Sap is the moisture in wood and all material it holds in solution. The amount and kind of materials dissolved in the water of the sap vary with the species, part of tree, and time of year, but constitute only small amount of the sap. In drying lumber it is principally the moisture that evaporates, leaving the soluble materials behind, although some acid and volatile oils also pass off in the form of vapor. The soluble materials in the sapwood are mostly sugar and a very small amount of mineral matter, while in the heartwood they are tannins, resin, dyestuffs, and other materials.