66 TEXTBOOK OF BOTANY 



exemplify walls that started as pectic compounds and cellulose and 

 became lignified. Cell walls also possess colloidal properties and swell 

 in water. They are composed of small submicroscopic particles between 

 which molecules of water may penetrate and be held tenaciously. 



The cell as a whole. From this description of the parts of the cell and 

 the study of cells with a microscope, it should be possible to picture 

 the cell as a structural unit, and at the same time as a dynamic unit, dis- 

 playing those properties and processes commonly associated with living 

 organisms. Some of these structures and processes may be briefly sum- 

 marized. 



1. All plants consist of one or more cells. 



2. Cells are formed by the division of previously existing cells, and 

 less frequently by the fusion of cells. 



3. The properties associated with life reside in the protoplasm, which 

 is a complex colloidal system of proteins, carbohydrates, fats, and 

 lipoids permeated by a water solution of acids, bases, and salts, 

 many of which are highlv reactive. Owing to the enormous sur- 

 faces between the water and the colloidal particles the proto- 

 plasmic system contains surface energv and electrical energy not 

 characteristic of solutions. 



4. The protoplasm therefore is a chemical and physical system, no 

 one constituent of which is living; the qualities that distinguish 

 protoplasm from non-living svstems result from the unique organi- 

 zation of atoms, ions, molecules, and colloidal particles with their 

 associated chemical, electrical, and surface energies. 



5. The protoplasmic system sustains itself in the presence of food, 

 and can combine food substances into its own structure and 

 thereby increase its mass. In other words, it grows. 



6. In the development of the plant cell, vacuolation occurs and the 

 cytoplasm at maturity becomes a sack-like layer surrounding a 

 solution of sugars, salts, acids, and very dilute colloidal dispersions 

 in the water of the vacuole. 



7. During its development, the usual plant cell also becomes sur- 

 rounded by a cell wall — at first primarily of pectic material, later 

 of cellulose layers — which may be altered in several ways. The 

 cell wall has a high water content during the active life of the 

 cell. 



8. The large proportion of water pervading all parts of the cell con- 



