BIOLOGY TRANSPIRATION Significance and Factors affecting Transpiration Factors Affecting the Rate of Transpiration •On a bright sunny day, stomata open fully, so transpiration is increased. •On a cloudy day, stomata open partially, so Intensity of sunlight transpiration is reduced. •At night, stomata close; hence, transpiration is greatly reduced or negligible. •Increase in temperature of the air increases the rate Temperature of transpiration. •Transpiration increases with rapid or active air Velocity of wind movement. Humidity •If the air is humid, the rate of transpiration is reduced. •Increase in the CO2 level in the atmosphere over Carbon dioxide normal 0.03% causes stomatal closure. Hence, it decreases the rate of transpiration. •With decrease in atmospheric pressure, the rate of Atmospheric pressure transpiration increases. www.topperlearning.com 2 BIOLOGY TRANSPIRATION Adaptation in Plants to Control Excessive Transpiration Plants which grow in dry climate have evolved a variety of adaptations to curtail transpiration. Morphological Leaves may be modified into spines as in cactus or into needles as in pines. Adaptations Spines Needles Leaves may be folded or rolled up. Rolled up Leaves Leaves may be shed. Example: deciduous trees. Deciduous Trees Anatomical The number of stomata is reduced, and they may be sunken in pits. Adaptations Sunken Stoma www.topperlearning.com 3 BIOLOGY TRANSPIRATION Structure of Sunken Stomata A thick waxy cuticle develops on the leaves. Example: Banyan tree, evergreen trees. Banyan Tree Shrubs and grass develop a waterproof covering of cork or bark. A multiple epidermis may develop in some leaves. www.topperlearning.com 4 BIOLOGY TRANSPIRATION Significance of Transpiration Cooling Effect Suction Force Distribution of Water and Mineral Salts Evaporation reduces the As water evaporates from leaves, Higher the rate of transpiration, temperature of leaf a suction force is created. This greater the rate of absorption of surfaces which is useful force helps to draw more water water and solutes from the soil. for plants on hot sunny up through the stem which days. causes roots to absorb more water from the soil. Differences between Transpiration and Evaporation Transpiration Evaporation Loss of water in the form of water Loss of water from the surface of vapour from aerial parts of the plant. water bodies. It is a physical process controlled by It is a physical change controlled internal and external factors. by the temperature and humidity of atmosphere. It is a slow process. It is a fast process. www.topperlearning.com 5 BIOLOGY TRANSPIRATION Guttation and Bleeding Guttation Sometimes, due to high root pressure, water is forced out through pores at the end of leaf veins which are called hydathodes. The water appears as tiny drops along the margins or the tip of leaves. This loss of excessive water is called guttation. Bleeding Loss of water, i.e. cell sap, through a cut stem is called bleeding. Differences between Transpiration and Guttation Transpiration Guttation Water is lost in the form of Water is lost in the form of water vapour. droplets. Pure water is converted into Water contains mineral salts water vapour. dissolved in it. Water vapour is lost through Water is lost through stomata, lenticels and cuticle. hydathodes. It occurs in the presence of It occurs at night or in early sunlight. morning. It has a cooling effect on the It does not have any significant plant. effect on the plant. www.topperlearning.com 6 .