Water loss, whether due to transpiration or perspiration, requires a lot of heat to break the hydrogen bonds between water molecules in the liquid state. This, in turn, means that a lot of heat is lost with evaporation, cooling the organism from which the water evaporates.
Work Step by Step
Review Ch. 3 and think about how sweating makes you feel— cooler when too hot. Transpiration has the same effect for plants for the same reasons of physics and chemistry.