From swirl to defect in wood grain.

When physicist Eric M. Kramer looked at the complex spirals and whorls sometimes visible in wood just under a tree's bark, he was reminded of patterns that can occur in liquid crystals or magnetic systems. Now at Simon's Rock College in Great Barrington, Mass., Kramer has applied theory developed for understanding inorganic pattern formation to the problem of tree growth. "Observations of wood grain and tree growth can provide important clues to growth processes taking place at the cellular level," he contends. Kramer describes his approach in the September JOURNAL OF THEORETICAL BIOLOGY.

The vascular cambium is the layer of cells just beneath a tree's bark. As the tree grows, these cells divide and the cambium layer moves outward. The daughter cells left behind turn into xylem, or wood. The xylem structures are typically aligned parallel to the axis of the trunk or tree branch, forming a pattern known as straight grain. More complicated arrangements can also occur, including a chaotic structure described as whirled grain.

Kramer focused on whirled-grain patterns seen in knot calluses of eastern cottonwood trees. Such sharp discontinuities in grain direction look like defects in a partially ordered liquid-crystal system, Kramer says. He categorized the types of defects that can occur. To clarify how defects form and how they are eventually eliminated from the vascular cambium, Kramer's next step is to study how a whirled-grain pattern changes as a tree grows. The ultimate goal of this research is a thorough understanding of the factors governing the direction of wood grain, Kramer says.