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.
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.