Water Loss in Plants

Leaves contain spongy and palisade cells. Water on the surface of these evaporates and leaves the leaves by diffusion. Water is then drawn out of the xylem cells within the leaves to replace water lost by this evaporation. The xylem cells form continuous tubes down to the roots and, as water is lost from the leaves, more is drawn up from the roots - rather like when you drink through a drinking straw. Transpiration is continuous and so there is a slow but continuous flow of water through the xylem tubes.

You are expected to know the factors that affect transpiration rate for tests and exams. Most of it is just common sense - anything that changes the speed of water loss to evaporation will affect transpiration - changes in temperature, humidity and the speed of the wind blowing over the leaves. One factor that is less obvious is light. When the light is bright, the stomata open more so that more carbon dioxide can enter the leaves to enable more photosynthesis to take place. Because the stomata are open more, more water can escape from the leaf, increasing transpiration.

There are times when plants need to conserve water in order to survive e.g. when the roots are damaged or water is short. In such situations, gardeners can remove some leaves - fewer leaves mean less transpiration and less water loss. Plants can reduce transpiration for themselves by wilting - you may have noticed this happening for yourself on hot days. Thin needle-like leaves or thick waxy coatings on fleshy leaves and stems are other adaptations to reduce transpiration in plants which live in desert conditions or places where the ground water is frozen for long periods of time.