What is Transpiration in Plants
- Plants absorb water through their roots and lose it from their leaves by evaporation.
- The loss of water from the leaves draws more water up through the plant.
- This continual flow of water through a plant, driven by evaporation, is known as Transpiration in Plants.
Transporting Water in Plants
Transpiration is a process in which plants lose water through its leaves, the water is replaced through the roots.
The uninterrupted stream of water from the roots to the leaves is called the Transpiration stream. Read More…
Process of Transpiration in Plants
Mechanism of the process of Transpiration is as follows-
- Roots absorb water from the soil through osmosis. Water travels from high concentration area to low concentration area via a semipermeable membrane called osmosis.
- The roots are covered in specialized cells that stick out into the soil. They have a large surface area to absorb both water and minerals from the soil.
- Water is passively transported into the roots and then into the xylem.
Capillary action in the Xylem Vessels
Water is drawn up the xylem in the stem by three factors:
- Root pressure
- Capillary action
- Transpiration pull
The forces of cohesion and adhesion cause the water molecules to form a column in the xylem
- Evaporation at the surface of the leaf keeps the water column moving.This is the strongest force involved in transpiration.
- Water evaporates through the stomata of leaves..
- Water moves from the xylem into the mesophyll cells evaporates from their surfaces and leaves the plant by diffusion through the stomata.
- Stomata are tiny holes in leaves through which water evaporates. Carbon dioxide enters the leaves through these holes, and oxygen diffuses out.
- Leaves contain spongy mesophyll cells that are covered with a film of moisture. When this moisture evaporates, it diffuses out through the stomata.
- Tiny tubes called xylem tubes run through the stem. The loss of water from the leaves pulls more water through these tubes, replacing what has been lost from the mesophyll cells.
- Transpiration is the movement of water up through a plant, driven by evaporation from the leaves.
- Plants regulate transpiration by opening their stomata (pores in leaves).
Plant roots are adapted to maximize the absorption of water and minerals from the soil. If a plant loses more water from its leaves than it gains from its roots, the plant starts to wilt (droop) and may eventually die.
Root hair cell
The root is covered in a layer of cells called the epidermis. Root hairs are long extensions of the epidermal cells. These hairs create a large surface area for absorbing water and minerals from the soil.
Cells on the surface of roots are covered in millions of fine hairlike extensions that stick out into the soil. These take in water and minerals from the soil. Water is absorbed by osmosis and mineral ions are absorbed by active transport.
Water is pulled upwards from the roots through to the stem and leaves. If there isn’t enough water, the vacuoles in cells throughout the plant shrink. Pressure is not put on the cell walls to keep the plant upright, so the plant starts to wilt
Rate of Transpiration
More transpiration happens during the day than at night because the stomata are closed in the dark.
Four main factors that affect the rate of transpiration are as follows
- Wind speed
- Light intensity Read More….
There are a number of methods for measuring transpiration. Some of the methods are as follows
- A small lightweight potted plant can be weighed before and after the end of a certain period of time.
- The soil surface and the pot should be fully covered to prevent evaporation from the surfaces other than the plant.
- The loss in weight by the plant during that time is due to the loss of water by transpiration.
- An improvement in the weighing method can be made by using a glass bottle linked by a rubber tube to a graduated side tube, filled with water.
- The water level in the side tube falls to demonstrate loss of water through transpiration from the leaves.
- This would indicate the volume of water loss that can be compared with the loss in weight with the help of a weighing machine.
- Another weighing experiment can be done by using a test-tube filled with water and inserting a leafy shoot (no roots) in it and pouring some oil on the surface to prevent loss of water from the test tube by evaporation.
- Place the test tube in a small beaker and weigh them together.
- Remove the intact test tube and keep it straight in the test tube stand for a few hours.
- Weigh it again by keeping it in the same beaker. Any difference in weight will indicate loss of water by the shoot (due to transpiration).
- Since there are no roots to actively absorb water, the water loss through transpiration will be much less.
- A Potometer is a device that measures the rate of water intake by a plant, and this water intake is almost equal to the water lost through transpiration.
- Rate of transpiration can be estimated by using an apparatus called a potometer to measure how quickly a plant takes up water.
- By potometer one can change light intensity, temperature, humidity, and wind speed to see how environmental factors affect the rate.
Also Check – Potometer- How Potometer works, How to use Potometer and Potometer Experiment
Types of Transpiration
Transpiration from the Aerial parts of a plant occurs from three different regions :
- Stomatal Transpiration – From the leaves through the stomata. Read More ….
- Cuticular Transpiration – Directly from the surface of the leaves and stems.
- Lenticular Transpiration– From the lenticels which are the minute openings on the surface of old woody stems. Read More….
The major part of the transpiration occurs through the stomata, whereas the other two types of Transpiration contribute very little.
How Transpiration affects the Climate
Transpiration affects climate, it increases the moisture in the atmosphere and brings rain. Transpiration is the secret of forests that contribute to bringing rain.
It has been shown experimentally that plants give out large quantities of water during transpiration.
A full grown single sunflower plant is estimated to lose about half a liter of water per day in the form of water vapor.A single maize plant loses about 2 liters of water per day. A large apple tree may lose about 30 liters of water per day. These figures give an idea of the huge quantities of water released into the atmosphere by vast stretches of fields and particularly forests.
Thus, transpiration increases the moisture in the atmosphere and brings rain. In this way, Transpiration from plants affects climate.