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Tuesday, February 28, 2012

How Transpiration Influences the Survival of the Plants

In the end of my previous post I brought a word called transpiration, But I didn't explained it. So this post will do that task.

You have the experience that a wilted plant revitalizes if water is supplied to it. You can assume that main reason for the wilting of plants is the non-availability of water. Why do plants face a dearth of water?

You can use this method to test how water goes away from a plant. Select a plant and cover a twig with polythene bag around it. Then keep it for some time. Now take it away and put some anhydrous copper sulphate into the polythene bag.

There are droplets of a liquid formed on the inner walls of the polythene bag. These droplets turn anhydrous copper sulphate blue, indicating that the droplets  are water. Hence, you can conclude that plants give out water. You cannot see the water coming out. In which case water should have come out in the form of vapor. This vapor has condensed and got deposited on the inner walls of the bag.

Loss of water from a plant in the form of vapor is known as Transpiration. The main way in which transpiration takes place is through the stomata. Hence this is called stomatal transpiration. If transpiration takes place through the cuticle of the leaf or any other part of the plant it is known as cuticular transpiration. Transpiration can occur through the air pores available in the roots and the stem of the plant. This is known as airpore transpiration.

The amount of water that goes out of the plant in the form of vapor is very large. This water has not taken part in any metabolic process. This quantity amounts to 95% of the total amount of water taken in by the plant.

Factors Affecting Transpiration

As transpiration is the process of water going out in the form of vapor, the factors that given evaporation affects transpiration too. Environmental factors that affect the evaporation of water are:
  • Environmental temperature.
  • Humidity of the atmosphere.
  • Speed of wind.
  • Intensity of light.
  • Atmospheric pressure.
These factors affect transpiration too. In addition the supply of soil, water too has an affect. To test the effect of these factors on transpiration, You should be able to measure the rate of transpiration. A potometer is used to measure the rate of transpiration in the laboratory.

The twig used in the potometer should be cut and fitted inside the water. This is done in order to prevent any air particles entering the twig. The twig should cut using a sharp knife. A small air bubble should be allowed to enter the capillary tube.

When water evaporates through the leaves, the bubble of air moves towards the twig. A the capillary tube is graduated, the distance moved by the air bubble can be measured. You can use this apparatus to measure the rate of transpiration of a plant under different environmental situations.

Effects of the environmental temperature

The rate of evaporation of water increases with temperature. The amount of water vapor that can remain in the atmosphere too increases with temperature. Therefore more water can go out of the leaves with the increase of temperature.

Relative humidity of air

The decrease in humidity of the atmosphere means decrease of the water vapor in the atmosphere. Water molecules can enter the atmosphere easily as a result. The diffusion rate of water from the leaf to the atmosphere decreases with the increase of the percentage of water in the atmosphere.

The effect of the wind speed

The water particles around the leaf will get blown away when there is wind. As a result more and more water particles can join the atmosphere. The rate of diffusion of water increases with the speed of the wind. Hence the rate of transpiration increases when the speed of wind increases.

Effects of the atmospheric pressure

The evaporation rate of a liquid increases in a vacuum. Th rate of evaporation decreases with the increase in pressure. Hence the rate of transpiration increases with the decrease of atmospheric pressure.

The Adaptations in Plants to Reduce the Rate of Transpiration


Plants absorb water from the soil when water leaves the plant by transpiration. But the plants on land will not get enough water. The xerophytes face the lack of water severely. Hence the plants show adaptations to reduce transpiration.

1.) Dropping of leaves, leaves turning into thorns and having fleshy stems.

A very small number of leaves appear in certain plants like cactus and euphorbia. These leaves also drop off when young. Some of the leaves and auxiliary buds modify into thorns. The transpiration reduces as there are no leaves. In addition the stem is fleshy and a milky white thick liquid is formed. This liquid keeps behind the water. The leaves of aloe are fleshy.

2.) Sunken stomata.

The stomata in plants like cycas and oleander are at a lower level than the cells of the epidermis. As they are sunk, the sotmata do not make a direct contact with the atmospheric air. Hence, the effect of the environmental factors is less and the transpiration is less.

3.) Bristles on the leaf surface.

There are hair like bristles on the surface of some leaves. These are formed by the cells of the epidermis. There is a wet, air strata between these bristles. Hence the loss of water is less and transpiration is less. eg: Pumpkin, Nettle

4.) Formation of a thick cuticle.

The cuticle of some leaves is very thick. As it has a wax on the surface of the leaf blade, the blade is shiny. Transpiration reduces as a result. eg: margosa, plumeria

5.) Rolling of leaves.

The leaves of certain grass types roll in and close down the stomata when there is a dry climate. As there are more stomata on the upper surface, these get closed down due to the rolling. Therefore transpiration reduces.

6.) Deciduousness (Dropping of leaves during certain times of the year)

The leaves of certain trees drop during the dry season. The transpiration reduces as there are no leaves. eg: rubber, terminelia

The effect of transpiration on plants

The following advantages are gained by plants due to transpiration.
  1. Certain substances are conducted to upper regions of the plant.
  2. The damage caused to plant leaves is reduced because of evaporation.
  3. Water is distributed within the plant body.
  4. The osmotic pressure inside the cells is maintained as a large amount of water is absorbed.
  5. More water enters to the plant.
  6. Plant is cooled.

Guttation

On certain days in the morning even though there is no rain, you can see small droplets of water at the tips of leaf blades such as grass, alocasia and paddy. This water has come out of the plant leaves.

There are some holes known as hydathodes at the tip of the small veins. The water pushed up by root pressure comes out of these holes. This is called as guttation. It has been found out that the function of hydathodes is only guttation. As root pressure cannot push water to a great height guttation occurs in plants that grow short. Guttation occurs in alocasia, amorph phallus and grass.

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