Mechanisms of water movement from soil through plants to the atmosphere.

The higher water potential of soil water compared to the water potential of roots induces water to flow from the soil into plant roots. As water enters roots from the surrounding soil, it joins a column of water that extends from the roots through the water-conducting cells, or xylem, of the stem to the leaves. Hydrogen bonds between adjacent water molecules bind the water molecules in this water column together. Consequently, as water molecules at the upper end of this column evaporate into the air at the surfaces of leaves, they exert a tension, or negative pressure, on the entire water column. This negative pressure further reduces the water potential of plant fluids and helps power uptake of water by terrestrial plants.