In plants and trees, water is sucked up the stem from the roots where it evaporates at the leaves. In travelling through the plant, the water transports the nutrients from the soil that the plant needs to grow. The process whereby water is sucked up is called Transpiration Pull - transpiration meaning breathing.
Transpiration pull only works in plants and trees because their stems (or trunks and branches) contain bundles of many very fine tubes, made of woody material, called xylem.
The transpiration process begins at the surface of the leaves of the plant or tree. Water continually evaporates from the pores of the leaf and is lost into the atmosphere. This evaporation causes million of minutely small droplets of water to form inside the pores. Each tiny droplet is contained within its own skin by the phenomenon of surface tension which means that the molecules of water at the surface of the droplets bind together so they are able to resist outside pressure. It is the same phenomenon that makes it possible for insects to walk across the surface of a pond.
The resulting surface tension from millions of droplets causes a positive pressure within the xylem which in turn pulls water upwards from the roots and soil, against the force of gravity.
Transpiration pull is not the only force acting on plants and trees however. There is also the phenomenon of root push or root pressure. Water can move from one place to another depending on what is known as its water potential - analogkfds;fous in some respects to the voltage of an electric current.
If the water potential of the cells in the root of a plant or tree is more negative than the water potential of the surrounding earth, then water can move from earth into the roots. This in turn creates a pressure on the sap of the plant, up the xylem towards the leaves.
