What is transpiration?
Transpiration is the loss of water vapour from a plant, mostly through tiny pores in the leaves called stomata. Water evaporates from the moist surfaces of cells inside the leaf, fills the air spaces, and diffuses out into the drier air. Plants lose more than 90% of the water they absorb this way. Transpiration cools the leaves, carries dissolved minerals up from the roots, and keeps cells supplied with water.
Water’s journey through a plant
- Water enters root hair cells from the soil by osmosis.
- It crosses the root and enters the xylem, a network of hollow, dead tubes.
- As water evaporates from the leaves, it pulls on the water column below. Water molecules stick to each other (cohesion) and to the xylem walls (adhesion), so the whole column is drawn upward — the transpiration stream.
- In the leaf, water evaporates into air spaces and exits through the stomata.
Stomata and guard cells
Each stoma is surrounded by two guard cells. When they take up water and become turgid, they bend apart and open the pore; when they lose water, they close it. Stomata usually open in light, to let in carbon dioxide for photosynthesis, and close in darkness or when the plant is short of water. In most broad-leaved plants, the majority of stomata are on the cooler, shaded lower surface of the leaf.
Factors affecting the rate
| Factor | Effect | Why |
|---|---|---|
| Light intensity ↑ | Rate increases | Stomata open wider |
| Temperature ↑ | Rate increases | Warm air can hold more water vapour, so evaporation and diffusion are faster |
| Humidity ↑ | Rate decreases | A smaller diffusion gradient between leaf and air |
| Wind speed ↑ | Rate increases | Humid air next to the leaf is blown away |
| Very dry conditions | Rate may level off or fall | Stomata partly close to prevent wilting |
Measuring with a potometer
A potometer measures how fast a cut shoot takes up water. The shoot is sealed into a water-filled tube, and a single air bubble is introduced into a narrow capillary. As the shoot draws up water, the bubble moves along a scale. The rate of water uptake is very close to the rate of transpiration, although a small amount of water is used in photosynthesis or kept by the cells. A fair test changes one factor at a time and repeats each measurement.
The model
The simulator uses the vapour pressure deficit (VPD) — how far the air is from being saturated — as the driving force for water loss:
e_s(T) = 0.6108 · exp(17.27·T / (T + 237.3)) kPa VPD = e_s(T) · (1 − RH)Water must pass through two “resistances” in series: the stomata (which depend on light, temperature, dryness and coatings) and the still boundary layer of air around the leaf (which shrinks with wind). The rate is proportional to the combined conductance multiplied by VPD.
Key terms
- Stoma (plural stomata)
- A pore in the leaf surface, opened and closed by guard cells.
- Xylem
- Plant tissue that carries water and minerals from roots to leaves.
- Cuticle
- A waxy, waterproof layer on the leaf surface.
- Relative humidity
- The amount of water vapour in the air compared with the maximum it could hold at that temperature.
- Wilting
- Drooping that happens when cells lose turgor because water is lost faster than it is replaced.
Real-world applications
- Farming and irrigation: farmers estimate crop water needs from temperature, humidity, wind and sunlight — essentially the same factors.
- Desert plants: cacti have few, sunken stomata, thick cuticles and often open stomata only at night to save water.
- Gardening: watering in the cool evening and sheltering seedlings from wind reduce water loss.
- Climate: transpiration from forests returns huge amounts of water to the atmosphere, helping form clouds and rain — the Amazon generates much of its own rainfall.
Common misconceptions
- “Roots pump water up the plant.” Most of the pull comes from evaporation at the leaves.
- “Transpiration is just wasted water.” It moves minerals, cools leaves and is unavoidable when stomata open for CO₂.
- “Stomata are on the top of leaves.” In most broad-leaved plants, most are underneath.
Key takeaways
- Transpiration is water vapour loss through stomata, which pulls water up the xylem.
- Light, temperature and wind increase the rate; humidity decreases it.
- Guard cells balance CO₂ uptake against water loss by opening and closing stomata.
- A potometer measures water uptake, a close estimate of transpiration.
Model note: the conductance values are simplified and scaled to typical potometer readings; real plants vary widely by species.