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How do stems move water upward from roots to leaves?
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Plant
Grandidier's baobas (Adansonia grandidieri) in Madagascar.
Stems move water from roots to leaves primarily through a process known as the cohesion theory, or the transpiration-cohesion hypothesis.21
This mechanism relies on the following principles:
- Transpiration Pull: As water evaporates from the leaves through stomata, it creates a negative pressure (tension) that pulls water upward through the xylem tissue.213
- Cohesion and Adhesion: Water molecules are held together by cohesion, forming a continuous, unbroken column. Simultaneously, adhesion allows these molecules to stick to the walls of the xylem vessels and tracheids, preventing the column from breaking under tension.21
- Passive Movement: Water moves passively along a gradient of decreasing pressure from the roots to the leaves.2
While root pressure can push fluids upward, it is generally insufficient to move water to the tops of tall trees, especially when transpiration rates are high. In such cases, the transpiration-driven pull is the primary force responsible for the ascent of sap.24