PALEOBIOLOGY

Leaf Story

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Science  23 Jul 2004:
Vol. 305, Issue 5683, pp. 452
DOI: 10.1126/science.305.5683.452c

The first tiny megaphyll leaves appeared in vascular land plants 50 million years before this leaf type became widespread. Why did adoption of such an advantageous photosynthetic structure take so long?

Osborne et al. performed a morphometric analysis of 300 plant fossils from the Devonian and Carboniferous Periods in order to assess trends in leaf size and to estimate the rates of convective and evaporative heat loss. They used these data to test the theory that high levels of atmospheric CO2 delayed the increase in leaf size by restricting stomatal development; larger leaves, possessing only limited cooling capacity, would have intercepted more solar energy and thereby suffered lethal overheating. As CO2 decreased, leaf blades were able to grow in size, and stomatal number rose abruptly. This analysis therefore supports the idea that the evolution of leaves was constrained by atmospheric CO2 concentrations. — HJS

Proc. Natl. Acad. Sci. U.S.A. 101, 10360 (2004).

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