30 



ELEMENTS OF BIOLOGY 



type of protoplasm, although it is continuous with the underlying 

 substance. This boundary, known as the plasma membrane, may be, 

 and in many cases is, surrounded by a non-living pellicle or cell 

 WALL, formed by substances constructed by the chemical processes 

 within the cell. 



CELL WALL 



N 

 U 

 C 



u 

 s 



CHROMATIN 

 KARYOSOME 



PLASMA MEMBRANE 



NUCLEAR MEM 

 BRANE 



NUCLEAR SAP 



PLASMOSOME 



LININ 



PLASTID 



GOLGI BODIES 



CENTROSOME 



CENTROSPHERE 



MITOCHONDRIA 



CYTOPLASM 



VACUOLE 



METAPLASM' 



Fig. 4. — Diagram representing the structure of a cell. It must be understood that 

 only the structures visible under the microscope are illustrated; these are compara- 

 tively coarse bodies. The colloidal nature of the seemingly structureless clear ground 

 substance can be demonstrated only by special tests. 



Thus it is seen that the term protoplasm is a word of broad 

 meaning, since there are many types of protoplasm within a single 

 cell. But its significance is still wider, for the protoplasm of each 

 variety of plant and animal has its own peculiarities and the proto- 

 plasm of no two individual organisms is identical. Cells, however, 

 are definitely organized bodies common to all organisms and the 



