[Chap. VII CELLS AS BIOLOGICAL UNITS 



Table 1. Names of Plastids and Their Characteristic Contents 



65 



plasm is denser and forms a more rigid gel, but sometimes it is a fluid 

 sol surrounded by a membrane. Chemically it differs from the cytoplasm 

 in that the proportion of proteins is less and its proteins are far more 

 complex.' They are relatively high in phosphorus and possibly on this 

 account more reactive. There seems to be good reason to regard the 

 nucleus as the center of many cell activities; as we shall see in a later 

 chapter, it contains certain very minute bodies that are the carriers of 

 many of the hereditary factors of the plant. 



The vacuole. At maturity the vacuole occupies most of the space in- 

 side the cell walls; it is filled with "cell sap," a solution of s,ugars, salts, 

 acids, and other soluble compounds. It mav also contain colloidally dis- 

 persed proteins, carbohvdrates, and other less soluble substances. Some 

 of the substances in solution readily pass from the vacuole into the sur- 

 rounding cytoplasm or from the cytoplasm to the vacuole; others do not. 

 Sometimes crystals of salts accumulate in the vacuole, or the cell sap may 

 become colored with pigments as in the cells of many flowers, fruits, 

 and red autumn leaves. 



The cell wall. The outermost part of a plant cell is the cell wall, 

 formed at the surface of the enclosed protoplasm. During cell division 

 the first wall between the daughter cells is composed of pectic material, 

 and on the inside of this pectic wall ( middle lamella ) successive layers 

 composed of cellulose or of cellulose and pectic compounds are de- 

 posited. Sometimes other substances, such as cutin and lignin, accumu- 

 late in cell walls and thereby change their properties. Most cells on the 

 outer surface of plants contain cutin in the outer wall. The cells in wood 



3 f Phosplioric acid 



[ Nucleic acid < + 



Nucleo-proteins \ + [ Sugars 



Protein base 



