OUTLINES OF BOTANY. 
XXV 
the same genus are collectively called the specific character of the plant ; those by 
which its genus is distinguished from other genera of the Order, or its Order from other 
Orders, are respectively called the generic or ordinal character, as the case may be. 
The habit of a plant, of a species, a genus, etc., consists of such general characters as 
strike the eye at first sight, such as size, colour, ramification, arrangement of the 
leaves, inflorescence, etc., and are chiefly derived from the organs of vegetation. 
184. Classes, Orders, Genera, and their several subdivisions, are called natural when, 
in forming them, all resemblances and differences are taken into account, valuing them 
according to their evident or presumed importance ; artificial , when resemblances and 
differences in some one or very few particulars only are taken into account indepen- 
dently of all others. 
185. The number of species included in a genus, or the number of genera in an 
Order, is very variable. Sometimes two or three or even a single species may be so 
different from all others as to constitute the entire genus ; in others, several hundred 
species may resemble each other so much as to be all included in one genus ; and there 
is the same discrepancy in the number of genera to a Family. There is moreover, un- 
fortunately, in a number of instances, great difference of opinion as to whether certain 
plants differing from each other in certain particulars are varieties of one species or be- 
long to distinct species ; and again, whether two or more groups of species should con- 
stitute as many sections of one genus, or distinct genera, or tribes of one Order, or 
even distinct Natural Orders. In the former case, a3 a species is supposed to have a 
real existence in nature, the question is susceptible of argument, and sometimes of ab- 
solute proof. But the place a group should occupy in the scale of degree is very arbi- 
trary, being often a mere question of convenience. The more subdivisions upon cor- 
rect principles are multiplied, the more they facilitate the study of plants, provided 
always the main resting-points for constant use, the Order and the Genus, are compre- 
hensive and distinct. But if every group into which a genus can be divided be erected 
into a distinct genus, with a substantive name to be remembered whenever a species 
is spoken of, all the advantages derived from the beautiful simplicity of the Lmnsean 
nomenclature are gone. 
Chap. III. Vegetable Anatomy and Physiology. 
§ 1, Structure and Oroivth of the Elementary Tissues. 
186. If a very thin slice of any part of a plant be placed under a microscope of high 
magnifying power, it will be found to be made up of variously shaped and arranged 
ultimate parts, forming a sort of honeycombed structure. These ultimate parts are 
called cells, and form by their combination the elementary tissues of which the entire 
plant is composed. 
187. A cell in its simplest state is a closed membranous sac, formed of a substance 
permeable by fluids, though usually destitute of visible pores. Each cell is a distinct 
individual, separately formed and separately acting, though cohering with the cells 
with which it is in contact, and partaking of the common life and action of the tissue 
of which it forms a part. The membranes separating or enclosing the cells are also 
called their walls. 
188. Botanists usually distinguish the following tissues : — 
(1) Cellular tissue, or parenchyma, consists usually of thin-walled cells, more or less 
round in form, or with their length not much exceeding their breadth, and not taperinu 
at the ends. All the soft parts of the leaves, the pith of stems, the pulp of fruits, and 
all young growing parts, are formed of it. It is the first tissue produced, and continues 
to be formed while growth continues, and when it ceases to be active the plant dies. 
(2) Woody tissue, or prosenchyma, differs in having its cells considerably longer than 
broad, usually tapering at each end into points and overlapping each other. The cells 
are commonly thick-walled ; the tissue is firm, tenacious, and elastic, and constitutes 
