Understanding what makes up the structure of wood is much easier if you have words to name the features that we can see. This glossary explains the words and their meanings.

Annual Ring: seasonal growth that is highly visible in RING POROUS hardwoods, especially oak and ash.

Bark: the skin of the tree outside the CAMBIUM; divided into living inner bark and dead protective outer bark.

Cambium: a layer of tissue that is the source of cells that grow and divide to form the wood and bark of a tree.

Cell: the basic structural unit of wood, consisting of an outer wall surrounding a central cavity.

Diffuse Porous: a type of hardwood tree that forms vessels of roughly the same diameter throughout the growing season.

Extractives: complex chemical substances that form during the transformation of SAPWOOD cells into HEARTWOOD cells; they darken the wood and make it less porous.

Fiber: hardwood cells formed in the latter part of the growing season in ring porous wood that gives the tree strength; characterized by thick walls and a small cavity.

Fusiform Initials: mother cells in the CAMBIUM that grow and divide to form vessels, fiber and parenchyma in hardwoods and a variety of tracheids in softwoods. Growth Ring: increment of wood added during a single growth period. In temperate regions, the growth period is usually one year, in which case the growth ring may be called an ANNUAL RING. In tropical woods, growth rings may not be discernible or are not annual.

Hardwood: timber from broad-leaved trees; designation does not necessarily reflect the hardness of the wood.

Heartwood: the older, nonliving central wood of a tree, usually darker and harder than the younger SAPWOOD.

Inclusions: any material that plugs a vessel, notably gum, resin or tyloses.

Parenchyma: cells mainly involved with food storage and distribution; light-colored tissue when viewed through a hand lens; distributed in a variety of very specific patterns in certain species, making it a good recognition factor.

Pith: the soft, spongy central cylinder of tissue in the trunk, branches and twigs about which the first growth takes place.

Pits: a thinning in the side wall of the cell that allows water to flow from TRACHEID to tracheid.

Photosynthesis: the synthesis of complex organic materials needed as food from carbon dioxide, water and inorganic salts, using sunlight as the source of energy, aided by chlorophyll.

Rays: plate-like thin layers of tissue that extend out radially; serves for food storage and conduction.

Ray Initials: mother cells in the CAMBIUM that grow and divide to make rays in both hardwoods and softwoods.

Reaction Wood: abnormal wood that comes from a tree with a severe off-center PITH, usually caused by severe curvature or leaning.

Ring Porous: type of hardwood tree in which vessels formed at the beginning of the growing season are much larger than vessels laid down at the end of the season.

Sap: fluid that carries nutrients and water to various parts of the tree.

Sapwood: younger, softer, living outer portion of wood that lies between the CAMBIUM and the HEARTWOOD; less durable, and usually lighter in color than the heartwood

Silver Grain: the emergence of ray tissue on the surface of a board.

Softwood: timber taken from a needle-bearing tree; designation does not necessarily reflect the softness of the wood.

Tracheids: elongated cells that serve for support and upward conduction of sap. See PITS.

Tyloses: film-like material found in the heartwood vessels of some hardwoods; forms regularly in white oak.

Vessels: specialized tubular structures in hardwoods for conducting sap upward.