Sap, the watery juice of plants. Sap is mostly water in which minerals and certain foods are dissolved. It is present in most parts of living plants. Sap that occurs in the rounded structures known as vacuoles of ordinary cells is called cell sap. Sap is also present in the cavities of specialized conducting cells in certain plants. Plants with conducting cells are called vascular plants; ferns, conifers, and flowering plants are examples.
The water in sap may come from the soil, and in most vascular plants the water and dissolved minerals are absorbed through the roots. Plants that live in water (water lilies and seaweeds, for example) absorb water directly through their outer layers. Some algae, fungi, and bacteria absorb water directly from the moisture in the air. Epiphytes—plants that grow attached to the surfaces of living or dead trees or other plants, to rock, or even to buildings—also get their water from the moisture of the air. Plants that are parasitic on other plants absorb sap directly from their hosts.
In vascular plants, sap is carried in special conducting tissues called xylem and phloem, besides occurring in the vacuoles and in other parts of the plant. Xylem and phloem usually contain many specialized conducting cells, permitting the rapid movement of sap throughout the plant. For example, the xylem carries water and dissolved minerals absorbed from the soil through the roots and stems to the green leaves. Here the water from the sap is used in the manufacture of food. The phloem carries the food to parts of the plant where it is needed for growth or where it can be stored for future use.
Some commercially important products are made from sap—sugar and molasses from the sap of sugarcane, maple syrup from the sap of the sugar maple, and sorghum syrup from the sap of the sweet sorghum.