ORGANS AND FUNCTIONS DEFINED, 



13 



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 head, trunk, limbs, 

 eyes, ears, etc., and internally stomach, intestines, liver, lungs, 

 heart, brain, and many other parts having the most diverse struc- 

 tures. These parts are known as organs, and the living body is 

 called an organism because it possesses them. 



The word organism, as here used, applies only 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 denned can scarcely be distinguished or are 

 entirely wanting. Such living things are nevertheless called organisms lie- 

 cause of the existence of different parts which are analogous in function to 

 the organs of plants and animals. 



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 

 called 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 plant fast 

 and absorb various substances 



from th(3 Soil; the Stem supports FIG. 3. (After Sachs.) Cross -section 

 ,11 i n through a group of dead, thick-walled 



the leaves and flowers and con- wood s ceUs froui the stem of maize. The 



ducts the Sap; the leaves breathe cens contain only air or water. (Highly 



and elaborate portions of the food ; 



and the reproductive organs of the flower serve to form and bring 



to maturity seeds destined to give rise to a new generation. 



Heterogeneity of the kind just indicated is one of the most 

 characteristic features of living things, and is not known in any 

 mass of lifeless matter, however large and complex. 



Organs composed of Tissues. Differentiation. In the next place, 

 it is to be observed that the organs themselves, when fully formed, 

 are not homogeneous, but are in turn made up of different parts. 

 The human hand is an organ which consists of many parts, differ- 



