at the cell and tissue levels is expected to improve our understanding of how these properties evolved, and of the environmental and functional constraints involved