THE CELL 21 
there remains in the cells only a watery substance 
with a few yellow bodies. Sometimes this liquid 
turns red, giving the vivid tints to leaves-so noticeable 
in autumn, 
13. The Cell Wall.—The cell wall is chiefly composed 
of a substance called CELLULOSE—a carbohydrate of the 
general formula n(C,H,,0,). It is a substance closely 
allied to starch, having, in fact, the same percentage 
composition, After treatment with sulphuric acid it is 
turned blue by iodine. It is insoluble in water, acids, 
and alkalies. When perfectly pure it is a white sub- 
stance—cotton being nearly pure cellulose. 
Cellulose walls may be changed in nature by the de- 
position of suberin, cutin or wood substance. When 
SUBERIN is deposited the cell walls become elastic, and 
give a bright yellow colour with caustic soda, and a 
yellow colour with chlor-zinc-iodide. The suberin also 
renders the wall less permeable to water. It is met 
with in cork. Curtin is very similar to suberin, and 
is met with in epidermal cells. 
Lignification is caused by the deposition of various 
substances, whose natures have not yet been determined, 
The wall becomes hard, inelastic and permeable to 
water. Lignified membranes stain yellow with chlor- 
zinc-iodide. They are usually met with in wood. 
14. Cell Contents.—Besides protoplasm, a typical cell 
contains many other substances, mostly organic com- 
pounds, but also a few inorganic. The most important 
is the CrLL Sip, a very dilute solution in water of the 
various Inorganic compounds found in soil, such as 
phosphates, sulphates, and nitrates of calcium (see par. 
