YOUTH, MATURITY, OLD AGE 



163 



143. Youth, Maturity, Old Age. — The life of man is 

 divided into three general periods, which are youth, the 

 period of maturity, and the period of old age. These same 

 terms are given when describing the life of animals and 

 plants. 



Youth is the period when living protoplasm always 

 grows, if furnished with proper food. This is the time 

 when boys and girls grow taller and 

 heavier each year ; when the tree grows 

 new leaves and the limbs become longer; 

 and when the small puppy is turning 

 into a full grown dog. During this 

 period of change the boys and girls, the 

 tree, and the puppy are all nourished 

 by food and this makes it possible for 

 them to grow. 



Maturity is the period when man 

 ceases to grow taller, although he con- 

 tinues to eat food as he did during the 

 period of youth. The living proto- 

 plasm in his body does not increase in 

 amount. The same can be said of the 

 tree, for it does not grow taller ; and 

 the puppy of last year has become a 

 full grown dog. During this period 



Figure 178. — Ali- 

 mentary Canal of 

 Frog. 



Compare with Fig- 

 ure 179. In what 

 are they alike? In 

 of maturity, each living organism is what different? 



able to repair its body as fast as the 



body wears out. The period of maturity varies in all 



living things ; in some butterflies lasting but twenty-four 



hours, in man continuing for about twenty-five years. 



Old age in man begins when the bodv wastes faster 



than it is repaired, and in the tree when growth Is over 



and decay begins. During this period of old age all 



living things use food as they did in youth and maturity, 



