CLASSES, ESSENTIAL OHAEACTEES, AND NOMENCLATURE. 411 



reference, so that mh-orders and tribes are formed, consisting of certain 

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

 the order Kosac^se, or the Eose family, includes the genera Rosa, 

 Eubus, Potentilla, Pragaria, Prunus, etc., which all agree in certain 

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

 such as the true Eoses, with aohenes contained in a hollow torus ; the 

 Amygdalese, with drupaceous fruit, comprehending the plum, almond, 

 peach, etc. ; the Potentillese, with achenes on a convex receptacle, 

 embracing the Oinquefoil, Strawberry, etc. 



Classes. — Orders having some general characters in common are 

 united together in classes ; and sub-classes are formed in the same way 

 as sub-orders. This is the general plan upon which botanical classifica- 

 tion proceeds. 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. 



EssENTiAi Ohaeactbes. — 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 

 the purpose of diagnosis. The essential characters of genera, when in 

 Latin, are put in the nominative case, while those of species are in 

 the ablative. Professor Henslow was instrumental in introducing into 

 schools an excellent method of teaching the young to notice and de- 

 scribe, the parts of plants. His method has been very generally 

 adopted, and with the best results. For describing the parts of a 

 plant tables are constructed with the names of the organs, and blank 

 spaces are left for the student to fill in the characters in methodical 

 order — 1. Eoot ; its form and structure ; 2. Stem ; its form and 

 structure; 3. Leaves, simple or compound, petiole, and lamina, vena- 

 tion, form, margin and apex, stipules. 4. Inflorescence ; indefinite or 

 definite; peduncle, pedicels, bract. 5. Parts of the flower; calyx, 

 corolla, stamen, pistil ; number of parts in each whorl ; their insertion 

 and relative position ; separation or cohesion ; adhesion ; the parts of 

 which each organ in a whorl is composed. 



NoMENCLATUEE. — The names of genera are variously derived, 

 from the structure or qualities of the group, from the name of some 

 eminent botanist, from the classical name of the plant, from old 

 mythological names, and from English names which are Latinised, 

 etc. ; while specific names have reference also to the country where 

 the plant is found, the locality in which it grows, the form of the 

 leaves, roOt, stem, or the colour of the flowers, etc. The general riile 

 is, that the name shall consist of a substantive and an adjective, the 



