PROTOPLASM AND PLANT-OELLS. 7 



13. How New Cells are Formed. — Most plant-cells in 

 some stage of their growth are capable of producing new 

 cells. This power is moglly 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 into two. 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 



