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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Upward movement of water in the plant

Water movement is active or passive? When the water moves against the ground in the stem, it needs some energy for moving up.

Root Pressure

The ions in the soil move towards the vascular tissues of the roots in active transport. Due to the change in the potential gradient, water also moves and enhances the xylem pressure. This positive pressure in the xylem is called as root pressure, which is responsible for movement of water to a certain height in the stem.

Let us see how the root pressure functions. A small soft stem is chosen for the test when there is lots of moisture in the atmosphere. The stem is cut at its base during the early part of the day which ends up in the release of a few drops of solution oozed out from the stem. The water drops coming out of the stem is due to the root pressure. If any rubber tube is fixed under the stem then the exudates can be gathered and rate of exudates can be measured. The ingredients of the exudates also can be evaluated.

The root pressure can be observed during the nights and even in the morning when the evaporation is less. The edges of the grass blades and leaves exudes water droplets from the vein openings of many herbs. This type of water loss is known as guttation. The water transport process can at best be stimulated by root pressure.  Root pressure itself is not solely responsible for the movement of water to the top of the tall trees. Root pressure aids in establishing the continuation of chain of water molecules in the xylem which might frequently be broken due to intensive tension formed by the transpiration pull. Most of the plants have the water movement aided by transpiration pull rather than by root pressure.

Transpiration Pull

Though there is no specific circulatory system in the plants, water movement through the xylem can be faster and can reach even upto 15 meters of height in an hour.  There was a big question regarding this aspect for many years. People were wondering whether the water is reaching  the plant top by a ‘push’ or ‘pull’. Many research studies have proved that water was ‘pulled’ towards top and it was due to the transpiration process in the leaves. This model was known as cohesion-tension transpiration pull of water. The force that is responsible for this ‘pull’ is called transpiration. It is observed that only about one percent of water that is absorbed into the plant leaves are used for plant growth and photosynthesis, and the rest is evaporated through the stomata by a process called transpiration. 

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planthabitBasic information on plant biology

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