INTRODUCTION TO BOTANY. XXVli 
unbranched from the base to the apex, and are connected by 
cross-bars or unbranched veins. 
The first of these types is generally characteristic of Exogens, the latter 
of Endogens ; but there are various intermediate conditions, and some 
Endogens have been called Dictyogens, because they have netted-veined 
leaves ; several Exogens also have straight-veined leaves. 
171. Leaves usually extend horizontally, and have an upper and an under 
surface, differing in anatomical structure. In the cellular stratum of the 
upper surface the cells are closely set and placed vertically, with their 
smallest ends next the surface; in the lower stratum the cells are more or 
less horizontal, more loosely placed, and very generally have large empty 
spaces between them. 
172. Leaves are functionally the most active of the vegetable organs. 
In them the process of digestion or assimilation is chiefly conducted. 
173. Assimilation is the name given to the process which accom- 
plishes these following results :— 
1. The chemical decomposition of the oxygenated matter of the sap, 
and of the carbonic acid which is absorbed by the leaves; re- 
sulting mainly in the liberation of pure oxygen, the fixation 
of carbon and the elements of water in the tissues of the plant. 
Thus, through the powers of life, and under the influence of solar light 
and heat, oxygen, hydrogen, nitrogen, and carbon, or some of these, uniting 
in certain proportions, become gluten, chlorophyll, gum, sugar, or starch, ete. ; 
and in like manner all other vegetable compounds pass from dead matter 
into the condition of living substance. 
174. The oxygen liberated by plants during the process of assimilation 
passes into the air; and as assimilation is constantly going on during sun- 
light, 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 restores to the air that free oxygen gas which is consumed 
Pe animals, and in all processes of combustion is indispensable to animal 
e. 
III. Ciassirication. 
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 sufficiently 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 particulars, such as colour of flower, size of leaf, 
etc. ; but such differences, observation teaches us, may occur in seedlings, 
raised from one individual. 
177. When a number of individuals of a species differ from the majority 
in any striking 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, cut- 
tings, 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 Linnzeus, has been universally adopted, 
viz. one common, substantive name is given to a number of species which 
