226 GENERAL MORPHOLOGY OF PLANTS . 



and includes the lowest plants, many of which are of micro- 

 scopic size. 



358. Classes, orders, families, etc. All of the subking- 

 doms are still farther subdivided into divisions of lesser grade, 

 each lower grade having plants which are more closely related 

 than those constituting the higher and broader grade. The sub- 

 kingdoms are divided into Classes, the Classes into Orders, the 

 Orders into Families, the Families into genera, and the genera 

 into species. There are also subclasses, suborders, subgenera and 

 subspecies. The name of a plant requires the use of both the 

 genus and species names. For example, the white oak is a species 

 of oak (one of the kinds of oak) ; so the red oak, scarlet oak, etc., 

 are species of oak. The oaks, however, taken collectively con- 

 stitute a genus, the name of which is Quercus. The species name 

 of the white oak is alba, but the name of the white oak is Quercus 

 alba; of the red oak, Quercus rubra; of the scarlet oak, Quercus 

 coccinea, etc. ; the genus name being placed first and the species 

 name next. A genus, therefore, is a group of closely related 

 species; a family is a group of closely related genera; an order, of 

 closely related families; a class, of closely related orders, etc. 

 It is more difficult to define a species, for until within recent years 

 a species was supposed to be composed of a group of individuals 

 which always reproduced their kind, i.e., that the individuals of 

 their progeny were all like their parents. This seems to be true 

 for certain species, but there are a number of species the indi- 

 viduals of which give rise in their progeny to certain individuals 

 very unlike the parent plant.* This has given rise to the term 

 subspecies or elementary species, all the elementary species of the 

 same origin making up what is often called a collective species, 

 It is unnecessary and impracticable, however, in this work to 

 deal with elementary species. The species will be treated in the 

 broader or collective sense. 



* This phenomenon is called mutation, i.e., a sudden and quite marked 

 change, and the differing individuals are called mutants, because they are 

 quite markedly different from the parent and appeared suddenly by means 

 of seed See mutation, paragraph 662. 



