Put simply, sap is to plants what blood is to animals. Sap is the fluid that carries water and nutrients around the plant, and without it the vascular plants could not survive, any more than animals could survive without blood. (Vascular plants are those with sap, and sap’s similarity to blood is reflected in the name vascular, since the blood system’s scientific name is the vascular system.)
There are three kinds of sap: xylem sap, cell sap, and phloem sap.
Xylem sap is the sap inside the xylem, which can be compared to the arteries in animals. It is basically a system of vessels or pipes, fibres, and other kinds of cells that carry the sap and its load of minerals up from the roots to the rest of the plant.
Cell sap is the watery fluid inside the cells of the plant. Much of the water in this sap is lost through photosynthesis or transpiration.
Photosynthesis is a process taking place in the leaves. It uses the energy of the sun to turn water and carbon dioxide into glucose, the basic sugar used as a building block of new plant tissues.
Transpiration is an extremely important process in which water evaporates from the leaves into the atmosphere, where it can form clouds and then rain. It is then replaced with more water, drawn up from the roots as sap. Without plants, especially trees, the water trapped deep underground would stay there. The evaporation of water at the leaves is believed to be the main force that draws water up from the roots.
Phloem sap is the fluid carrying the sugars produced in photosynthesis, other foodstuffs such as amino acids to other parts of the plant, where the foodstuffs can be stored for later use or used to build tissues. The direction of phloem sap movement can change, depending on the circumstances, but is generally downwards. In some plants the roots or tubers underground store a great deal of the sugars in phloem as food storage. Unlike xylem, which in trees generally stops being used to transport sap after about a year (leaving the annual rings), phloem is always alive. The best known phloem sap is maple syrup, which is sweet because it contains the sugars produced by photosynthesis.
While sap in plants has been likened to blood in animals, the similarities are limited. Blood circulates for example, whereas sap does not really circulate. Xylem sap moves upwards through the plant from the roots and is very different in its contents to the phloem sap, which generally moves downwards from the leaves, whereas blood in veins is almost identical to blood in arteries except for the amount of oxygen it carries. Even so, plants have sap for the same reasons as animals have blood: to carry nutrients from one part of the organism to another.