The top of a tree may be many metres from the ground. Yet the leaves at the top need water just as much as the lower branches. So how do they get the water they need?
Water loss from the leaves
The stomata on the surface of plant leaves can be opened and closed by the guard cells that surround them. Plants open their stomata to take in carbon dioxide for photosynthesis. However, when the stomata are open, plants lose water vapour through them as well. This loss of water vapour is called evaporation.
As water evaporates from the surface of the leaves, more water is pulled up through the xylem to take its place. This constant movement of water molecules through the xylem from the roots to the leaves is known as the transpiration stream. It is driven by the evaporation of water from the leaves. So anything that affects the rate of evaporation will affect transpiration.
The effect of the environment on transpiration