GROWTH 



139 



enlargement of those already present; and the method by which 

 new cells are formed and added to the plant body deserves care- 

 ful study in a consideration of the growth process. 



The Production of New Cells. — In the previous discussion of 

 the plant cell (Chapter IV) we noted that it consists of a small 

 mass of living substance or protoplasm in which two parts may 

 be distinguished, the undiffer(>ntiated cijtoplasm and the denser 





Fig. 69.— Cell division by mitosis. A, resting cell, the chromatin of the 

 nucleus in a fine network. B, the chromatin is gathered into a long thread. 

 C, this thread breaks up into chromosomes. D, each chromosome splits into two 

 lengthwise. {B, C, and D are called jirophascs). E, metaphase. The split 

 chromosomes arrange themselves in a plane across the equator of the cell, and the 

 spindle, with its two poles, is formed. F, ariapha^ie. The chromosome halves 

 separate, one complete set (eight in this case) going to one pole and the other set 

 to the other pole. G, telophase. Each new group of chromosomes arranges 

 itself into a thread and a new cell wall begins to appear between the groups. 

 H, two complete new cells, each with a nuclear content equal and similar to that 

 of A. 



and more or less spherical nucleus. About the whole is a cellulose 

 wall, deposited by the living substance within much as a clam- 

 shell is deposited by the living clam. In growing tissues where 



