APPENDIX 365 



size of flowers, and other minor points, constitute mere 

 varieties, and have no special names appUed to them. The 

 Une between varieties and species is not clearly defined, and 

 in the nature of things can never be, since progressive de- 

 velopment, through unceasing change, is the law of all 

 life. 



In botanical descriptions, the name both of the species 

 and the genus is given, just as in designating a person, like 

 Mary Jones or John Robinson, we give both the surname 

 and the Christian name. The genus, or generic name, 

 answers to the surname, and that of the species to the 

 Christian name — except that in botanical nomenclature 

 the order is reversed, the generic, or surname, coming first, 

 and the specific or individual name last; for example, 

 Ipomea is the generic, or surname, of the morning-glories, and 

 purpurea the specific one. 



How to use the key. — Any good manual will answer the 

 purpose. Gray's " School and Field Book " is, perhaps, the 

 best available at present for the states east of the Missis- 

 sippi. Reference to the floral analyses in sections I-IV of 

 Chapter VII will make its use clear. Suppose, for instance, 

 we want to find out to what botanical species the morning- 

 glory or the sweet potato belongs. Turning to the key, 

 we find the sub-kingdom of Phaenerogams — flowering or 

 seed-bearing plants — divided into two great classes, Angio- 

 sperms and Gymnosperms, as explained in 18. A glance will 

 show that our specimen belongs to the former class. Angio- 

 sperms, again, are divided into the two subclasses of Dicotyle- 

 dons and Monocotyledons (18, 171). We at once recognize 

 our plant, by its net-veined leaves and pentamerous flowers, 

 as a dicotyledon (171, 229), and turning again to the key, 

 we find this subclass divided into three great groups : Sym- 

 petalous (211), called also Monopetalous and Gamopetalous ; 

 Apopetalous, or Polypetalous (211), and Apetalous — having 

 no petals or corolla. A glance will refer our blossom to the 

 sympetalous or monopetalous group, which we find divided 



