The Orientation and Localization of Cortical Microtubules in Differentiating Conifer Tracheids during Cell Expansion

Arrays of cortical microtubules (MTs) on radial walls in differentiating tracheids of Taxus cuspidata were randomly oriented when primary walls formed. The orientation of MTs changed progressively from longitudinal to transverse as cells expanded. During formation of primary walls, MTs in differentiating tracheids disappeared locally at sites of future intertracheal bordered pits. In further differentiated tracheids, circular bands of MTs were observed around the edges of developing bordered pits.