CHAPTER XIX 

 SOME IMPORTANT FAMILIES OF ANGIOSPERMS 



298. Classification of Plants. So far as possible, plants 

 are classified according to their relationships, so that a system 

 of classification is much like a genealogical tree. Plants 

 which are very like one another, are said to be members of the 

 same species; for instance, our common apple trees belong 

 to the species known as Pyrus mains. Some species are com- 

 posed of plants that are different enough from one another 

 to be distinguished as varieties, but not different enough to 

 constitute separate species. Thus there are many varieties 

 of apples, some of which may have developed in the wild 

 state, but most of which are the result of breeding. Species 

 that are nearly related to one another are placed in the same 

 genus. For example, the common apple (Pyrus malus), 

 the Siberian crab-apple (Pyrus baccata) , and the pear (Pyrus 

 communis) belong to the genus Pyrus. Several or many 

 genera make up a family, and several families, in turn, are 

 grouped together to form an order. The genus Pyrus 

 belongs to the rose family (or Rosaces;} ; this family and some 

 others make up the order Ro sales. Since something new is 

 constantly being learned about the relationships of plants, 

 changes are made from time to time in their classification. 

 New species are found, and the arrangement of species into 

 genera, families, and orders must be changed to fit the newly 

 discovered facts. However, the seed plants have been 

 studied so long and so much is now known about them that 

 the general outlines of their classification may be con- 

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