The mechanism which controls the orientation of cells in the vascular cambium of trees, and hence the direction of wood grain, is poorly understood. We argue that disordered grain patterns known as "whirled" grain provide significant constraints on proposed theories of cambial orientation. We present observations of whirled grain in knot calluses of Populus deltoides (eastern cottonwood), and argue that point- and line-discontinuities in the grain direction are evidence for topological defects in the vascular cambium of this species. Topological defects are a common means of classifying patterns in the physics of partially ordered systems. In this case, the symmetry of the defects implies (1) cambial orientation is controlled by a vector quantity and (2) the (unspecified) orientation mechanism tends to minimize the divergence of the grain. These constraints correctly account for the normal growth habits of cylindrical branch. We also discuss the time evolution of whirled grain. In the knot callus, a high density of defects originate within 1mm of the surface of the encased branch and thereafter undergo elimination from the cambium through a process of annihilation. Copyright 1999 Academic Press.