LIVING SUBSTANCE 19 



the same with the leaf itself we would find at once 

 that the leaf bears to its waxen counterfeit the same 

 relation that a mosaic figure does to a photograph. 

 Instead of being homogeneous in structure it is 

 composed of innumerable little units, the aggre- 

 gate of which makes up the mass of the leaf. 



These structural units of organization of proto- 

 plasm are called cells. The word owes its deriva- 



FIG. 3. Section of a leaf, showing its cellular composition : a, a 

 breathing pore or stoma ; b, upper layer of " palisade cells " which con- 

 tain most of the chlorophyll ; c, epidermal cell. (Bailey.) 



tion to the fact that the discoverer of the first cells 

 described found, in examining a thin slice of cork, 

 that the cork was made up of little boxes like 

 the cells of the honeycomb. Similar observations 

 were made later on a great variety of tissues until 

 it was established that all plants and animals are 

 composed of cells as structural units in much the 

 same way that a house is built of individual bricks 

 or stones. 



Of course the shapes and structures of these 

 units vary greatly in accordance with the kinds of 

 tissues in which they are found or the activities they 



