Figure 1.

Schematic diagram of coupling between leaf CO₂ assimilation and the geochemical carbon cycle. Atmospheric CO₂ concentration, c_a, on geological timescales is determined by the balance between major sources (volcanism) and sinks (deep ocean sedimentation). The fossil record suggests that leaves have adapted to megacycles in atmospheric CO₂ concentration by adjusting leaf diffusive conductance to CO₂, achieved through changing the size and density (number) of stomatal guard cells on the leaf surface. Stomata on the leaf surface are represented as guard cell pairs in outline containing circular red nuclei.