Transpiration

Most transpiration by plants involves water that evaporates from the moist membranes of a tissue known as spongy mesophyll, which occur in the minute cavities beneath the tiny leaf pores called stomata. Stomata can be closed tightly using bordering cells known as guard cells. However, in most plants stomata are kept open much of the time. This is done so that carbon dioxide, which is needed for photosynthesis, can diffuse into the leaf, and oxygen, a waste product, can diffuse out. Therefore, transpiration by plants can be viewed as an unavoidable consequence of having moist mesophyll membranes exposed to the atmosphere. It is important to understand that, in general, any area of vegetation with a multilayered canopy of foliage will evaporate much larger quantities of water than can an equivalent non-foliated area, such as the surface of a lake or moist soil.

A similar explanation can be developed for animals, who must also maintain moist respiratory surfaces open to the atmosphere, for example in the lungs, from which water can freely evaporate. Transpiration can be an important problem for both plants and animals, because it may be difficult to replenish their water losses in relatively dry environments, so that dehydration can occur. If severe, dehydration can kill plants and animals. In some respects, then, transpiration can be regarded as a necessary evil that organisms must endure in order to exchange oxygen and carbon dioxide with the atmosphere.

However, there can sometimes be important benefits of transpiration. It takes about 540 calories of thermal energy to evaporate 1 gallon of water at an ambient temperature of 68°F (20°C). This is a rather large amount of energy, and as a result transpiration can be an important way by which some animals and plants cool themselves. For example, when humans are hot, they sweat to distribute water onto their exposed skin, which is then cooled by the subsequent evaporation.