Transpiration

Transpiration is the loss of water in the form of water vapour from the aerial parts of a plant, chiefly through the stomata of the leaves. Roots absorb far more water than the plant needs; only a tiny fraction is used in photosynthesis and to keep cells turgid. The rest, often more than ninety per cent, escapes by transpiration. It happens in three ways: stomatal transpiration through the stomata (by far the largest), cuticular transpiration through the waxy cuticle of the leaf, and lenticular transpiration through the lenticels of woody stems. Although it looks like simple water loss, it is the engine that drives water all the way up a tree.

Each stoma is a tiny pore bordered by two bean-shaped guard cells. The inner wall of each guard cell (facing the pore) is thick and the outer wall is thin. When water enters the guard cells they become turgid; the thin outer walls stretch more than the thick inner walls, so the cells bow apart and the pore opens. Water vapour then diffuses out into the air. When the guard cells lose water they become flaccid, straighten and close the pore. This is why stomata are usually open in the day, when photosynthesis makes the guard cells take up water, and closed at night, reducing water loss.

The rate of transpiration is controlled by both external conditions and the leaf itself. Bright light opens the stomata and raises the rate. High temperature increases evaporation and speeds it up. High humidity in the surrounding air slows it down, because diffusion stops when the air is already saturated. Moving air (wind) sweeps away the moist layer next to the leaf and increases the rate. Plenty of water in the soil keeps the rate high, while dry soil makes guard cells flaccid and slows it. Larger leaf area and more open stomata also raise the rate.

Water absorbed by the roots travels up through the xylem to the leaves; this upward movement is the ascent of sap. The chief force behind it is the transpiration pull. As water evaporates from the leaf cells, it creates a suction (tension) that pulls the continuous column of water upward. The water column does not break because water molecules cling to one another (cohesion) and to the xylem walls (adhesion) - the cohesion–tension theory. Transpiration is sometimes called a necessary evil: it wastes a great deal of water, yet it pulls up water and minerals, cools the leaf and keeps cells turgid.