10 



THE STRUCTURE OF LIVING THINGS. 



head, trunk, limbs, eyes, ears, etc., and internally stomach, in- 

 testines, liver, lungs, heart, brain, and many other parts of 



Fig. 2.— Cross-section through part of the young leaf of a fern (Pteris cujuilina)^ 

 showing thick- walled cells ; most of the walls are double. The granular sub- 

 stance is protoplasm. Most of the cells contain a large central cavity (vacuole) 

 filled with sap, the protoplasm having been reduced to a thin layer inside the 

 partitions. Nuclei are shown in some of the cells, and lifeless grains of starch 

 in others : ?i, nuclei ; s, starch ; r, vacuole ; u\ double partition-wall. ( X 500.) 



the most diverse structure. These parts are known as oi^ganSy 

 and the living body, because it possesses them, is called an or- 

 ganism. 



The word organism, as here used, applies best to the higher animals 

 and plants. It will be seen in the sequel that there are forms of life so 

 simple that organs as here defined can scarcely be distinguished. Such 

 living things are nevertheless really organisms because they possess 

 parts analogous in function to the well-defined organs of higher form. 

 (See p. 157.) 



Since organisms are composed of unlike parts, they are said 

 to be heterogeneous in structure. They are also heterogeneous 

 in action, the different organs performing different operations 

 Q,2XiQA functions. For instance, it is the function of the stomach 

 to digest food, of the heart to pump the blood into the vessels, 

 of the kidneys to excrete waste matters from the blood, and 

 of the brain to direct the functions of other organs. A similar 

 diversity of functions exists in plants. The roots hold the 



