Transportation of water in terms of transpirational pull:

Cohesion-tension Theory:

1. According to this theory the mechanism by which water (along with dissolved materials) is carried upward through the xylem is transpirational pull.
2. Transpiration creates a pressure difference that pulls water and salts up from their roots.

Mechanism of transpiration pull:

When a leaf transpires (loses water), the water potential of its mesophyll cells drops. This drop causes water to move by osmosis from the xylem cells of the leaf into the mesophyll cells. When one water molecule moves up by the xylem of the leaf, it creates a pulling force that continues to the root. This pulling force created by the transpiration of water is called transpirational pull. It also causes water to move transversely (from root epidermis to cortex and pericycle).

Reasons for the creation of transpirational pull:

Following are the reasons for the creation of transpirational pull.

1. Water is held in a tube (xylem) that has a small diameter.
2. Water molecules adhere to the walls of the xylem tube (adhesion).
3. Water molecules cohere to each other (cohesion) and do not contain dissolved gases (which would otherwise come out of water and form bubbles).

Note:

These attractions allow an overall tension among water molecules and form 'columns' of water. The columns of water move from root to shoot and the water content of the soil supplies water to the ‘columns.