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  • Transpiration
    • the loss of water vapour by evaporation from aerial parts of the plant
    • Transpiration Stream
      • as water leaves the xylem in the leaf, it must be replaced below. Water moves up the Xylem from the roots to replace the water lost
      • Useful to the plant:
        • water is required to enable cells to grow and elongate
        • water is required in the leaf for photo-synthesis
        • water keeps the cells turgid
        • evaporation of water can keep the plant cool
        • the flow of water can carry useful minerals up the plant
    • Measuring Rate
      • Potometer can be used to estimate the rate of water loss
        • actually measures the rate of water uptake but about 99% of water taken up is lost in transpiration
      • important that there are no air bubbles
      • water lost by the leaf is replaced from the water in the capillary tube
        • the movement of the meniscus at the end of the water column can be measured
      • to study different       environmental conditions on the rate of transpiration, you can place the equipment  in different situations
    • Altering Factors
      • Light - in light stomata open for gaseous exchange
      • Cuticle - waxy reduces evaporation
      • Water Availability - if there is little water in the soil then the water cannot be replaced so water loss is reduced when stomata are closed or when leaves shed water
      • Wind - carries water vapour away that have just diffused out of the leaf - maintaining high water potential gradient
      • Number, size and position of Stomata - lower surface, water is lost less readily
      • Relative Humidity - higher decreases water loss, smaller water potential gradient
      • Number of Leaves - more = more surface area over which vapour can be lost
      • Temperature -  increased increases water loss
        • 1.increase rate of evaporation from the cell surfaces so that the water potential in the leaf rises
        • increase the rate of diffusion through the stomata as they have more kinetic energy
        • decrease the water vapour potential   in the air, allowing more rapid diffusion
    • Water Loss
      • If water loss by transpiration is greater than water uptake from the roots, the plant cells will lose turgidity
        • the leaves of woody plants will wilt and the plant will eventually die
        • non woody plants will wilt and die
    • How does it occur
      • water enters the leaves in the xylem and passes to the mesophyll cells by osmosis and evaporates from the surface to from water vapour
        • the spongy mesophyll cells have large air spaces between them that help the water vapour to diffuse through the leaf tissue
          • As vapour collects in these air spaces, the water vapour potential rises, and once to above outside the leaf, water molecules will diffuse outside the leaf
            • the stomata make an esy route out of the leaf, as they are open during the day
      • 1. osmosis from the xylem to mesophyll cells
        • 2. evaporation from the surface of the mesophyll cells into the intercellular spaces
          • 3. diffusion of water vapour from the intercellular spaces out through the stomata


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