Plants are mostly made up of water – about 80% by weight. So we might expect plants would need around four grams of water for each gram of dry mass to achieve their ideal level of hydration. That may be so, but they need a lot more water to grow. To produce one gram of new dry mass, a plant needs about 300 grams of water.

Plant leaves are covered in microscopic valves called stomata. Stomata open to let in carbon dioxide from the air, which plants need for photosynthesis and growth.

But when the stomata are open, the moist internal tissue of the leaf is exposed to the drier outside air. This means water vapour can leak out whenever the stomata are open.