28 BIOLOGY FOR BEGINNERS 



The comparison is sometimes made between a plant or animal 

 and a book, as follows: 



The elements correspond to the letters. 



Compounds correspond to words. 



Cells correspond to sentences. 



Tissues correspond to paragraphs. 



Organs correspond to chapters. 



The plant or animal corresponds to the whole book. 



To illustrate this method of structure we may look at the hand. 

 It is made of millions of cells, as shown by the microscope, each 

 having its characteristic shape and the usual cell parts: protoplasm, 

 nucleus, and wall. 



Numerous as these cells are, they can be classified into a com- 

 paratively few kinds. Groups of similar cells are called tissues, 

 and we find in the hand, muscle tissue, bone tissue, nerve tissue, 

 skin tissue, and some others. Each of these tissues has its special 

 use. The muscle is used for motion; the bone, for support; and 

 so on. All together they are combined into one organ', whose 

 general function is prehension (grasping things). 



In a similar way with plants, the cell is the unit of structure, 

 and in a stem, for instance, there are several kinds of cells. These 

 are grouped into wood tissue, bark tissue, tubular tissue, and pith 

 tissue, each made of similar cells and each with different functions. 

 However, they are all grouped together to form the plant organ, 

 called the stem, with its general functions of support and circulation 

 of sap. 



Relation of Structure to Use. Organic things are composed of 

 the same elements, combined in similar compounds, which appear 

 as living protoplasm, whether of animals or plants. This proto- 

 plasm performs very similar functions in either case, but by very 

 different organs. The plant gets its food by way of leaves and 

 roots, while an animal like the cat uses its claws, teeth, and swift- 

 ness. Our whole course in biology deals with the essential life 

 functions of plants and animals, but, in order to study these func- 



