The Cohesion-Tension Theory explains how water is pulled up through the xylem over great heights. It relies on the high cohesive forces between water molecules (hydrogen bonding) and adhesive forces between water and the xylem cell walls.
As water evaporates from the mesophyll cells into the sub-stomatal air spaces, it creates a negative pressure or tension in the leaf xylem. This tension is transmitted downward through a continuous column of water to the roots, effectively 'pulling' water upward.
The Water Potential Gradient () is the fundamental thermodynamic driver. Water moves from the soil (highest ) through the plant to the atmosphere (lowest ), ensuring a passive but highly efficient transport system.
| Feature | Transpiration | Guttation |
|---|---|---|
| State of Water | Water vapor (gas) | Liquid droplets |
| Exit Point | Stomata, Lenticels, Cuticle | Hydathodes (leaf margins) |
| Driving Force | Transpiration pull (negative pressure) | Root pressure (positive pressure) |
| Timing | Primarily during the day | Night or early morning |
| Purity | Pure water | Contains dissolved minerals/sugars |
Identify the Variable: When analyzing graphs, always check if the x-axis represents an environmental factor (like light intensity) or time. Transpiration rates typically peak at midday when light and temperature are highest.
Potometer Limitations: Remember that a potometer measures water absorption, not just transpiration. Some water is used for photosynthesis or maintaining cell turgidity, though this is usually less than 1% of the total uptake.
Units and Precision: When calculating rates, ensure units are consistent (e.g., or ). Always mention the necessity of a 'control' (e.g., a plant in still air vs. a plant in wind) to validate experimental results.
Active Transport Myth: A common error is assuming water is 'pumped' by the plant. Transpiration is a passive process driven by solar energy (which causes evaporation); the plant does not expend ATP to move water through the xylem.
Stomatal Closure: Students often think stomata only close at night. In reality, stomata may close during the day if the plant is under extreme water stress (wilting) to prevent excessive dehydration, regardless of light availability.
Humidity Relationship: Many mistakenly believe high humidity increases transpiration. In fact, high humidity reduces the vapor pressure gradient between the leaf and air, significantly decreasing the transpiration rate.
