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, etc. 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 



