124 CELLS AND TISSUES 



is the rule. Consequently, as a cell grows, a size is soon attained 

 at which division must occur. By division the cell becomes two 

 cells of half the parent size, and each of the new cells has all of the 

 structures of the parent cell and the ability to repeat the proc- 

 esses of growth and division. 



It is by the growth, division, and differentiation of cells that 

 both plants and animals become adult individuals. In the ferti- 

 Hzed egg, the first stage of an individual's existence, cell division 

 begins usually in a few hours after fertilization and continues 

 throughout the life of the plant, although interrupted at various 

 times. Although the cell divisions are countless in number in the 

 higher plants, they all proceed in the same way throughout the 

 plant, except in the anther and ovary where a peculiar type of 

 division to be discussed later occurs. 



In some simple plants, as Bacteria and the Yeast Plant where 

 cell division is of a simple type, the processes of division may oc- 

 cupy only a few minutes, but in the higher plants where cell divi- 

 sion is more complex, the processes of division often require two 

 or more hours, and so far as we know the processes are continuous 

 throughout the entire period. Most of this time is occupied by 

 the division of the chromatin about which cell division centers. 



Although cell division consists of a continuous series of events, 

 a few stages in the process, as shown in Figure 110, will suffice to 

 give an understanding of cell division as it occurs in the liigher 

 plants. Thus starting with the chromatin in a granular condi- 

 tion and scattered through the nucleus, the first step in division 

 is the organization of this chromatin into a thread which then is 

 segmented into segments known as chromosomes. The number 

 of chromosomes into which the thread segments is definite for 

 each plant or animal, although varying much in different species, 

 ranging from two in some worms to more than one hundred in 

 some Ferns. However, in many of our common plants and ani- 

 mals the number ranges from sixteen to forty-eight. In man 

 there are forty-six or forty-eight, in Tomatoes twenty-four, and 

 in Wheat sixteen. The chromosomes, which have no definite 

 arrangement when first formed, soon arrange themselves in a 

 plane across the cell. As they assume this arrangement, the 

 nuclear membrane disappears, thus allowing the chromosomes to 

 come m contact with the fibers, known as spindle fibers, which 

 seem to be special provisions of the cytoplasm for bringing about 



