Transpirational Pull

What is Transpiration Pull ?

What is Transpiration Pull ?

As the water is lost from the leaf surface by transpiration, more water molecules are pulled up due to the tendency of water molecules to remain joined (cohesion), and thus to produce a continuous column of water through the stem.

Transpirational Pull Theory

According to Root Pressure Theory, the roots absorb water and exert a pressure, the root pressure, which pushes the water upward. The root pressure develops in the tracheary element of xylem (i.e., tracheids and vessels) as a result of metabolic activities of roots. The root pressure theory is applicable in small herbs but not in tall trees.

Also Check – 15 Differences between Transpiration and Evaporation

Transpiration pull and cohesion-tension theory explains the upward movement of water. The main force responsible for upward movement of water is transpiration pull generated in the leaves which pulls the water column filled in the xylem tracheids and vessels. 

Also Check – All About Transpiration in Plants

The major events of upward movement of water and minerals are as follows:

  1. The water filled in xylem tracheids and vessels is not pure. It has several mineral elements dissolved in it. The minerals needed by plants are absorbed from the soil through root hairs and other epidermal cells of root tip. They are taken up in the inorganic form, such as nitrates, phosphates, etc.
  2. The water and minerals, absorbed by root hair and other epidermal cells, move through the root cortex, endodermis, pericycle and reach the xylem.
  3. Water and minerals (i.e., the sap) move upward in the tracheids and vessels of xylem. Vessels are found in the xylem of only flowering plants. That means, the flowering plants (Angiosperms) possess both tracheids and vessels. The non-flowering plants (such as Cycas, fern, etc.) possess only tracheids which serve as the main conducting tissues.
  4. Xylem tracheids and vessels are dead cells and thick-walled. The tracheids are long, thin, spindle-shaped cells with tapering ends. They have pits in their thick cell walls through which the water flows from one tracheid to another. The xylem vessels are tubular structures extending from roots to the top of the plants.
  5. The cells are placed one above the other with their end walls perforated forming a continuous tube. Thus, the lumen of tracheids and vessels are filled with sap and this sap moves upward from one tracheid to another and from one vessel to another through pits and perforations. 
  6. The xylem of the root is connected with that of stem, petiole and veins of leaves. All these cells make a continuous network of tubes through which the sap can move from roots to the leaves.
  7. The water and minerals (sap) filled inside the xylem capillaries forms a continuous column which cannot be broken or pulled away from the xylem walls. Thus, the unbroken water column is just like steel rope which extends from leaves to the roots. If this column is pulled from the top, the entire rope of sap moves upward. In plants, the pull is generated by the process known as transpiration.
  8. Loss of water in the form of vapors through transpiration is replenished by movement of fresh supply of water to the leaf. This creates a suction which pulls the water column (sap) upward.


Also Check – Transpiration Stream

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