Observations on Individual Callus Cells of Nicotiana tabacum in Liquid Culture

Abstract

Individual parenchyma cells of tobacco callus that had been shaken apart were followed by time-lapse phase-contrast photomicrography for periods up to 13 days. Approximately 50 cells, along with several cell groups, were cultured in observation chambers which contained 1-3. 5 ml of conditioned culture medium. Such cells had diameters ranging from 50 to 200 [mu] and were highly vacuolate. During the first few hours of observations, the cells adhered to the upper surface of the lower cover glass. Only those cells which adhered to the glass divided. Cells were chosen for photography by the rapidity of their cytoplasmic streaming, since it was found, empirically, that the fastest streaming ones were most likely to divide. All cells observed were asymmetrical in shape and often showed peculiarities of form associated with their former adherence to other cells. All divisions recorded were asymmetrical, that is, one of the derivatives was always larger than the other. Furthermore, divisions occurred in any plane, ranging from at right angles to the cover glass to parallel to it. The derivatives of the first division generally did not enlarge, but those of subsequent ones did and showed a tendency to become spherical. It is assumed that the original wall was of sufficient strength to resist not only changes in turgor, but the normal tendency to enlarge, and that the walls of derivative cells has, necessarily, undergone transformations. In some cases prophases occurred with "fixed tension" of cytoplasm, and there was temporary disappearance of cytoplasmic strands during telophases. In others, no diminution of cytoplasmic streaming or number of cytoplasmic strands occurred during the course of mitosis and cytokinesis. Most new walls were laid down at minimal surfaces and at right angles to the original cell wall; occasional exceptions, however, were noted in the laws of Errera (1888) and of Sachs (1887). No resemblance of the patterns of cell division, starting from a single cell, could be adduced to embryogeny. First, the patterns of division in these isolated callus cells were entirely at random, and the result was always a small callus; and second, the sizes of the cell and its derivatives were always much larger than that which ever occurs in vivo embryogeny.