| My dissertation research critically examines the evolutionary history of the angiosperm family Winteraceae from a physiological perspective. The Winteraceae have long been regarded as the least-modified descendents of the first flowering plants because they lack xylem vessels. Because vessels are considered a key innovation in allowing angiosperms to rise to ecological dominance, are portrayed as declining relicts, limited to wet forest habitats where tracheid-based wood does not impose a significant hydraulic constraints, and evolved essential leaf-level features to prevent excess water loss. However, phylogenetic analyses indicate that the vesselless Winteraceae wood may be secondarily derived. These observations motivate an examination of the functional significance and selective pressures underlying the pattern and direction of xylem evolution in Winteraceae.; I begin by examining the physiological importance of waxy, stomatal plugs on leaf water loss rates in the winteraceous tree Drimys winteri. Instead of being adaptations for drought, gas-exchange measurements reveal that stomatal plugs modify the humidity response of the guard cells such that, under high evaporative-demand, 'plugged' leaves lose water at a faster rate than leaves from which the plugs were removed. Chlorophyll fluorescence measurements suggest that stomatal plugs enable leaves to maintain photosynthesis even when leaves are exposed to mist, a condition that commonly occurs in the everwet cloud forests that Drimys inhabits. In chapter three, measurements of xylem sap flow in an elfin cloud forest in Costa Rican on Drimys granadensis support an inhibitory effect of stomatal plugs on the ability of leaves to regulate water loss. I found that transpiration is often more related to wind speed (through its influence on boundary layer conductance) than sunlight.; Finally, I investigated the importance of freezing stress as a pressure driving the loss of vessels in Winteraceae. I found that the tracheids of Winteraceae were resistant to embolisms by freeze-thaw. In contrast, vessel-bearing angiosperms exhibited substantial loss in xylem hydraulic capacity when frozen. Integrating Winteraceae's phylogenetic relationships, fossil and paleobiogeographic history, I hypothesize that as ancestors of modern Winteraceae passed through the temperate conditions then present in Southern Gondwana during the Early Cretaceous, they were exposed to selective pressures against vessel-possession. |
