LIVING THINGS 213 



such as the stomachs of animals or the leaves of trees. It is 

 the occurrence of these organs in all forms of life except the 

 very lowest that causes us to name the group to which they 

 belong the organic kingdom. 



179. Growth. In a sense, crystals may be said to grow in 

 that they increase in size, but this increase is always from 

 without by the addition of similar molecules. All living 

 things grow by the addition of matter taken into their bodies 

 and there built up into new molecules. In living things also 

 there is a regular cycle of development. Beginning with a 

 single cell, the organism increases in size by the repeated 

 division of its cells until it reaches a determinate size and 

 becomes mature. It then reproduces, or gives rise to new 

 individuals, for a certain time, and finally declines to old 

 age and ceases to exist. Of the two groups, the animals are 

 more closely circumscribed both as regards length of life and 

 size, but though some plants may continue to live for several 

 centuries, there are certain limits as to their size beyond which 

 they rarely pass. Animals again, usually have very definite 

 shapes, though the simplest of them as well as the simplest 

 plants are not thus restricted. Though the plant body, as a 

 whole, has not as definite a form as has the animal body, 

 some organs, such as flowers, fruits and seeds, are constant in 

 this respect. 



180. Food Making. The material which living things build 

 into their bodies and from which they obtain their necessary 

 energy is called food. The food materials are the chemical 

 elements in the soil and air, and plants are the only organisms 

 in nature that can combine them into foods. In consequence, 

 the entire animal world is dependent upon plants for its 

 existence. The energy in food making is derived from sun- 

 light by the green parts of plants. In these parts, the cells 

 contain small green bodies called chloroplasts which stop 

 some of the rays of light and change them into an available 



