The Fossil Record of Plant Physiology and Development—What Leaves Can Tell Us

Abstract

Plants provide unmatched opportunities to evaluate long debated evolutionary patterns in terms of the detailed biology of the fossil organisms. Leaves serve here as an example of how those advantages can be exploited. Over the history of vascular plants, three important transitions in leaf evolution—the origin of laminate leaves, the progressive loss of seed plant morphological diversity, and the evolution of more angiosperm-like leaves—also represent major shifts in leaf development and physiology. These transitions often occurred in parallel in different lineages, such as the evolution of marginal growth in each of at least four independent origins of laminate leaves during the Devonian and Carboniferous. Each also entailed dramatic reorganizations of leaf hydraulics. For example, the length of the finest distributary vein order varies from up to tens of centimeters down to hundreds of microns in successive groups of dominant seed plants. Angiosperms impose an additional trend upon these patterns with the evolution of their uniquely high vein densities. Vein density strongly influences and can provide a proxy for other physiological characteristics, such as assimilation and transpiration rates. The large increase in transpiration capacity accompanying the evolution of angiosperm leaf traits may even play an important role in feeding precipitation and thereby altering local climate.