APPENDIX 365 
size of flowers, and other minor points, constitute mere 
varieties, and have no special names applied to them. The 
line 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 Phznerogams — 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 
