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 
