Microtubules have long been known to play a key role in plant cell morphogenesis, but just how they fulfill this function is unclear. Transverse microtubules have been thought to constrain the movement of cellulose synthase complexes in order to generate transverse microfibrils that are essential for elongation growth. Surprisingly, some recent studies demonstrate that organized cortical microtubules are not essential for maintaining or re-establishing transversely oriented cellulose microfibrils in expanding cells. At the same time, however, there is strong evidence that microtubules are intimately associated with cellulose synthesis activity, especially during secondary wall deposition. These apparently conflicting results provide important clues as to what microtubules do at the interface between the cell and its wall. I hypothesize that cellulose microfibril length is an important parameter of wall mechanics and suggest ways in which microtubule organization may influence microfibril length. This concept is in line with current evidence that links cellulose synthesis levels and microfibril orientation. Furthermore, in light of new evidence showing that a wide variety of proteins bind to microtubules, I raise the broader question of whether a major function of plant microtubules is in modulating signaling pathways as plants respond to sensory inputs from the environment.