Structure of Xylem Tissue and TACT Mechanism Explained

🌱 Structure of Xylem Tissue

Definition:
Xylem is a type of vascular tissue in plants responsible for the transport of water and dissolved minerals from the roots to the stem and leaves.
It also provides mechanical strength and support to the plant body.

🔹 Main Components of Xylem

Xylem is made up of four different types of cells:

Tracheids

Long, thin, and tapered cells with thick lignified walls.
Have bordered pits at their ends that allow side-to-side movement of water.
Found in all vascular plants (ferns, gymnosperms, and angiosperms).
Function: Transport of water and provide mechanical support.

Vessels

Shorter and wider than tracheids.
Arranged end-to-end to form continuous tubes for water flow.
Have perforation plates at their ends for efficient water movement.
Found mainly in angiosperms (flowering plants).
Function: Fast and efficient water transport.

Xylem Fibres

Elongated dead cells with thick lignified walls.
Function: Provide mechanical strength to the xylem and the plant body.

Xylem Parenchyma

Living cells with thin cell walls.
Store water, starch, and other nutrients.
Help in lateral transport of water and nutrients and also in repair of tissues.

💧 Movement of Water through Xylem (TACT Mechanism)

The movement of water in plants, from roots to leaves, mainly occurs through the xylem by a process explained by the TACT mechanism.

TACT stands for:

T – Transpiration
A – Adhesion
C – Cohesion
T – Tension

🌬️ 1. Transpiration

It is the loss of water vapour from plant leaves through stomata.
As water evaporates, it creates a negative pressure (suction force) in the leaf.
This “pull” draws water upward from the roots through the xylem.
Transpiration is the main driving force behind water transport.

🧲 2. Adhesion

Water molecules are attracted to the walls of xylem vessels.
This adhesion helps water cling to the sides of xylem tubes, preventing breakage of the water column.
It helps maintain a continuous flow of water, especially in tall trees.

🤝 3. Cohesion

Water molecules stick together due to hydrogen bonding.
This creates an unbroken chain of water molecules from roots to leaves.
Cohesion ensures that when water is pulled at the top, the entire column moves upward smoothly.

🌀 4. Tension

The negative pressure created by transpiration at the leaves is called tension.
This tension helps pull the cohesive water column upward through the xylem.
It maintains continuous upward movement of water even against gravity.

🌿 Summary of TACT Mechanism

Process	Description	Function
Transpiration	Water loss through stomata	Creates upward pull
Adhesion	Water sticks to xylem walls	Maintains water column
Cohesion	Water molecules stick together	Keeps water column continuous
Tension	Negative pressure from leaves	Pulls water upward

🧠 Key Points to Remember

Xylem = Dead + Living cells (Tracheids, vessels, and fibres are dead; parenchyma is living).
Movement of water is a physical process, requiring no energy (ATP).
The TACT mechanism depends on properties of water and transpiration pull.