What makes sap go up a tree?

One theory ascribes it to osmotic pressure. Liquids and dissolved substances pass through membranes in living things. This is called osmosis. Dissolved chemicals in contact with a membrane press against it. This is osmotic pressure. Yet another holds that it is because of transpiration and the cohesion of water. Evaporation of water from leaves is transpiration. The attraction between water particles is cohesion.