Plants move water from soil to leaves through a continuous pathway called the transpiration stream. This process matters because water carries dissolved minerals, cools leaves, and keeps cells firm enough to support plant tissues. In tall trees, water must rise many meters against gravity without a pump like an animal heart.
The main driving force comes from evaporation at leaf surfaces, especially through tiny pores called stomata.
The cohesion-tension theory explains how transpiration can pull water upward through xylem vessels. As water evaporates from moist cell walls in the leaf, it creates negative pressure, or tension, that pulls on the column of water below it. Cohesion between water molecules and adhesion to xylem walls help keep the column continuous as it rises from roots to leaves.
Stomata can open and close to balance carbon dioxide intake for photosynthesis with water loss to the air.
Understanding Biology: Transpiration and Water Transport
Water enters a plant through root cells with partly permeable membranes. Roots often move mineral ions into their tissues using energy from respiration. This lowers the water potential inside the root, so water enters by osmosis.
A layer called the endodermis acts as a checkpoint near the centre of the root. Its waterproof Casparian strip forces water to cross cell membranes, where the plant can control which ions enter the xylem. Sometimes roots create enough pressure to push water upward a short distance.
This can produce guttation, which is droplets at leaf edges in cool, humid conditions. Root pressure is far too weak to supply a tall tree by itself.
Inside a leaf, water coats the walls of mesophyll cells facing air spaces. Evaporation removes water from this thin film first. The remaining water forms curved surfaces in tiny gaps within the cell walls.
These curved surfaces create a strong pulling effect. The large internal surface area of a leaf makes this effect much greater than its flat outside shape suggests. Xylem vessels are built for this job.
They are mostly hollow, dead cells joined end to end, with thick walls strengthened by lignin. This prevents the vessels from collapsing when the water column is under tension.
Stomata are controlled by pairs of guard cells. In suitable conditions, guard cells take up ions. Water enters by osmosis, the cells become more curved, and the pore opens.
During drought, a plant hormone called abscisic acid helps guard cells lose ions and water. The pore then closes. Closing stomata saves water, but it reduces carbon dioxide entry.
This can slow photosynthesis and growth. Transpiration therefore involves a constant compromise between conserving water and gaining the carbon dioxide needed to make sugars.
Wind, warm air, bright light, and low humidity usually increase water loss. Still air creates a humid layer around a leaf, which slows evaporation.
The water column can break during severe drought, freezing conditions, or physical damage. A break creates an air bubble called an embolism. Air bubbles stop water moving through that part of the xylem because gases compress much more easily than water.
Plants reduce this risk with narrow vessels and pits in xylem walls that limit bubble spread. Some plants can refill damaged vessels when water is plentiful. Others grow new xylem each year.
Wilting is another useful sign of water shortage. Cells lose water, their internal pressure falls, and soft tissues can no longer stay firm. A wilted plant may recover after watering if its xylem has not been badly damaged.
Students often investigate water transport with a potometer or by measuring the mass of a potted plant over time. A potometer measures water uptake, which is close to transpiration but not exactly the same. Some absorbed water is used in photosynthesis or stored in tissues.
Good experiments control leaf area, temperature, light, airflow, and humidity. A layer of oil on the soil can reduce evaporation from the pot, making the measurement fairer.
Coloured water in celery or white flowers can show the xylem pathway clearly, though it does not measure the rate of water movement. When interpreting results, focus on the changing water loss gradient between the moist leaf interior and the surrounding air.
Key Facts
- Transpiration is the evaporation of water from plant leaves, mainly through stomata.
- The main water pathway is soil to root hairs to root xylem to stem xylem to leaf xylem to mesophyll to air spaces to stomata.
- Cohesion is attraction between water molecules, and adhesion is attraction between water molecules and xylem walls.
- Cohesion-tension theory: evaporation in leaves creates negative pressure that pulls a continuous water column upward through xylem.
- Transpiration rate = water lost / time, often measured in mL/min or g/h.
- Water potential moves from higher water potential to lower water potential, often written as water moves down a water potential gradient.
Vocabulary
- Transpiration
- Transpiration is the loss of water vapor from a plant, mostly through stomata in the leaves.
- Xylem
- Xylem is the vascular tissue that transports water and dissolved minerals from roots toward stems and leaves.
- Stomata
- Stomata are tiny adjustable pores in the leaf epidermis that allow gas exchange and water vapor loss.
- Cohesion-tension theory
- Cohesion-tension theory is the explanation that evaporation from leaves creates a pulling force on cohesive water columns in xylem.
- Water potential
- Water potential is a measure of the tendency of water to move from one place to another, with water moving from higher to lower water potential.
Common Mistakes to Avoid
- Saying roots push most water to the top of tall trees is wrong because root pressure is usually too weak to lift water many meters. Transpiration pull is the main force in tall plants.
- Thinking xylem cells actively pump water is wrong because mature xylem vessels are dead, hollow tubes. Water movement is driven by physical forces such as tension, cohesion, and adhesion.
- Confusing stomata with chloroplasts is wrong because stomata are pores for gas exchange, while chloroplasts are organelles that carry out photosynthesis. Guard cells control the size of the stomatal opening.
- Assuming plants should always keep stomata open is wrong because open stomata allow carbon dioxide in but also increase water loss. Plants must balance photosynthesis with dehydration risk.
Practice Questions
- 1 A plant loses 18 mL of water through transpiration in 6 hours. What is its average transpiration rate in mL/h?
- 2 A tree is 24 m tall. If water in the xylem rises at an average speed of 3 m/h, how long would it take for water to travel from the roots to the top leaves?
- 3 On a hot, dry, windy day, explain how stomatal opening, evaporation rate, and transpiration pull would likely change compared with a cool, humid day.