il 
CELL WALL 5 
which gives firmness to the cell and acts as a protection 
to it. Plastids are very frequent constituents of cells 
although large groups of the lower plants, the so-called 
fungi, lack them entirely. Most cells contain spaces 
within the cytoplasm filled with watery solutions. These 
are called vacuoles, and the contained solutions are 
known as cell sap. At its outer surface as well as at the 
surfaces in contact with the larger vacuoles and the 
nucleus the cytoplasm forms a denser layer, free from 
granules, which holds the cytoplasm in shape, prevents 
passage of certain substances into or out of the cyto- 
plasm, ete. This is the plasma membrane. The plasma 
membrane about the nucleus is usually, however, called 
the nuclear membrane. The layer next to the vacuoles 
is frequently spoken of as the tonoplast. 
9. The cell wall consists usually of cellulose or related 
substances, i.e. of some of the more complex carbohy- 
drates. These are composed of carbon, hydrogen and 
oxygen in the proportion, usually, of six parts of carbon, 
ten of hydrogen and five of oxygen. In many of the 
fungi and some other plants the cell wall is composed of 
a form of chitin, containing nitrogen in addition to the 
substances mentioned. This has been called fungus 
cellulose, although not related to cellulose chemically. 
In the walls of older cells there are frequently deposited 
various other substances such as silica in the diatoms 
and in the epidermal cells of joint rushes and grasses, 
suberin and cutin in the walls of cork and epidermal cells, 
respectively, hadromal, or perhaps vanillin and conif- 
erin in wood cells, etc., these being in part the so-called 
“lignin” of earlier botanical works. Aside from cellu- 
lose the chief constituent of cell walls is pectose, chemi- 
cally very similar to it and frequently mixed with it. 
Under the influence of certain not well understood 
