In plants, the water is lost in the form of water vapour from the stomata of the leaves. As the film of water is continuous over the cells, thus a transpirational pull is generated by the loss of water from the stomatal pores of leaves. This is called the cohesion-tension model of water transport.

During daytime, the water lost through transpiration (by the leaves to the surroundings) causes the guard cells and other epidermal cells to become flaccid. They, in turn, take water from the xylem. This creates a negative pressure or tension in the xylem vessels, from the surfaces of the leaves to the tips of the roots, through the stem. As a result, the water present in the xylem is pulled as a single column from the stem. The cohesion and adhesion forces of the water molecules and the cell walls of the xylem vessels prevent the water column from disrupting.

Factors affecting transpiration

The rate of transpiration depends on the following factors: 

External factors - 

i. Temperature
ii. Humidity
iii. Light
iv. Wind speed

Internal factors

i. Number and distribution of stomata,
ii. Percent of open stomata
iii. Water status of the plant
iv. Canopy structure. 

Transpiration is useful to the plants in the following way:

i. Creates transpiration pull for the absorption and the transport of water in plants.
ii. supplies water for photosynthesis.
iii. Transports minerals and nutrients to the plants.
iv. Has a cooling effect on the leaf surface. 
v. Maintains the shape and structure of the plants cells by keeping them turgid.