THE DIVISION OF THE PROTOPLAST 65 



are equally apportioned to the two daughter nuclei (Fig. 44). As a 

 result, the organization and capacities characteristic of the original 

 nucleus are exactly reproduced in the two new ones: somatic mitosis is 

 eqiiational. From this it follows that the essential organization present 

 in the nucleus of a fertilized egg is reproduced in all the nuclei of the adult 

 soma, for all these result from a succession of equational mitoses. A 

 simple quantitative mass division of the nucleus without respect to its 

 differentiated components would disrupt the system, and normal develop- 

 ment could not continue. In the chapter on meiosis we shall encounter a 

 form of nonequational division, but it is an orderly process of such nature 

 that a complete outfit of materials is still maintained. 



Cytokinesis. — The division of the cytoplasmic portion of the pi-o- 

 toplast is variously correlated in time with mitosis. In some tissues 

 no cytokinesis follows, in others it follows after all signs of recent mitosis 

 have disappeared, whereas in the root meristem and other somatic 

 tissues of higher plants it commonly begins immediately, even before 

 mitosis has been completed. In this last case mitosis and cytokinesis 

 appear like two parts of one process, for the region of the cell in which 

 cytokinesis commences is still occupied by the remains of the mitotic 

 spindle. As a result, c^-tokinesis in these tissues is of a type characterized 

 by the development of a cell plate. Cytokinesis in many other plant 

 cells and in animals is accomplished by constriction or furrowing. 



Studies on living cells, notably those of stamen hairs, show that 

 cytokinesis by cell-plate formation begins as follows. The spindle 

 becomes less prominent near the two early telophase nuclei and widens 

 at the equator into a barrel-shaped figure, the phragmoplast. Some 

 chemical change within it is indicated by the fact that it now stains 

 like the cytoplasm with chrysoidine, a vital dye, whereas during meta- 

 phase and anaphase it did not. Meanwhile, even before widening begins 

 in some instances, small droplets appear near the equator and gradually 

 unite to form a continuous cell plate across the phragmoplast (Figs. 

 45, 46). In some cells the cell plate appears as a continuous film from 

 the start. In fixed material the developing cell plate commonly appears 

 at first like a series of granules or spindle-fiber swellings at the eciuator. 

 The phragmoplast continues to fade away near the nuclei and to widen 

 at the equator, while the cell plate extends at its margins until the latei-al 

 walls of the cell are reached. The remains of the phragmoplast then 

 disappear. 



That the young cell plate is composed of fluid is shown by the fact 

 that upon plasmolysis the two new cells easily round up from each other, 

 leaving fluid but no definite membrane between them. Veiy soon, how- 

 ever, the cell plate undergoes both physical and chemical alterations, 

 and if the two cells are then separated by plasmolysis a firm membi-ane 



