16 A TEXTBOOK OF GENERAL BOTANY 
special characters of many different kinds of cells will be evident 
after a study of the different parts of a plant. All living cells 
are alike, however, in their fundamental characteristics. 
Cell walls. Cell walls are important not only because they 
serve as containers of the protoplasm but also because they 
enable the plant to assume a definite shape. If it were not for 
the cell wall the protoplasm, being liquid or semiliquid, would 
simply spread out on the ground, as would any other liquid. 
The heartwood of trees consists largely 
of thickened cell walls of dead cells 
(Fig. 130). Ifa piece of wood is exam- 
ined under a microscope, the walls of 
the individual cells can be readily seen. 
The cell wall is typically composed of 
cellulose, which is a chemical compound 
having the formula (C,H,,O,), and 
belonging to the group of organic com- 
pounds known as carbohydrates. Cellu-. 
lose is hard and colorless, absorbs water 
readily, and is insoluble in water. Cot- 
ton and filter paper are very good exam- 
ples of nearly pure cellulose. Cellulose is well suited to the 
formation of cell walls, because it is rather hard and strong and 
because water can pass through it. Owing to its hardness and 
strength it gives firmness and strength to the cell wall. Since 
water is one of the essential constituents of plant cells, it is of 
great importance that the cell wall should be of some substance 
through which water can pass, so that water can readily enter 
a cell or pass from cell to cell. 
_ Cells that are specialized for certain purposes sometimes 
have other substances in addition to the cellulose in the cell 
wall. Cell walls that are considerably thickened, so as to give 
great strength, frequently contain a substance known as lignin. 
Such cells are said to be “ignified. Wood is composed largely 
of lignified cells. Water passes through lignin very readily, 
so that the addition of lignin strengthens the wall without 

Fig. 11. A section of a 
group of cells from an 
onion root tip. (x 600) 
