Water vapor rises by convection, and having the stomata facing downward minimizes water loss when the stomata are open. Except for stomata, the rest of the leaf's surface is covered with a cuticle that is nearly impermeable to water. The cuticle minimizes water loss from surfaces that are not employed in CO 2 uptake. We generally think of water loss as being deleterious to plant survival. However, transpiration can also be beneficial to plants that live in hot climates.
Transpiration requires that liquid water evaporate, which has a cooling effect.
Transpiration cools the mesophyll of the leaves, and in hot climates this cooling effect helps to protect the leaf from being damaged by the heat. Return to BIO214 Homepage .