concept

Stomatal index

The ratio of stomata to total cells across a patch of leaf surface — a way of measuring pore density that is not thrown off by whether the leaf grew large or small. What makes it valuable: a plant decides how many pores to build according to how much CO₂ it lives in. Carbon-rich air, fewer pores; thin air, more. Ian Woodward demonstrated this directly in 1987. Which means the measurement can be inverted: counting pores on a fossil leaf yields the CO₂ concentration of a sky tens or hundreds of millions of years ago. This is a genuine paleoclimate proxy — a place where physics forced an organism's hand, so its anatomy became a measurement.