Every single part of the human body receives a constant supply of blood which is pumped by the heart. In plants and trees, every single part receives water and nourishment, which we call sap. But a tree has no pump because it has no heart. So how does the sap go up a tree?
Science still cannot explain this mystery exactly. Of course, there are several theories about it, but no single theory seems to offer a complete answer. Scientists believe that there are several forces at work to make this possible.
One explanation has to do with “osmotic pressure”. In living things, liquids and dissolved materials pass through membranes. This is called “osmosis”. When there are dissolved chemicals in contact with a membrane, they press against the membrane. This is called “osmotic pressure”. If there are many particles in a solution, more particles press against the membrane and seep through than in solutions with fewer particles.
Minerals and water used by plants come front the roots. Since the soil contains more minerals than the plant, the osmotic pressure causes the minerals to enter the plant. The dissolved minerals remain in the plant cells. The water evaporates. In this way, water from the soil continuously moves upward through plants.
Another way of explaining how sap goes up a tree has to do with “transpiration” and the cohesion of water. The evaporation of water from leaves is called “transpiration”. The attraction of one water particle to another is called “cohesion”.
Transpiration provides the upward “pull”. As water evaporates from the cells of the leaves, it creates a vacuum in the cells directly below the surface. So these cells draw on the cells below them for a new supply of sap. And this continues right down to the roots of the tree. Cohesion holds the water particles together as they move up.