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. On the basis of descent or phylo- 

 genetic relationships the Plant Kingdom is divided into four 

 groups called divisions. Divisions are divided into classes, 

 classes into orders, orders into families, families into genera, and 

 genera into species. Species are the units in the phylogenetic 

 classification. Species are aggregates of individuals which are 

 alike in their important characteristics and are therefore regarded 

 as the same in kind. Of course the individuals of a species may 

 vary in a number of minor features. Thus White Oaks vary 

 much in size, shape, and a number of other ways, but in essential 

 features all White Oaks are alike and all belong to the species 

 White Oak. Sometimes some of the individuals of a species 

 show some important differences, and then the species is sub- 

 divided, the subdivisions being called varieties, strains, or races. 

 Also for a similar reason it sometimes happens that orders are 

 subdivided into suborders, classes into subclasses, and families 

 into subfamilies. 



The scientific names of the phylogenetic groups are Greek or 

 Latin terms and commonly express some characteristic of the 

 group. With some exceptions due to historical causes, the 

 groups are named according to a rather definite plan. The names 



