196 



A YEAR IX SCIENCE 



Nucleus 



Cell wall- 



complicated structure, the nucleus. Protoplasm is the 

 living part of the cell. These three parts, the cell wall, 

 the protoplasm, and the nucleus, are the cell. 



The simplest form of plant or animal life has only 



one cell. Others have sev- 

 eral cells grouped together, 

 and still others consist of 

 millions of cells. When a 

 plant consists of but a few 

 cells, these cells are very 

 vacuoie similar in structure. How- 

 ever, when millions of cells 



Protoplasm-^ $ are present,- as in trees, 



there is much variation in 

 them. Some of them are 

 of one kind and form the 

 Fig. 75. Diagram of a plant bark of the tree, others 



form the wood, others form 



tubes through which liquids travel, and still others are 

 used to store food. Each group of similar cells is called 

 a tissue. For example, wood is one kind of a tissue; 

 the outer covering of a leaf is another kind of tissue ; 

 and bark is still another tissue. In all cases, while the 

 plant is alive, the tissues are composed of cells which 

 contain the living substance, protoplasm. 



Composition of protoplasm. What is protoplasm? 

 Exactly what is its composition? Why is it living? 

 How does it grow? These and similar questions we 

 can not fully answer. They still remain to us the un- 



Chloroplast 



