Studies of monoplastidic mitosis in hornworts (Bryophyta) using transmission electron microscopy and indirect immunofluorescence staining of microtubules have revealed that two mutually perpendicular microtubule systems predict division polarity in preprophase. Events of cytoplasmic reorganization in preparation for division occur in the following order: migration of the single plastid to a position perpendicular to the division site, constriction of the plastid where its midpoint intersects the division site, development of an axial system of microtubules parallel to the elongating plastid isthmus, and appearance of an atypical preprophase band of microtubules (PPB). The PPB is asymmetrical with a tight band of microtubules on the side over the plastid isthmus and a broad band of widely spaced microtubules over the nucleus. The axial system contributes directly to development of the spindle. In prometaphase, the axial system separates at the equator and additional microtubule bundles project from polar regions, creating two opposing halfspindles. The PPB is still present during asymmetrical organization of the spindle and microtubules extending from the broad portion of the PPB to poles appear to be incorporated into the developing spindle. Dynamic changes in the microtubular cytoskeleton demonstrate (1) intimate relationship of plastid and nuclear division, (2) contribution of preprophase/prophase microtubule systems to spindle development in monoplastidic cells, and (3) dynamic reorientation of microtubules from one system to another.