PROTOPLASM AND PLANT.-CELLS. vA 
13. How New Cells are Formed.—Most plant-cells in 
some stage of their growth are capable of producing new 
cells. This power is mostly confined to their early thin- 
walled state, new cells being rarely formed after the walls 
have attained any considerable thickness. There are two 
general methods, viz., (1) by the Division of cells, (2) by 
the Union of cells. 
14. In the Division of a cell it may simply constrict its 
sides so as to pinch itself into two parts. In other cases 
the protoplasm first divides itself through the middle, and 
the two halves then help to form a partition-wall of cellu- 
lose between them. Both of these modes of division are 
known as Fission. 
15. In some cases of Division the protoplasm divides 
itself into two, four, or many parts, which then become 
spherical. in shape. Each part then covers itself with a 
cell-wall of its own; and the old cell-wall of the original 
cell, not being of further use, soon decays or breaks away. 
This kind of Division is known as Internal Cell-formation. 
16. Cell-division always results in an increase in the 
number of cells, and is the usual process by which plants 
are increased in size, and in the number of their cells. 
Growth may be very rapid, even where the cells simply 
divide successively intotwo. Thus a single cell may give 
rise in its first division to two cells, next to four, then eight, 
then sixteen, thirty-two, sixty-four, etc. etc. By the twen- 
tieth division the cells would exceed a million in number, 
17. The process of cell-formation by Union is exactly 
opposite to that by Division. Two cells which were sepa- 
rate unite their protoplasm into one mass, which then 
forms a cell-wall around itself. Thus instead of doubling 
the number of cells at every step, there is here an actual 
