Transpiration pull

Transpiration Pull

Despite having no heart, plants move water upward through the xylem at surprisingly high rates — up to roughly 15 metres per hour. For a long time it was debated whether this water is "pushed" from below or "pulled" from above; the consensus now is that water is mainly pulled, with transpiration from the leaves supplying the driving force. This whole explanation is known as the cohesion–tension–transpiration pull model of water transport.

Water passing through a plant is largely transient — less than about 1% of the water reaching the leaves is actually used in photosynthesis and growth. The overwhelming majority is instead lost through the leaf stomata, a loss called transpiration. This loss can itself be demonstrated simply, for example by enclosing a healthy plant in a sealed polythene bag and watching water droplets condense on the inside, or by using cobalt chloride paper, which visibly changes colour on absorbing water, pressed against a leaf surface. The tra …