Removal of excess amount of water from plants, Transpiration

 

• The removal of excess amounts of water through the stomata by aerial parts of plants is called transpiration.Potometer is used to measure the transpiration.

• It is noted that just two or  three percent of water  that is absorbed by the root of plants, is used by plants for their  growth, development and metabolism.

• Remaining amount of water is eliminated by the  transpiration and guttastion .

Read about the stomata 

Significance  of Transpiration: 

• It develops  transpiration pull for absorption and transport of  water in plants. It also help in transport of  minerals from the soil to others  parts of the plant

• It lowers the temperature up to ten to fifteen degrees and cools the surface of the leaf. 

• It also maintains the shape and structure of the plants by keeping cells turgid.

• Rate of transpiration is affected by both external and internal factors.

• Temperature, light humidity,Wind speed are external factors that affect transpiration whereas some internal factors in plants also affect the transpiration.

• They are- Number and distribution of stomata,Water status of the plant, Canopy structure etc.

Transpiration pull

• When there is  low concentration of water vapour in the atmosphere  than the substomatal cavity and intercellular spaces in plants.

• The water is diffused  into the surrounding air from the plants in the form of droplets through the process of evaporation.

• As water evaporates through the stomata, As a  result, it causes   the pulling of water  into the leaf from the xylem.

• This pulling of water is called as  ‘transpiration pull’ or ‘suction pressures’.This is responsible  for the lifting  of water over sufficient  height through xylem.

• It indicates Water is withdrawn in the plant by the driving force that is generated due to the transpiration by  the leaves. This is also referred as cohesion-tension-transpiration pull model of water transport.

• The ascent of xylem sap means water through transpiration pull depends mainly on the following physical properties of water.

• Mutual attraction between water molecules termed Cohesion.

• Attraction of water molecules to polar surfaces called Adhesion.

• Surface Tension means Water molecules are attracted to each other in the liquid phase more than to water in the gas phase. These properties provide water with high tensile strength.

• Capillarity action - The ability of Water  to rise in thin tubes like tracheids and vessels of xylem.

Types of Transpiration in plants 

• Stomatal, Leticular and cuticular  are three different types of transpiration in plants:

• When the stomata are open,the water in leaves changes into vapour and evaporates due to low concentration in the atmosphere.

• Most of the water is removed  from the plants by this method. The removal or  evaporation of water through  the stomata of the plants is called Stomatal Transpiration

• Lenticels are small openings in the bark of branches and twigs.

• Evaporation of water from the lenticels of the plants is known as lenticular transpiration. Lenticles are not present in all the plants.

• The evaporation of water from the cuticle of the plants. The cuticle is a waxy covering on the surface of the leaves of the plants.

• During dry conditions when the stomata are closed, more water is transpired through the cuticles, called cuticular respiration.