The relationship between cell expansion, cortical microtubule orientation, and patterned secondary-cell-wall deposition was investigated in xylogenic cell suspension cultures ofZinnia elegans L. The direction of cell expansion in these cultures is pH dependent; cells elongate at pH 5.5–6.0, but expand isodiametrically at pH 6.5–7.0. Contrary to our expectations, indirect immunofluorescence revealed that cortical microtubules are oriented parallel to the long axis in elongating cells. Pulse labeling of the walls of isolated cells with the fluorochrome Tinopal LPW demonstrated that xylogenic Zinnia mesophyll cells elongate by tip growth in culture. These results confirm that cortical microtubules in developing tracheary elements reorient before bundling to form transverse cortical microtubule bands. This rearrangement may allow the secondary cell wall pattern to conform to cell shape, independent of the direction in which the cell was expanding prior to reorientation.