3 Why transpiration matters (two key roles from the book)

13 3.3 Why transpiration matters (two key roles from the book)

Cooling the plant. Evaporation of water absorbs a large amount of heat (the latent heat of vaporization of water is high because of hydrogen bonds — Module 3). The leaf therefore loses heat and its temperature falls. This protects leaf tissue and enzymes from overheating, just as sweating cools a person.
The "transpiration pull" that lifts water. Water evaporating from the leaf cells is replaced by water pulled from the leaf's xylem vessels. Because water molecules stick together (cohesion, through hydrogen bonds) and to the vessel walls (adhesion), a continuous column of water is pulled upward from the roots through the xylem tissue to the upper parts of the plant, carrying dissolved minerals with it. This "pulling force" is a suction created by evaporation at the top; it can lift water to the tops of trees tens of metres tall without any pump.

Additional consequences (consistent with the book and your later lessons): transpiration helps bring minerals to the leaves, maintains the water content of cells, and contributes a large amount of vapour to the atmosphere.
Over land, particularly forests, transpiration accounts for a large part of the vapour that returns to the air.
When forests are cut (deforestation), local rainfall often falls, which is one reason why plants are described as a part of the water cycle.