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6.2: Introduction

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  • Plants are nature’s great water filters. They absorb water from the soil through their roots (if they have roots), use this water to maintain homeostasis, and whatever is left evaporates from open stomata across the epidermis of the plant. Each water molecule that leaves the plant is electrically charged and, due to these charges, tugs on the molecule behind it. The vast majority of water absorbed by most plants will exit via this process, creating a continuous vacuum of water through the plant, from the soil to the atmosphere. This evaporation of water from plant tissues is called transpiration. This process is highly dependent on the current environment, but to provide some perspective, an acre of corn transpires about 3,500 gallons of water per day.

    Why do plants transpire if they are losing so much water everyday? Why not close their stomata to prevent water from escaping? There are many reasons why plants have evolved to transpire instead of retaining water.

    1. Transpiration drives the flow of water and dissolved nutrients through the plant. If plants stop releasing water through the stomata, they will stop pulling in the nutrients dissolved in that water essential for plant function.
    2. Transpiration provides evaporative cooling. As water leaves the plant tissues into the atmosphere, it takes energy with it in the form of heat. Much like when we sweat, this allows the plant to cool and maintain homeostasis. This is particularly valuable in hot environments.
    3. Leaving stomata open is required for photosynthesis (except in certain plants, more on this in Lab 10: Photosynthesis). Carbon dioxide enters the plant through the stomata and oxygen is released as a waste product. If the stomata are closed, the plant cannot form sugars.

    → At the beginning of this lab as a class, set up and number 4 potted plants of similar size and type. On each plant, put a bag over one of the branches (or main stem) and tape it closed around the base. Note: the bags should be the same type or the mass of each bag should be recorded beforehand. Water plants 1, 2, and 3 until the soil is saturated. Put plant 1 under a light or next to a sunny window, plant 2 in the dark, and plants 3 and 4 somewhere away from a window but where light is still present. You will check on these later in the lab.

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