THE ORIGIN OF MAN 13 



tissues are made up of cells, so that all organisms 

 have a cellular structure and a cellular origin. 

 The simplest organisms are unicellular and the 

 entire life process goes on in such single cells, 

 each a unit mass of living matter; as such all 

 plants and animals begin. In the great majority 

 of organsims, however, the body is made up of 

 hundreds upon hundreds of cells combined into 

 tissues and organs. A cell, therefore is a unit 

 mass of living matter, usually with a nucleus. 



In a general way it may be said that the cells of 

 plants have firm, more or less thick walls enclos- 

 ing protoplasm, with granules of green coloring 

 matter. This stiffness of wall structure isolates 

 the living substance, and consequently independ- 

 ence of action, that is, movement, in plants, as 

 compared with that in animals, is diminished. 



By means of the green coloring matter, plants 

 have the power of producing their own nutritive 

 substances from certain constituents of the air 

 and water and from the salts contained in the 



