12.4.2 Importance of Transpiration
Transpiration plays an important role in the life of plants as it provides the forces or tension to pull water and minerals ions up the xylem vessels from the roots to the leaves. Sometimes, when the water supply is short, more water may be lost from the plants by transpiration than is absorbed from the soil.
In such a condition the cells loses water. As a result of this, the plant loses their turgor and become flaccid. The plant has the ability to withstand such a condition if it lasts for a short period. But if such a condition persists for long periods, the plant wilts even dies. In such conditions many plants have ability to control the rate of transpiration by closing their stomata.
Stomata
The stomata are very minute openings in the epidermis of the leaves. A stomata (singular of stomata) consists of a small central opening surrounded by a pair of modified epidermal, sausage- shaped, cells called guard cells. The guard cells regulate or control the size of the stomal opening, by changing their own shape. When the guard cells are turgid, their inner margins curve apart from each other because their inner walls are thicker than outer ones.
Thus by the curving if the inner margins of guard cells the aperture between them opens. On the other hand, when the guard cells loose their turgidity, they become flaccid and their inner margins become straightened and come to lie close together. In this way, the stoma is closed. If the stomata are closed, transpiration is almost completely stopped.
12.4.3 Measurement of the Rate of Transpiration
The rate of transpiration can be measured by many methods. It is generally believed that the rate of absorption of water from the soil is nearly equal to the rate of transpiration. Of the many methods for the measurement of the rate of transpiration, one makes use of potometer, a glass apparatus that works on above-mentioned principle.
Practical Work: Comparison of the rate of transpiration by potometer
Take a leaf shoot and fix it in of the arm of a thoroughly water- filled potometer. If there are any air bubbles, these should be excluded. All connection should be made air tight with the help of vaseline or plaster of Paris. Now introduce the air bubble in to the capillary tube of the apparatus. Place the setup at a well-lit place.
As soon as the transpiration starts, the air bubble will begin to move along the capillary tube, the rate of movement of the air bubble gives the measure of transpiration. Rate of transpiration can be compared under different condition such as light /shade; under the fan /away from fan etc.
Practical Work: Comparison of transpiration from too surfaces of leaf by cobalt paper
The lower surface of the leaf has more stomata than the upper surface, so more water transpires from the lower surface than the upper surface. This can be demonstrated by the cobalt chloride method. In this method the amount of transpiration of water is estimated by the change in colour of the cobalt chloride paper. When dried cobalt chloride paper is exposed to humid air or moisture it will gradually change its colour from blue to pink the normal the colour of dry (anhydrous) cobalt chloride paper is blue.
The cobalt chloride paper can be prepared by treating filter paper disc with 3% slightly acidic solution of cobalt chloride. After treatment the filter paper discs are thoroughly dried. In order to perform this experiment, take to equal sized discs of blue cobalt chloride filter papers place one disc on the upper and one on the lower surface of the leaf of a potted plant. You will see that after a few minutes, the cobalt chloride paper starts changing colour from blue to pink. You will note that the paper fixed on the upper surface of the leaf takes much longer time to turn pink from blue than that of the paper on the lower surface of the leaf. The rate of change of colour indicates the rate of transpiration, which is much higher from the lower surface than from the upper surface. This is because of the presence of stomata on the lower surface.