340 GENERA, ORDERS, AND CLASSES. 



708. Genera. Certain species are more nearly allied than others, and 

 are conveniently grouped together so as to form a distinct kind or genus. 

 A genus then is an assemblage of nearly related species, agreeing with 

 one another in general structure and appearance more closely than 

 they accord with any other species. Thus, the various species of Roses 

 compose one genus, which is distinguished by marked characters. 

 Occasionally, a subgenus is formed by grouping certain species, which 

 agree more nearly with each other in some important particulars than 

 the other species of the genus. The characters of the genera are taken 

 exclusively from the parts of fructification, while all parts of the plant 

 furnish specific characters. In the name of a plant, the genus is given 

 as well as that of the species. The latter was called by Linnaeus the 

 trivial name. Thus, a particular species of Rose is called Rosa spino- 

 sissima; the first being the genus, and the second the specific or trivial 

 name. As regards the definition of genera and species, and nomen- 

 clature in general, no one has conferred so much benefit on science as 

 the great Linnaeus. This may be considered as among his highest 

 titles to fame. 



709. Orders. Several genera agreeing in more general characters, 

 although differing in their special conformation, are grouped together 

 so as to form an order or family. As genera include allied species, so 

 orders embrace allied genera. Subdivisions are also made to facilitate 

 reference, so that suborders and tribes are formed consisting of cer- 

 tain genera, more nearly related in particular characters than others. 

 Thus, the order Rosaceae, or the Rose family, includes the genera 

 Rosa, Rubus, Potentilla, Fragaria, Prunus, &c., which all agree in 

 certain general characters; and the order is divided into various sub- 

 orders, such as the true Roses, the Amygdalese, comprehending the 

 Plum, Almond, Peach, &c. ; the Potentilleae, embracing the Cinquefoil, 

 Strawberry, Raspberry, &c. (1[ 854). 



710. Classes. Orders having some general characters in common, 

 are united together in classes, and subclasses are formed in the same 

 way as suborders. This is the general plan upon which botanical 

 classification proceeds. With the exception of the individual species, 

 all the divisions are more or less arbitrary. In making them, however, 

 the object of the enlightened botanist is to follow what he considers 

 to be the natural affinities, and thus to trace, as far as possible, the 

 order which pervades the vegetable creation. 



711. Essential Characters. Each of the divisions of a system is 

 accurately defined, the characters being as short as is consistent with 

 precise diagnosis. Such characters are called essential, and they em- 

 brace only those points by which the group is distinguished from the 

 others in the same section. The complete description of an individual 

 species, from the root to the flower and fruit, is called the natural 

 character, and embraces many particulars which are not requisite for 



