Loss of water by evaporation is considered as Transpiration and it takes place through the stomata present in the leaves. Apart from the water loss, exchange of oxygen and carbon-di-oxide also occurs through these pores called stomata. Stomata usually remain open during the day and closed during the night. The opening and closing of stomata are due to the change in the turgidity of the guard cell. The inner wall of the guard cell near the stomatal aperture is elastic and thick. The guard cells flanking the stomatal aperture bulge towards the thin outer walls due to increase in turgidity. The inner walls are forced to form a crescent shape. The stomatal opening is supported by the arrangement of microfibrils in the walls of the guard cell. The stomata are opened easily by arranging the cellulose microfibrils radially rather than longitudinally. If the guard cells lose turgidity, water is lost easily and the inner walls that are elastic will retain the original shape. The lost turgor creates flaccid guard cells and leads to the closure of stomata.

The dorsiventral leaf of all dicotyledons are found to have more stomata below the surface of the leaf. The isobilateral leaf of monocotyledons are known to have equal number of stomata on both the surfaces of the leaf. The external factors that influence transpiration are light, wind speed, temperature, and humidity. Transpiration is affected by certain plant factors like stomata number, stomata distribution, number of opened stomata, canopy structure, and plant water levels, etc.

stomatal aperture

                       Stomatal Aperture with Guard cells


The movement of xylem sap due to transpiration is based mainly on certain physical properties of water such as surface tension, cohesion and adhesion. Cohesion represents attraction between water molecules. Adhesion represents attraction of water molecules to the polar surfaces of the tracheary elements. Surface tension represents the attachment of water molecules with each other in liquid phase to be higher than the attachment between water molecules in gas phase. The above water properties provide high tensile strength, which is the force that is used to resist the pulling force. Tensile strength also enhances the capillarity or the ability of water to rise through very thin tubes. The movement of water in tracheids and vessels in plant showed capillary movement due to the tiny diameter.

Water is very much essential in the process of photosynthesis. The xylem vessels that extend from the roots to the leaf veins aid in the transport of necessary water. The question is that what could be the force that is functioning in the transport of essential water into the parenchyma cells of the leaf? The thin continuous film of water that is flowing in the capillary path is pulled by the force of transpiration towards the leaf from the xylem vessels.

In the atmosphere, lower concentration of water vapour in comparison to the intercellular spaces and substomatal cavity allows the diffusion of water into the surrounding environment. This diffusion creates a “PULL”. The investigation and experimental evaluation has revealed that transpiration force can generate pressures enough to drag a column of water of the size of xylem approximately to the height of 130 meters.

The below diagram illustrates the leaf water movement.

transpiration in leaf


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