Homoiohydry is the capacity of plants to regulate, or achieve homeostasis of, cell and tissue water content. Homoiohydry evolved in land plants to a lesser or greater degree during their transition to land more than 500 million years ago, and is most highly developed in the vascular plants. It is the consequence of a suite of morphological innovations and strategies that enable plant shoots exploring aerial environments to conserve water by internalising the gas exchange surfaces, enclosing them in a waterproof membrane and providing a variable aperture control mechanism, the stomatal guard cells, which regulate the rates of water transpiration and CO2 exchange. In vascular plants, water is acquired from the soil by roots and transported via the xylem to aerial portions of the plant. Water