196 INTRODUCTION TO BOTANY 
species that belong to it. A careful study of figure 164 will 
give some notion of the relative resemblances of two struc- 
tures in these six species of Quercus. In speaking of plants it 
is customary to use both genus and species name (that is, the 
binomial system of names), as Quercus alba; or, more often, 
we use the common name, as white oak. Elementary botany 
formerly concerned itself chiefly with attempts to learn the 
genus and species names and the proper classification of seed 
plants. We are now concerned more with how plants live than 
with what their botanical names are. 
183. The leading groups of plants. Those genera (plural 
of genus) which resemble one another are grouped together 
into a family; families with sufficiently close resemblances 
are grouped into an order; orders are grouped into classes; 
and classes into great groups. A study of the following divi- 
sions of the plant kingdom will enable you to see the relations 
that these divisions bear to one another, and the relations of 
genus and species names to the whole plan of classification. 
Plant kingdom 
Great groups 
Classes 
(Sometimes sub-classes) 
Orders 
Families 
Genera 
Species 
(Sometimes varieties) 
The four great groups of plants are 
Spermatophytes, the seed plants in which are: 
Angiosperms, or plants with inclosed seeds ; 
Tymnosperms, or plants with exposed seeds. 
Pteridophyles, the fern plants, including the common ferns and other 
fern-like plants that are not so common. 
Bryophytes, including liverworts and mosses. 
Thallophytes, including the lowest and simplest plants — the fission 
plants, the algee, and the fungi. 
Some of these groups will be very briefly considered in the 
chapters immediately following. 
