Understanding the Core Principle
The Cohesion-Tension Theory describes the primary mechanism by which water ascends from the roots to the leaves of plants, often against significant gravitational pull. It posits that water movement is driven by a continuous column of water maintained under tension (negative pressure) throughout the plant's xylem, primarily caused by transpiration (evaporation of water from leaves).
Key Components: Cohesion and Adhesion
This theory relies on two critical properties of water: cohesion and adhesion. Cohesion is the attraction between water molecules, forming an unbroken column due to hydrogen bonding. Adhesion is the attraction between water molecules and the xylem vessel walls, preventing the water column from breaking and helping it "stick" to the sides as it moves upwards.
The Role of Transpiration in Creating Tension
Transpiration, the process where water vapor evaporates from stomata (small pores) on the leaves, creates a pulling force or "tension" at the top of the water column. As water molecules leave the leaf, they pull on adjacent water molecules below them through cohesive forces, transmitting this tension all the way down to the roots, drawing more water up from the soil.
Importance for Plant Life and Ecosystems
The Cohesion-Tension Theory is vital for the survival of terrestrial plants, enabling them to transport essential water and dissolved minerals from the soil to every cell, especially in tall trees. This continuous water flow is also a significant component of the global water cycle, linking atmospheric moisture to soil moisture through plant activity.