The Cellular Basis of Growth 



25 



CELL DIVISION 



Growth of plants and animals, in the last analysis, is an increase in 

 amount of living stuff in them, but this growth is almost always accom- 

 panied by an increase in the number of their cells. This takes place by 

 the process of cell division, which thus assumes much significance for 

 problems of growth and differentiation. The precise method by which 

 new cells are formed was not understood for some time after the cell 

 theory was established. In the seventies of the last century a number 

 of botanists and zoologists, Strasburger prominent among them, made 

 clear the mechanism of mitosis and the leading part played by the 

 nucleus in cell division. 



Division does not take place in all parts of the plant individual. In 

 higher forms it is limited chiefly to apical and lateral meristems and to 



r 





Fig. 3-1. Division of a vacuolate cell showing the development of the phragmosome, 

 which precedes the cell plate. ( From Sinnott and Bloch. ) 



the growing regions of determinate organs, though there may be cell 

 division under certain conditions in other parts of the plant. 



Division is usually studied in small-celled meristematic regions where 

 the cells are not vacuolate or have only small vacuoles. In many cases, 

 however, particularly in the rib meristems of root and shoot and below 

 wounds, cells that are relatively large and in which a vacuole occupies 

 the bulk of the cell may continue to divide. In such cases the nucleus 

 moves from near the wall to a position in the center of the cell, where it 

 is held by strands of cytoplasm. Here it undergoes mitosis. The position 

 where the cell plate, and later the cell wall, will form is usually indi- 

 cated early bv a plate of cytoplasmic strands, the phragmosome, which 

 extends across the cell and in which the nucleus is embedded (Sinnott 

 and Bloch, 1941; Figs. 3-1, 3-2). The cell plate itself is laid down later 

 by the phragmoplast, a group of fibers which are a continuation of the 

 fiber system between the nuclei at telophase. This spreads across the cell, 

 following the course of the phragmosome where the latter is present. In 



