concept

Water-use efficiency

The score of the bargain at the pore: carbon gained per unit of water spent. The figure deserves to be better known — a typical land plant loses several hundred molecules of water for every single atom of carbon it gains. Not as waste, but as the unavoidable cost of eating. Nearly all the water that moves through a tree, and a large fraction of all the water that moves through a continent, is spent on this one exchange. The higher the CO₂ in the air, the kinder the ratio: the concentration gradient does the work instead of the aperture, so a plant can eat its fill through the barest crack.