Abstract text: The vascular cambium is the least studied meristem of plants because it is located deep within the tissues. For the same reason, intrusive growth is the least studied type of plant cell growth. Yet, cambium activity and intrusive growth are responsible for the formation of the largest carbon sink on Earth, wood, and determine its structure, properties, and performance. Flax can serve as a model for studying wood formation because the developmental principles and responses of secondary xylem to stimuli are similar to those of tree species. We analyzed the flax variety CDC Bethune and an rdf mutant with reduced length of secondary xylem fibers derived from it. Three distinct biomechanical zones of fibers were identified: tips, extensions, and bodies. The extensions were found to be the most actively growing part. Fiber cell walls in the rdf mutant were stiffer than those in the control, which is consistent with a more chaotic orientation of cell wall macrofibrils. These stiffer cell walls were likely the reason for the reduction in fiber length. Computational modeling showed that the initiation of intrusive growth depends on cell wall properties and the breakability of intercellular contacts rather than increased turgor pressure.