CHAPTEE II. 



THE STRUCTURE OF LIVING THINGS. ORGANISMS. 



LIFELESS tilings occur in masses of the most various sizes 

 and forms, and may differ widely in structure and chemical com- 



t/ */ 



position. Living things, on the other hand, occur only in rela- 

 tively small masses, of which perhaps the largest are, among 

 plants, the great trees of California and, among animals, the 

 whales ; while the smallest are the micro-organisms or bacteria. 

 Moreover, the individual masses in which living things occur 

 possess a peculiar and characteristic structure and chemical com- 

 position which have caused them to be known as organisms, and 

 their substance as organic. All organisms are built up to a 

 remarkable extent in the same way and of the same materials, 



PIG. 1. (After Sachs.) Longitudinal section through the growing apex of a young 

 pine-shoot. The dotted portion represents the protoplasm, the narrow lines be- 

 ing the partition-walls composed of cellulose (C 6 H,oO 5 ). (Highly magnified.) 



and we may conveniently begin a study of living things with the 

 larger and more complex forms, which exhibit most clearly 

 those structural peculiarities to which we have referred. 



Organisms composed of Organs. Functions. It is character- 

 istic of any living body- -for example, a rabbit or a gerannmi- 

 that it is composed of unlike parts, having a structure which 

 enables them to perform various operations essential or accessory 

 to the life of the whole. The plant has stem, roots, branches, 



leaves, stamens, pistil, seeds, etc. ; the animal has externally 



9 



