Perhaps the only really convincing evidence of a direct C02 effect occurring in nature is a change in stomatal indices of leaves (the stomatal index is the abundance of stomatal pores relative to normal epidermal cells in the leaf surface) over the past centuries. The stomatal index has been shown many times to decrease with increasing C02 concentration in experimental plants grown at different C02 concentrations. Ice core evidence, and old measurements of the C02 content of air, show that the atmospheric C02 concentration has been increasing since the early 1800s. Ian Woodward of Sheffield University was the first to show that over the last 200 years herbarium specimens of leaves of common trees (e.g., beeches Fagus, birches Betula) show decreasing stomatal indices that parallel the increase in C02. This finding has been repeated many times on herbarium specimens of other species gathered before and after the onset of the main C02 increase.
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