The evolution of water transport in plants stands as one of the major functional innovations in the history of life. Plants harness the vapor pressure gradient between air and water to transport water from the soil to the leaves, where it is exchanged for atmospheric carbon dioxide necessary for photosynthesis. The biophysical nature of this system is consistent across all clades of land plants and allows the ecology and physiology of extinct plants to be reconstructed in a quantitative way. In these pages, I provide an approach to this problem grounded in fluid dynamics modeling and plant anatomy. The first two chapters focus on the anatomically unusual Carboniferous seed plant Medullosa , which supported large leaf areas on a small stem that contained anomalous development of its vascular tissue. I show that medullosan stems provided a high-throughput, low-resistance, well-connected pathway of water to the leaves, but this organization restricted medullosans to tropical everwet environments. Further work shows that medullosans could support large leaf areas, even on very small stems, but that medullosan fronds must have operated with low safety margins, as angiosperms do today. Chapter three investigates the hydraulic architecture of Glossopteris, a Permian seed plant that dominates plant deposits of the Gondwanan continents. Previous work has shown that Glossopteris is the most conspicuous terrestrial plant casualty of the Permian-Triassic mass extinction; analysis of Glossopteris wood and leaves shows that it contained tissues that are among the lowest-conducting of any seed plant, living or extinct. However, these features are advantageous when resisting damage from freezing. Rapid environmental change at the Permian-Triassic boundary, caused by CO₂ outgassing and thermogenic methane produced by the Siberian Traps flood basalts, would have placed this plant in severe jeopardy. Finally, chapter four investigates the evolution of water transport cells across the seed plant Glade through the lens of morphometric analysis. Transport cell morphology has been canalized into two types, small tracheids with torus-margo pits and multicellular angiosperm vessels, reflecting the conflicting demands of fluid flow, structural support, and cavitation safety. Reconstructing extinct plants' function allows ecological and environmental inferences to be made in a quantitative way.