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Transpiration. 1) Discuss the role of stomata in transpiration (7) 2) What is meant by Transpiration Stream? (3) 3) Describe the factors affecting the rate of transpiration (10)

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Transpiration 1) Discuss the role of stomata in transpiration (7) 2) What is meant by Transpiration Stream? (3) 3) Describe the factors affecting the rate of transpiration (10) 1) Leaves are the primary photosynthetic organs of most plants. Leaf surfaces are equipped with small openings or pores called stomata, which allow carbon dioxide to enter the leaf and oxygen to escape to facilitate photosynthesis. In addition, water is lost through stomata during a process called transpiration. It is estimated that approximately 99% of the water absorbed by the roots of the plant is lost by the leaves in transpiration. Plants must exchange gasses through their leaves in order to conduct photosynthesis and respiration; they also must permit evaporation / transpiration in order to assist in the movement of water from the ground to leaves, where it is needed to build carbohydrates. Yet, if transpiration is uncontrolled, a plant may become dehydrated and die. The stoma is a pore formed by a pair of guard cells. The guard cells are located in both epidermal layers of the leaf, with a higher concentration on the underside (this is a strategy to reduce water loss). ...read more.


This is why it is called the transpiration 'stream'. 3) There are numerous factors affecting the rate of transpiration of a plant. These factors majorly influence the closing or opening of stomata within the leaves. The following are the major examples of factors (both environmental/external and internal) affecting any plant's rate of transpiration: ? Temperature: A high temperature increases the rate of transpiration within a plant, and a low temperature decreases. This is due to the fact that evaporation is stimulated by heat, so as to provide cooling down of the plant. The more water evaporates, the more is needed to replace it, keeping the needed amount of water within the plant needed for all of its metabolic processes, and therefore more water is taken up from the soil by the roots of the plant, and the transpiration rate is therefore increased. ? Humidity: A high humidity of the air surrounding a plant would account for a decrease in the rate of transpiration of the plant, and vice versa. This is because, once the air in the environment surrounding the plant is moist, there is no high difference in the osmotic potentials of the plant and the air, and no steep diffusion gradient is present to influence diffusion of water molecules out of the leaf. ...read more.


An example which could explain why this is so is that more of the leaf is exposed to sunlight, increasing the amount of photosynthesis going on in the plant, and therefore increasing the plants water needs. The roots take up enough water to fulfill these needs, and in doing so, they increased the transpiration rate of the plant. ? Cuticle Thickness: The thicker the cuticle, the more the rate of transpiration is reduced. This is so because the cuticle makes exchange of materials between the outside environment and the inside of the plant very restricted. So therefore, one of the materials which can now with more difficulty leave the plant leafs is water. As this water stays within the plant, there is no need to take up more from the soil, and so the transpiration rate is automatically decreased. ? Stomatal Density: The higher the number of stomata per unit area of a leaf, the higher the rate of transpiration. This is due to the fact that more stomata are exposed to factors such as wind or sunlight (which increase transpiration). It is also clear that for instance, in the case of a very moist soil, all the stomata will open, allowing water vapor out, again, in creasing the transpiration rate of the plant. ?? ?? ?? ?? ...read more.

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