i8 



General Botany 



Fig. 7. Pari of a moss leal that 



Portions of the cytoplasm may 

 be organized into definite struc- 

 tures called plastids, in which food 

 substances or coloring matters ac- 

 cumulate. Starch is formed in plas- 

 tids, as is also the green pigment of 

 leaves, and some plastids accumulate 

 fats and oils (Fig. 7). 



The nucleus is a small, round body 

 of greater density than the cyto- 

 plasm. It occupies the center of the 

 young cell, but as the vacuole en- 

 larges it is carried with the cyto- 



composed of a single layer of cells. .^^ ^j^^^ ^^ ^j^^ ^^Ij ^^jj jj^^ 

 The dark bodies shown m the cells ^ i • • 



are the plastids which contain the UUclcUS SCCmS tO be the Startmg pomt 



green coloring matter. q£ many of the activities of the cell. 



It is beheved to control and determine the course of develop- 

 ment of the cell and the formation of new cells in growth and 

 reproduction. 



Vacuoles. The cell sap inclosed by the cytoplasm is a small 

 drop of water containing sugars, salts, acids, and other soluble 

 substances. As will be seen later, the cell sap influences many 

 of the cell processes, especially those concerned with absorption 

 and growth. The vacuole is the reservoir of the cell into which 

 dissolved substances may pass from the cytoplasm and from which 

 substances may again move into the cytoplasm as they are being 

 utilized in cell activities. 



The cell wall. The walls of plant cells are composed of non- 

 living materials deposited by the cytoplasm. They support the 

 soft protoplasm of the cells in somewhat the same way that the 

 wax of the honeycomb supports the honey. They also give 

 stiffness to all parts of the plants. Most cell walls are composed 

 in part of cellulose, a substance closely related to starch and sugar. 

 You have seen pure cellulose in the form of cotton fiber. Filter 



