CHAPTER 18 • GROWTH AND DEVELOPMENT 



1 What makes a plant or animal stop growing? 



2 Is there anything besides feeding that makes the members of a 



species differ in size or development? 



3 Is there any advantage in being larger or smaller than the 



average r 



4 Can anything be done to quicken growth or to slow it ? 



5 Does every part of the body need exercise in order to develop? 



6 Why do people grow faster at some times than at others r 



7 Why do some parts of the body grow faster than others? 



8 Do any new organs develop after one is born ? 



9 Can a part that has stopped growing be made to start growing 



again ? 

 10 Do people become more alike or less alike as they grow older? 



Most of the people you know have grown in the last year or two. Nearly 

 everybody, but not quite all. For in addition to the universal fact that living 

 beings grow is a second general fact, namely, that they stop growing. More- 

 over, the parts of a plant or animal grow at different rates, so that shapes, 

 forms or proportions change. 



In some species of plants, like the famous redwood trees of California, 

 the individual may keep on growing for centuries. Some animals keep on 

 growing as long as they remain alive, as certain fish. But in most of the 

 familiar species the individuals reach a fairly definite Hmit of growth. They 

 may then continue to live for a time, but without becoming larger. On 

 the other hand, even after the body reaches full size, some parts may con- 

 tinue to grow, as do our hair and nails or the fruits on many shrubs and 

 herbs. 



Why does not the increase in size of living things continue through life ? 

 What determines the different rates of growth among different species or 

 among the different parts of one plant or animal ? How can the growth 

 of living things be controlled ? 



How Do Plants and Animals Increase in Size? 



The Steps in Growth When the conditions are suitable, an organism 

 grows by two distinct processes: 



Cells enlarge as they form new protoplasm by assimilating food. This 

 is true of one-celled organisms, as well as of many-celled organisms. A cell 

 may indeed increase in size by absorbing a quantity of water, just as a piece 

 of leather or wood swells when it absorbs water. But bv growth we usually 

 mean the making of more protoplasm. 



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