INTRODUCTION TO BOTANY. XXiil 
174. The oxygen liberated by plants during the process of assimilation passes, by 
the leaves, into the air ; and as assimilation is constantly going on during sunlight, 
the amount of oxygen thus poured into the air by plants is enormous, and indeed, 
so far as we know, vegetation is the only great operation in nature which gives to 
the air that free oxygen gas which is indispensable to animal life. 
In. CLASSIFICATION. 
175. It has been already said (2) that descriptions of plants should be arranged, 
as nearly as possible, under natural divisions, so as to facilitate the comparison of 
each plant with those most nearly allied to it.. The descriptions here alluded to are 
descriptions. of species ; the natural divisions of the Flora refer to natural groups of 
species, ; 
176. A species comprises all the individual plants which resemble each other suf- 
ficiently to make us conclude that they may all have descended from a common 
parent stock, These individuals may often differ from each other in striking parti- 
culars, such as colour of flower, size of leaf, &c.; but such differences, observation 
teaches us, may occur in seedlings raised from one individual. 
177. When a cered of sc era a species differ from the majority in any 
iking particular, they constitute a Variety. If the variety generally comes true 
from seed, it is often called a-race, 
178. A variety can be propagated with certainty only by grafts, cuttings, bulbs, or 
tubers, or other method which produces a new plant by the growth of buds taken 
from the old one. A race may very frequently, but not with certainty, be propagated 
by seed. 
179. The known species of plants (now nearly 100,000) are far too numerous to 
be studied without classification. To facilitate their study, an admirable system, 
invented by Linnzus, has been universally adopted, viz. : one common, substantive 
name is given to a number of species which resemble each other more than they do 
any other species ; the species so brought together are collectively called a Genus, 
and the common name is the generic name. Each species is further distingui 
from the others of the same genus by the addition of an adjective epithet, or specific 
name. Every species has thus a botanical name of two words. Latin, the lan- 
guage usually used for the purpose, the first word isa substantive usually of Greek 
origin and Latinized, and designates the genus ; the second, usually a Latin adjec- 
tive, indicates the species, 
180. The genera thus formed being very numerous, they have been grouped toge- 
ther on similar principles—associating those which resemble each other most nearly— 
into Families or Natural Orders, to each of which a name has been given. This is, 
however, for the purpose of study or comparison. In speaking of a species it is only 
necessary to give the generic and specific names. The name of a Natural Order or 
Family, in Latin, is an adjective plural, usually formed from the name of some one 
fewae genus, generally the best known, the first discovered, or the most marked. 
Thus Ranwnculacee is the Order of which the Ranunculus is the typical genus ; 
Order, is very unequal. Some genera pageant ot 
cies ; in others, several h red species a) : 
182. Orders are collected into Classes ; and guy wy rae large number 
of genera, or genera a large number of species, a er sub-division is required. 
ee cases of “he several groups most generally adopted are as follows, beginning 
with the most comprehenslve or highest divisions : 
Classes : Genera: 
Sub-classes or alliances. Sub-genera. 
Natural Orders : Sub-Sections 
- Tribes. — Species : 
Sub-tribes. ~ Varieties. 
Divisi 
Sub-divisions. 
