INTRODUCTION. XXvii 



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 hahit 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, arrange- 

 ment 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 difierences are taken into account, valuing them 

 according to their evident or presumed importance ; artificial^ when resemblances and 

 difierences in some one or very few particidars 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 

 difierent 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- 

 fortiuiately, in a number of instances, great difierence of opinion as to whether certain 

 plants difieriug fi'om 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, as a species is supposed to have a 

 real existence in natui'e, the question is susceptible of argument, and sometimes of ab- 

 solute proof. But the place a group should occupy in tlie scale of degree is very arbi- 

 trary, being often a mere question of convenience. The more subdivisions upon cor- 

 rect principles are midtiplied, the more they facilitate the study of plants, provided 

 always the main resting-points for constant use, the Oi'der and the Grenus, 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 simpUcity of the Linnsean 

 nomenclature are gone. 



Chap. III. Vegetable Anatomy aio) PHYSiOLoar. 

 § 1. Structure and Growth of the JElementary 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 entu*e 

 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 hfe and action of the tissue 

 of which it forms a part. The membranes separating or enclosing the cells are also 

 called their ivalls. 



188. Botanists usually distinguish the following tissues :— - 



(1) Cellular tissue, or parenchyma, consists visually of thin- walled cells, more or less 

 round in form, or with their length not much exceeding their breadth, and not taper- 

 mg at the ends. All the soft parts of the leaves, the pith of stems, the pidp of fruits, 

 and all young growing parts, are formed of it. It is the first tissue produced, and 



