The Theory of Cohesion Tension and the Movement of Sap

Ascent Of Sap Cohesion Tension Theory

The Ascent of Sap Cohesion Tension Theory is a significant concept in plant physiology that explains how water and nutrients are transported from the roots to the leaves. This theory proposes that the movement of sap occurs due to a combination of cohesive forces between water molecules and tension created by transpiration. It suggests that as water evaporates from the stomata on leaf surfaces, it creates negative pressure or tension, which pulls up more water through xylem vessels. The cohesion among water molecules helps maintain a continuous column of liquid, allowing for efficient upward transport against gravity. Understanding this theory is crucial in comprehending how plants sustain their growth and survive under varying environmental conditions.


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Professor Melvin T. Tyree is affiliated with the Aiken Forestry Sciences Laboratory, which is a part of the United States Department of Agriculture Forest Service. The laboratory is located in South Burlington, VT, USA at 705 Spear Street with zip code 05403.

Understanding the concept of cohesion-tension theory

The introduction explains the cohesion-tension theory (C-T theory) proposed by Boehm in 1893 and Dixon and Joly in 1894. According to this theory, the upward movement of water in trees is solely caused by the transpirational pull exerted by continuous water columns present in the xylem conduit that runs from the roots to the leaves.

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1. The cohesion-tension theory (C-T theory) was developed by Boehm and Dixon & Joly.

2. This theory suggests that water ascent in trees occurs due to transpirational pull.

3. Continuous water columns within the xylem conduit facilitate this process.

4. The pathway for water movement extends from roots to leaves.

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Tyree and Zimmermann (2002) proposed the Cohesion-Tension Theory to explain how sap is transported upwards in plants. This theory suggests that water molecules are cohesive, meaning they stick together, forming a continuous column of water from the roots to the leaves. As water evaporates from the leaves through tiny openings called stomata, it creates tension or negative pressure within this column of water. This tension pulls more water up from the roots, creating a continuous flow known as transpiration stream. The cohesion between water molecules and tension generated by evaporation work together to drive sap ascent in plants.

What does the ascent of sap phenomenon entail?

The ascent of sap in plants refers to the upward movement of water and minerals from the roots to the aerial parts. Xylem tissue, which consists of non-living conducting cells, plays a crucial role in this process. In different groups of plants, these conducting cells include vessel members and tracheids.

The significance of cohesive force in sap ascent

The ascent of sap in plants is a fascinating process that involves the movement of water from the roots to the leaves. This movement is made possible by cohesive and adhesive forces. Cohesion refers to the attraction between water molecules, which allows them to stick together. Adhesion, on the other hand, describes the forces between water molecules and plant xylem tissue surfaces.

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As water evaporates from these stomata, it creates a negative pressure or tension within the leaf cells. This tension pulls more water up from below as it tries to replace what was lost through transpiration. The cohesion between water molecules helps ensure that this upward movement continues uninterrupted throughout the entire plant.

What is the reason behind naming it the cohesion-tension theory?

The cohesion-tension theory is based on the remarkable cohesive force of water, which enables it to withstand the tension caused by transpiration. This allows the water column in plants to remain unbroken even under conditions of drought or other stresses. The name “cohesion-tension theory” reflects this phenomenon and its significance in explaining how sap ascends through plants.