Immunofluorescence studies were carried out on tissue explants of Nicotiana tabacum embedded in polyethylene glycol (Hawes et al. 1983). The orientation of cortical microtubules with respect to the cell axis gradually changed from transverse to longitudinal during the first 6 h of culturing. These changes might occur by assembly of tubulins at" + "-ends of microtubules in new directions, rather than by reorientation of complete Mt-helices (Roberts etal. 1985). Before cell division preprophase bands of microtubules were formed parallel to the plane of the cell wall to be formed. Cells of the protrusions, from where floral buds will develop (Wilms & Sassen 1987), had randomly oriented microtubules. The initiation of protrusion formation occurred between 24 and 48 h after tissue explantation.