CLASSIFICATION OF PLANTS 291 



branches representing other groups are so located as to show the 

 relative complexity and relationships of the various other groups 

 included in the Plant Kingdom. 



The origin of a plant from simpler previously existing forms is 

 known as phylogeny, while the series of changes which a plant 

 or any^ living being passes through in attaining a mature con- 

 dition is called ontogeny. Plants are classified in a number of 

 ways, but chiefly upon their phylogenetic relationships. An- 

 other basis of considerable importance upon which plants are 

 classified pertains to their place of living and adjustments to 

 environment. Relationships of this kind are ecological. 



Part II is devoted, although briefly: first, to a study of the 

 structure, habits, economic importance, and phylogenetic rela- 

 tionships of the plants below the Flowering Plants; second, to 

 a study of some of the important groups of Flowering Plants as 

 to their phylogenetic relationships and economic importance; 

 third, to a consideration of plants as to their ecological relation- 

 ship; and fourth, to a special study of evolution, heredity, and 

 the breeding of plants. 



Classification of plants. — The various similarities and differ- 

 ences in the characteristic features of plants, due to evolutionary 

 or phylogenetic causes, afford botanists a basis for a systematic 

 classification of plants. The units in the phylogenetic classifica- 

 tion are species. A species is usually defined as a group of indi- 

 viduals similar in essential features and constant, that is, produc- 

 ing offspring like themselves. On the basis of similarities species 

 are grouped into genera, genera into families, families into orders, 

 orders into classes, and classes into divisions. The entire plant 

 kingdom, comprising 233,000 or more species, consists of four 

 divisions. In many cases within species there are groups of indi- 

 viduals differing in important features from other individuals 

 of the species, and such groups of individuals are called varieties, 

 strains, or races. Thus all of our common Apples belong to one 

 species (Pyrus mains), but there are many varieties of Apples, 

 and in Corn, Wheat, etc., there are many varieties, strains, and 

 races. For a similar reason, in some cases suborders within 

 orders, subclasses within classes, and subfamilies within families 

 have been formed. 



In naming the groups, the aim of botanists has been to follow 

 a rather definite plan, and the names are most all Latin terms, 



