158 PRACTICAL BOTANY 



all these are oaks, and bear to one another many resemblances 

 in form of tree, form of leaf, and in the proportion of parts, 

 there are sufficient differences in form to distinguish them 

 one from another. The botanist uses a genus name, which is 

 Quercus, to include all the oaks, and to this genus name he 

 adds a species (specific) name by which to indicate the par- 

 ticular kind of oak of which he is speaking. In names of 

 many plants the specific name suggests a prominent character- 

 istic of the particular kind of plant to which the name refers ; 

 as in Quercus alba, alba, meaning "white," refers to the whitish 

 bark of the tree; and in Quercus nigra, nigra refers to the 

 blackish bark. A list of six of the oaks, together with the 

 leaf outlines and drawings of the acorns (Fig. 149), should 

 help to make more clear the meaning of genus and species. 

 This meaning needs to be understood, since we usually speak 

 of plants by their common names or by their generic names, 

 and often it is necessary also to use the specific names. 



144. The leading groups of plants. Quite similar plants are 

 grouped together into one species, and species that closely re- 

 semble one another are grouped together into one genus. In 

 the same way similar genera (plural of genus) are grouped 

 together into one family, and families of close resemblances 

 are grouped into an order. Orders are grouped into sub-classes 

 or directly into classes, and the classes into great groups, of 

 which there are four. These four great groups together con- 

 stitute the plant kingdom. Other intermediate groups are 

 sometimes used. 



Of the four great groups of plants, the one which includes 

 the flowering, or seed plants, is the spermatophytes, a name 

 which means " seed plants." The spermatophytes are divided 

 into the angiosperms, or plants with inclosed seeds, and the 

 gymnosperms, or plants with exposed seeds. The group next 

 below the spermatophytes is the pteridophytes, or fern plants, 

 the group which includes the true ferns and certain other 

 plants which are rather rare. Next below the pteridophytes 

 is the Iryophytes, or moss plants, consisting of the mosses and 



