THE CELL 15 



very small bodies known as the nucleoli (singular, nuceolus). 

 The most important substance in the nucleus is the chromatin, 

 which will be described later (see page 129). The nucleus is 

 believed by many to exert a controlling influence over the activ- 

 ities of the cell. If the cell is divided into two parts, one with 

 and one without the nucleus, the former can produce a new wall, 

 while the latter is unable to do so and soon dies. The nucleus 

 is also believed to cause the oxidation within the cell and to be the 

 most important factor in heredity (see Chapter X). 



Cell Wall. — The outside covering of the protoplasm is known 

 as the cell wall. It is the product of the protoplasm, but is not 

 living material. It may be considered the skeleton of the cell. 

 It is frequently perforated with very minute pores for the pas- 

 sage of very delicate threads of cytoplasm which connect the 

 protoplasmic contents of neighboring cells. When young, and 

 sometimes throughout the entire life of the cell, the wall is 

 composed of cellulose (CeHioOs)^, which expands with the 

 growth of the cell. In most cases, it becomes modified into' 

 lignin (in bast and wood cells), cutin (in epidermal cells), and 

 suberin (in cork cells). These modifications are due to the im- 

 pregnation of the wall with other compounds and may serve 

 many purposes ; they may give strength, as in the bast and wood 

 cells, or may serve for protection against water and other factors, 

 as in the ease of the cutin covering of leavesi and the snberin of 

 the bark cells. Soflietimes parts of the cell wall become gelat- 

 inous and capable of absorbing great quantities of water. This 

 is very characteristic of the seed coats of certain seeds. In some 

 cases, such as the date, the cellulose serves for plant food in the 

 germination of the seeds. A knowledge of the structure and 

 chemical character of plant materials is of very great value in 



