It takes place when stomata opens for gas exchange while photosynthesis .

It helps the plants cool down as it absorbs the heat when it evaporates(same principle as that of when we sweat).

It also creates suction pull, which leads the water from roots in the xylem rise up. BUT HOW ????????????

It is the attraction of two molecules of the different kind.

It may be because of bonding .

When opposite charges attract each other adhesion may take place.

Like the water is clinging to the leaf.

When water is poured on clean glass, it tends to spread, forming a thin,

uniform film over the glasses surface. This is because the adhesive forces between water and glass are strong enough to pull the water molecules out of their spherical formation and hold them against the surface of the

glass, thus avoiding the repulsion between like molecules.

This is the attraction of molecules of same type.

The attractive forces exist between molecules of same substance.

Water droplets are formed because of cohesive forces.

This theory was proposed by an Irish botanist H.H. Dixon and John Joly (in 1894). They said that water is pulled from up, and not pushed from down .There are 2 important things of this theory. They are:

Cohesion of water and adhesion between water and xylem tissues

Transpirational pull

(vapor form)

(liquid form)

(vapor form)

As the water molecules attract each other (by cohesion) they cannot be easily separated away from each other.

Further, there is attraction between water molecules and the inner wall of xylem .

So , one can determine that water cannot be pulled away because of strong adhesive and cohesive forces between water and xylem and water and water respectively.

It is the pulling force responsible for lifting the water. As water is lost in form of water vapor to atmosphere from the stomata, there is a negative pressure created in the cells of leaves or it can be said that the concentration of water becomes less(in the leaves).