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- 
lized 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 higher 
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 in contact with the fibers, known as spindle fibers, which 
seem to be special provisions of the cytoplasm for bringing about 
