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.

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 guttation.

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 It indicates Water is withdrawn in the plant by the driving force that is generated due to the transpiration by  the leaves.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 like 1. mutual attraction between water molecules termed Cohesion. 2. Attraction of water molecules to polar surfaces called Adhesion. 3. 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. 4.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.