Body of Abstract: Grasses develop a shoot-borne root system, which originates from the nodes at the base of the shoot after the primary root system is established. In biophysical analyses, the physical properties of shoot-borne roots, particularly in the basal region where they emerge from the plant, are predicted to have an important role in promoting plant anchorage to the soil. This region features distinct anatomy, characterized by a ring of rigid sclerenchyma tissue formed via lignification and secondary thickening of the first several cortex cell files. We analyzed the development of this tissue layer in soil-grown Brachypodium distachyon plants, revealing that this sclerenchyma layer progressively thickens as the root matures, and therefore is unlikely to facilitate initial penetration into the soil. We further discovered that cortex sclerenchyma is absent in plants grown in agar or hydroponic media, suggesting that its development nonetheless depends on environmental stimuli experienced by roots growing into a solid substrate. To understand the genetic basis for this process, we assayed cortex thickening in plants overexpressing SWAM4, an activator of secondary cell wall thickening in stems, uncovering the fact that wall thickening is driven by common genes in both stem fiber cells and root sclerenchyma cells. Finally, we developed an automated brushing system to reproducibly simulate wind and other mechanical challenges to plant anchorage. Using this system, we found that cortex thickening is strongly induced by tension and compression forces transduced from the shoot to the root crown. Thus we have developed a foundation to characterize a system that governs cortex thickening in roots by employing known above-ground cell wall regulatory genes, but that, unlike in the stem, further depends on environmental factors to control the magnitude and localization of cell wall thickening.