What Are Rays in Wood?

The structure of wood is a subject that is interesting and complicated. There are many different "components" that come together to make wood function as a mechanism to provide support for trees and allow it to move sap throughout the tree. This article will focus on one type of cell that is sometimes visible to the naked eye on lumber: rays.

If you look down at a cut stump, the very center of the tree is where the pith is located.  Growth rings form circles around the pith, with one being created each year (usually). Rays are cells that run perpendicular to the growth rings. As you look down at the stump, the rays would run from the pith toward the bark.

Sap movement in the vertical direction in trees occurs in the xylem (from the base to the top of the tree) and phloem (from the top to the base of the tree). The phloem is found just under the outer bark of the tree.  The xylem is found inside the cambium layer and is what we think of as wood.  Because the sap moving down the tree is in the phloem and contains sugars that provide energy for a growing tree as well as water, growth regulators, and other substances, there needs to be a way to move it horizontally to reach areas such as the cambium layer, where cell division occurs.  This is where the rays come into play.  Rays allow for the movement of sap horizontally through the tree. The rays also serve as areas of storage for sugars that can be utilized by the tree later. 

All wood contains rays. Some are visible with the naked eye, while others are barely visible with a hand lens.  Species such as oak, sycamore, and beech have large rays that may be up to thirty or more cells wide. The feature in these types of species is visible to the naked eye and can be easily seen on any face of the lumber that has been surfaced, as well as the end-grain of the board. 

Ray fleck is the term used to describe rays that can be seen on the surface of a board. The fleck shows up best on the radial surface of a board. In quarter-sawn lumber, the radial surface of the board will be across the width of the board. In a flat-sawn (or plain-sawn) board, the radial surface will be on the edge of the board. Species that have large rays are sometimes intentionally quarter-sawn so that the ray fleck can be highlighted on the face of the board and used to make decorative furniture. 

Species such as maples, cherry, and birch have very small rays, which may only be 1-10 cells in width. These rays can be seen with the help of a hand lens by looking at the end-grain. If they are wide enough, they can also be seen on the radial surface of the boards, but the resulting ray fleck is not as dramatic as the fleck found on species with large rays.

Rays also influence some of the properties of wood. They can help to restrain movement, to a certain extent, in the radial direction when a piece of wood shrinks or swells. It is one of the reasons that wood shrinks less in the radial direction than it does in the tangential direction when it is dried. Rays can also influence the strength properties of the wood. Rays create a radial area of weakness in the lumber. In species with large rays, such as oak, drying the wood often creates small cracks on the surface of the board right beside a ray, called checking.  The larger the rays, the more prone the wood is to surface checking.

Those lines that cut across the growth rings or show up on the surface of the lumber have a vital purpose for trees. They provide a way to store sugars for future use and a way to get the needed sugars from the phloem to areas inside the tree. They can also make for some interesting wood projects.