Sap in plants is a mixture of water and dissolved mineral salts contained in the xylem vessels. When we speak of sap, we often think of a sugary liquid but in plants, the sugary solution in the phloem does not rise in plants but is created inthe leaves and travels from the site of production to other parts of plants, so when we speak of the ascent of sap, we mean the water and salts of the xylem vessels.
How sap rises is part of theprocess known as the transpiration stream. This is a complex system of water control in the plant. It starts with the root hairs taking in water from the soil. Water is held on to soil particles tightly by a force called capillary force and in some cases, water is taken in against considerable resistance. It enters the root hairs by a process called osmosis which is a special kind of diffusion where water andother selected substances only pass through a semi-permeable membrane into an area of less concentration. Water passes from cell to cell across the cortex of the root. It does this because as a cell loses water, a deficit occurs and so it draws water from the next cell. When it reaches the special area called the Caspian strip, water can only go one way ( until now it can go either way) and from here, it travels into the xylem vessels, which are long, dead cells of lignified tubes.
As water enters the plant, it creates root pressure. This force can push water up to a height of around 30 feet. At the other end of the plant, water vapour is lost to the atmosphere by transpiration where it evaporates from inter cellular spaces , into special areas called stomata and from there to the atmosphere. The openings of the stomata are controlled by two crescent shaped guard cells and these, unlike most cells of the lower epidermis where most stomata are found, contain large numbers of chloroplasts, which photosynthesis and make sugars. These create a concentrated solution in the guard cells to during light, they absorb water from adjacent cells, swell and take on a crescent shape, the two cells creating a pore in their middle - this is the stoma through which water vapour escapes.
This evaporation of water vapour inthe atmosphere creates a suction force and this lllleaf suction can puill water up to around 30 feet.
For trees with a total height of over 60 feet other special forces come into play. These are adhesion where the water molecules are attracted to the walls of the xylem vessel and cohesion where they are attracted to each other (water molecules have a slight charge). This means that so long as the column of water remains intact the column will stay cohesive and water can reach the uppermost leaves of even the tallest trees.
SO, the rising of the sap if something complex and yet so natural and taken for granted. In spring, some trees pull water up so fast that you can actually hear it rising if you listen to the trunk. What amazing things plants are!