ANATOMY OF THE CELL. FF. 
By far the greater portion of solid matter comprising the 
cell wall consists of cellulose, a substance of the same formula 
as starch, C,H,,O;. It resists the action of most solvents, 
yielding only to concentrated sulphuric acid, to certain ferments, 
and to copper oxide of ammonia. When treated with iodine 
and followed by weak sulphuric acid it turns blue, and with 
chlor-iodide of zinc it turns violet. The young walls of all 
plants consist largely of pure cellulose. With age, however, 
certain modifications take place in the cellulose of some cells 
which are designed for special functions ; the principal changes 
are the following: Lignification; Suberization; Partial or 
complete conversion into mucilage, Gelatination. 
3. Lignification of Wall. 
Lignin, or the woody portion of plants, consists of the same 
elements as cellulose, but with carbon largely predominating. 
The process by which cellulose is changed to lgnin is not 
known. ‘There are some grounds for supposing that lignin is 
a substance formed by the activity of the living protoplasm of 
the wood cell, and deposited among the cellulose micellae as an 
incrustation, which works a chemical change in them. In the 
walls of mature wood cells there are generally found three 
layers, primary, secondary, and tertiary; the primary lying next 
to the middle lamella, and the tertiary being in contact with 
the protoplasm. The amount of lignin in these layers varies, 
the primary generally containing more than the other two. 
4. Suberization and Cutinization of Wall. 
The walls of those cells which are designed to protect the 
plant from outward injury, such as too rapid evaporation and 
extremes of heat and cold, undergo a change by which the 
cellulose becomes suberin and cutin. These substances are 
very similar in their chemical nature, and are both very rich in 
