INTRODUCTION. XXV 



the same genus are collectively called the specific cliaracter of the plant ; tliose bj 

 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. 



184f. Classes, Orders, Genera, and their several subdivisions, are called natural ^vIlcn, 

 m forming them, all roseiublancea and differences are taken into account, valuing them, 

 according to their evident or presumed impDrtance ; artijicialy when resemblances and 

 differences in some one or very few particulars only ai^ taken into account indepen- 

 dently of all others. 



185. Tlie 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 

 J9 the same discrepancy in tlie number of genera to a Family. There is moreover, un- 

 fortunately, in a number of instances, gi^eat 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 distmct 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 ab- 

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

 twiry, being often a more question of convenience. Tlie more subdivisions upon cor- 

 rect principles ai-e multiplied, the more they facilitate the study oi 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, witli a substantive name to be remembered whenever a species 

 i8»3poken of, all the advantages derived from the beautiful simplicity of the Linntcan 

 nomenclature are gone. 



Chap. III. Vegetable Anatomy am> PiirsioLoor. 



§ 1. Sinicture and Growth of the Elementart/ Tissues. 



186. If a very thin slice of any part of a plant be placed imder a microscope of liigh 

 inagriifying power, it will be found to be made up of variously shajied and arranged 

 ultimate parts, forming a sort of honeycombed structure. These ultimate parts are 

 failed cells, and form by their combination the elementary iissues of which the entire 

 plant is composed. 



1B7. 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 

 indmdual, separately formed and separately acting, though cohering with the cells 

 '^'Jth ^yliich it is in contact, and partaking of the common hfe and action of the tissue 

 01 which it forms a pai't. The membranes aepai'ating or enclosing the cells are also 

 ^^^^d their cc-alls. ^ c. o 



ISS. Botanists usually distinguish the following tissues :- 



(i) Cellular thsue, or paretwhi/ma, consists usually of tldn- walled cells, more or less 

 »*ound 111 form, or with their length not much exceeding their breadth, and not tapering 

 at the ends, AH the soft parts of the leaves, the pith of stems, the pnl]^ of fruits, and 

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



^o be iorraed while growth continues, and when it ceases to be active the plant dies. 



1^) Woodt/ tissue, or prosenchifma, diifers in having its cells considerably longer than 

 ^J'oad, usually tapering at each end into points and overlapping each other. The cells 

 are commonly thick-walled ; the tissue is firm, teuacioLis, and elastic, and constitutes 



