XXXV111 INTRODUCTION. 



Genera. Subsections. 



Subgenera. Species. 



Sections. Varieties. 



183. The characters (3) by which a species is distinguished from all other 

 species of 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, such as size, colour, 

 ramification, arrangement of the leaves, inflorescence, etc., and are chiefly de- 

 rived from the organs of vegetation. 



184. Classes, Orders, Grenera, and their several subdivisions, are called na- 

 tural when, in forming them, all resemblances and differences are taken into 

 account, valuing them according to their evident or presumed importance ; arti- 

 ficial^ 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 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, unfortunately, in a number of instances, great dif- 

 ference 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 argument, and sometimes of absolute 

 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 

 correct 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 comprehensive 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 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 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 sub- 

 stance 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 icalls. 



