Transpiration is the loss of water from a plant by evaporation, which then diffuses out of the leaf via the stomata. It is important to note whether the student understands how and why water leaves the plant. The leaves of a plant are adapted for efficient photosynthesis, meaning they have a large surface area to absorb more sunlight. It is this energy obtained from the sunlight that changes water in leaves from a liquid state to a vapour state. Most of the water evaporation occurs in the spongy mesophyll cells. So, as water is lost, water from the leaf xylem is being transported to the cells via osmosis. Therefore, the transpiration stream is the flow of water (in an upward movement) through the xylem from roots to leaves.
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