The Cellular Basis of Growth 27 



( 1937), studying the distribution of mitoses in root tips, found that these 

 occurred not only in the small cells at the apex but in the progressively 

 larger ones back from this until division ceased. Incidentally, he reported 

 that in many cases dividing cells are not evenly distributed through the 

 meristematic region but tend to occur in several waves, moving backward 

 from the tip (Fig. 3-3). 



In developing cucurbit ovaries during the period of cell division there is 

 a progressive increase in the size of the dividing cells in each region 

 (epidermis to placental region), and this increase is greater in successive 

 tissues from the epidermis inward ( Sinnott, 1939; Fig. 3-4 ) . The daughter 

 cells from a division must therefore increase to a size somewhat greater 

 than that at which their mother cell divided before they themselves di- 

 vide again. The largest cells to divide were many times the volume of 

 the smallest ones. 



Fig. 3-3. Changes in cell length in microns (lower line) and frequency of mitoses 

 (upper line) at successive distances from the root tip (at left) in periblem of onion 

 root. ( From Wagner. ) 



Another factor in division, emphasized especially by zoologists, is the 

 ratio of nucleus to cytoplasm (the nucleoplasmic ratio). R. Hertwig 

 ( 1908 ) believed that, as a cell grows, the cytoplasm increases faster than 

 the nucleus so that a tension is set up which is finally relieved by the 

 division of the cell. This restores the equilibrium of nucleus and cyto- 

 plasm, since presumably the size of the nucleus is at once restored. Popoff 

 ( 1908 ) was able to remove some of the cytoplasm from certain cells by 

 micropipette and found that these divided more slowly than their un- 

 treated sister cells, as one would expect on Hertwig's theory. There is 

 little evidence from meristematic plant cells that a changing nucleo- 

 plasmic ratio is significant in cell division, though perhaps it may be. In 

 larger and vacuolate plant cells it is difficult to measure the cytoplasm 

 since it is distributed in a thin layer lining the wall. The cytonuclear 

 ratio (volume of cell to volume of nucleus), however, can be de- 

 termined. In vacuolate cells of progressively larger size the volume of 



