Green Leaves at Work 91 



are mingled various accidental substances in vary- 

 ing proportions. For while the plant or animal 

 lives new tissue is always being built up or old 

 and waste tissue is being resolved into its ele- 

 ments and cast out of the body. This unceasing 

 work is accompanied by unceasing changes in the 

 protoplasm, which makes up the bulk of the liv- 

 ing creature, and when death puts an end to these 

 forms of activity decomposition sets in, and the 

 protoplasm begins to change again. So the exact 

 proportions in which six lifeless substances are 

 blended in order to make the " basis of life" can 

 never be accurately known, and the jelly which 

 fills the cells of the summer leaves is one of the 

 great mysteries of the physical world. 



Because they are forever changing protoplasm 

 and its chemical allies are called " proteids." 



When protoplasm, existing alone, or mingled 

 with other substances, is surrounded by a wall, we 

 call the little bag and its contents a "cell." But 

 the living jelly is the chief part of the combina- 

 tion. The wall which encloses it is of secondary 

 importance, and is sometimes dispensed with alto- 

 gether, for the Nature-student makes the ac- 

 quaintance of cells, so called, which are merely 

 little naked masses of protoplasm. 



