XVI International Botanical Congess

Changes in stomatal density and index in fossil plants from the Devonian to the Tertiary are conformable with the synchronous changes in CO2 levels calculated largely from geochemical data. The massive fall in CO2 through the Devonian, driven by an expanding terrestrial flora, parallels the rise planated leaves derived from photosynthetic cylindrical axes of earlier vascular plants. This origin of the megaphyll leaf may have been driven at least in part, by that autogenic decline in atmospheric CO2, the thin laminate leaf gave a shorter diffusion pathway within the photosynthetic tissue, compensating for the declining external CO2 level.