Types of Transpiration

Water vapour can leave a plant by three anatomically distinct routes, and these three differ enormously in how much of the total water loss each one actually accounts for. Stomatal transpiration, occurring through the numerous microscopic pores concentrated on the lower epidermis of leaves, is overwhelmingly the dominant route and is responsible for roughly 90-95% of a plant's total water loss, because stomata are both extremely numerous and actively regulated by guard-cell turgor changes. Lenticular transpiration occurs through lenticels - small, raised, lens-shaped pores that develop in the periderm (bark) which replaces the epidermis of woody stems and trees once secondary growth has taken place, letting gas exchange continue after the original epidermis is gone - but this route is functionally minor, contributing only about 0.1% of total transpiration. Cuticular transpiration passes directly through the waxy or resinous cuticle covering the epidermis of leaves and other aerial organs, usually accounting for about 5-10% of the total, though this fraction is reduced still further in xerophytes, whose cuticle thickens specifically to cut down this loss. Physiologically, all three routes trace back to the same underlying driver: evaporation of water from mesophyll cells into the intercellular spaces lowers those cells' water potential, so water moves cell to cel …