A woody plant is any vascular plant that has a perennial stem that is above ground and covered by a layer of thickened bark - that is, the stem supports continued vegetative growth above ground from year to year.
A perennial plant with true woody stems contain wood, which is primarily composed of structures of cellulose and lignin which provide support and a vascular system used to move water and nutrients from the roots to the leaves and sugars from the leaves to the rest of the plant. Most woody plants form new layers of woody tissue each year, and so increase their stem diameter from year to year. The new wood is deposited on the outer parts of the stem under the bark on most plants but in some monocotyledons such as palms and dracaenas, the wood is formed in bundles from meristem cells within the trunk. In palms, the wood is formed in the center of the stems. The dermal layer has been modified to protect the stems from the elements with a thickened covering of dead tissue, generally called bark.
Some annual plants appear to form woody stems in their first year, but die at the end of the growing season. They are herbaceous stems without the dead bark covering.
Woody herbs are herbaceous plants that do not have the thickened bark covering, but develop hard stems with vascular bundles. They include such plants as Uraria picta and certain species in Family Polygonaceae. These herbs are not truly woody but have hard densely packed stem tissue. Other herbaceous plants have woody stems called a caudex, which is a thickened stem base often found in plants that grow in alpine or dry environments. In Tree Ferns the caudex is the stout woody trunk.