Transpiration is the process by which water moves through a plant and evaporates from its leaves through tiny pores called stomata. As water evaporates from the leaf surface, it creates a pulling force that draws more water up from the roots through the stem — like water being pulled up through a straw. Transpiration is the main driver of water movement in plants and also helps cool the plant. Stomata open to allow carbon dioxide in for photosynthesis but lose water in the process, creating a tradeoff that plants must balance.
Set up a transpiration experiment: place a plastic bag over a leaf on a living plant and observe water droplets collecting inside the bag after a few hours. This demonstrates that water is leaving the leaf. Discuss why: stomata must open to let CO₂ in for photosynthesis, and when they open, water vapor escapes. Use the "straw analogy" — as water evaporates at the top (leaves), it pulls more water up from below (roots). Connect to the water cycle: transpiration is a major way water returns to the atmosphere.
A tall tree can move hundreds of liters of water from its roots to its leaves every day — up to 100 meters against gravity in the tallest trees. How does it do this without a heart or a pump? The answer is transpiration — the evaporation of water from leaf surfaces — combined with a remarkable property of water: its molecules stick to each other.
Here is how it works. Inside each leaf are tiny pores called stomata (singular: stoma). Plants open their stomata to let carbon dioxide in for photosynthesis. But when the stomata are open, water vapor from inside the leaf escapes into the air — this is transpiration. As water molecules evaporate from the leaf surface, they pull on the water molecules behind them in the leaf's veins. Those molecules pull on the ones behind them, and so on, creating a continuous chain of pulling force all the way down through the stem to the roots. Think of it like sucking water through a straw — the suction at the top (evaporation at the leaves) pulls water up from below (absorption at the roots).
This pulling force works because water molecules are cohesive — they stick to each other through hydrogen bonds. A column of water in a plant's xylem (the tube tissue that carries water) is like a chain: pull the top link, and the entire chain moves upward. This mechanism, called the transpiration-cohesion-tension theory, explains how water reaches the tops of even the tallest trees without any pumping.
Transpiration also cools the plant, much like sweating cools your body. Evaporation requires heat energy, which is drawn from the leaf, lowering its temperature. On a hot day, transpiration can keep leaf temperatures several degrees cooler than the surrounding air. But there is a tradeoff: the same stomata that let CO₂ in also let water out. In hot, dry conditions, plants face a dilemma — keep stomata open for photosynthesis and risk drying out, or close them to conserve water and sacrifice food production. Many plants compromise by opening stomata in the cooler morning and closing them during the hottest part of the day. Desert plants have evolved extreme solutions: cacti, for example, open their stomata only at night, when the air is cool and humid, and store the CO₂ they collect for use during the day.
No topics depend on this one yet.