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Unformatted text preview: lecules are polar and are attracted to each other.
• Adhesion: water molecules also stick to other polar substances, such as the walls of xylem vessels.
• Both phenomena tend to draw water up narrow vessels. The highest a column of water can rise
before gravity overcomes the forces in the hydrogen bonds involved is 32 feet.
• Evaporation: contributes to the Cohesion-Tension theory
• Cohesion creates a “water chain” up the xylem vessels.
• Evaporation at the leaf surface removes water molecules, lowering the water concentration in the
• More water rises by osmosis.
• Evapo-transpiration is controlled by the opening and closing of the stomata of the leaf. Stomata
open in response to light and low carbon dioxide levels in the leaf. They close in response to water
• The pressure-flow theory is the current explanation of how plants transport sugars.
• Pressure-flow takes advantage of osmosis to move sugars in any direction the plant needs.
• Source: Sugars are made in the photosynthetic cells of the leaf.
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This document was uploaded on 02/25/2014 for the course BIOLOGY 103 at Western Oregon University.
- Winter '09