ELEMENTARY BOTANY. 
XXV 
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 independently 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 hun- 
dred 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, unfortunately, 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 belong to distinct species ; and again, whether two or more groups 
of species should ‘constitute as many sections of one genus, or distinct genera, or 
tribes of one Order, or even distinct Natural Orders. In the former case, as a species 
is supposed to have a real existence in nature, the question is susceptible of argu- 
ment, and sometimes of absolute proof. But the place a group should occupy in the 
scale of degree is very arbitrary, being often a mere question of convenience. The 
more subdivisions upon correct principles are multiplied, the more they facilitate 
Order and the Genus, are comprehensive and distinct. But if every group into 
the study of plants, provided always the main resting-points for constant use, the 
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 Linnsean nomenclature are gone. 
Chap. III. Vegetable Anatomy and Physiology. 
§ 1. Structure and Growth 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 ar- 
ranged 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 
tapering 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 pro- 
duced, 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 the principal part of wood, of the inner bark, and of the nerves and 
veins of leaves, forming, in short, the framework of the plant. 
notion that their functions are analogous to those of the vessels (veins and arteries) 
(3) Vascular tissue , or the vessels or ducts of plants, so called from the mistaken 
of animals. A vessel in plants consists of a vertical row of cells, which have their 
transverse partition- walls obliterated, so as to form a continuous tube. All phaenoga- 
mous plants, as well as ferns and a few other cryptogamous plants, have vessels, and 
are therefore called vascular plants ; so the majority of cryptogams having only cel- 
