an interesting tidbit from my biology textbook
We are starting with plant physiology this quarter & this was in my text:
"... transpiration has an added benefit of cooling a plant's leaves. The evaporation of water from mesophyll cells consumes heat, thereby decreasing the leaf temperature. [One] can hold a leaf between thumb & forefinger to estimate its temperature; if the leaf doesn't feel cool, that means that transpiration is not occurring and it must be time to water."
Also of interest: the authors say that a 15m maple tree, with an estimated 177,000 leaves (= a surface area of 675 sq meters or 1.5 basketball courts) loses, on a hot summer day, 220 liters of water PER HOUR via evaporation from the leaves!