Observation of topological defects in the xylem of Populus deltoides and implications for the vascular cambium

The mechanism which controls the orientation of cells in the vascular cambi um of trees, and hence the direction of wood grain, is poorly understood. W e argue that disordered grain patterns known as "whirled" grain provide sig nificant constraints on proposed theories of cambial orientation. We presen t observations of whirled grain in knot calluses of Populus deltoides (east ern cottonwood), and argue that point- and line-discontinuities in the grai n direction are evidence for topological defects in the vascular cambium of this species. Topological defects are a common means of classifying patter ns in the physics of partially ordered systems. In this case, the symmetry of the defects implies (1) cambial orientation is controlled by a vector qu antity and (2) the (unspecified) orientation mechanism tends to minimize th e divergence of the grain. These constraints correctly account for the norm al growth habits of cylindrical branch. We also discuss the time evolution of whirled grain. In the knot callus, a high density of defects originate w ithin 1 mm of the surface of the encased branch and thereafter undergo elim ination from the cambium through a process of annihilation. (C) 1999 Academ ic Press.