INTRODUCTION TO BOTANY. XXVll 



unbranchcd from the base to the apex, and arc connected by 



cross-bars or unbranchcd veins. 

 The first of these types is generally characteristic of Exogens^ the latter 

 of Eiulogens ; but there are various intermediate conditions, and some 

 Endogcns have been called Dictyogens, because they have netted-veined 

 leaves ; several Exog-ens also have straight-veined leaves. 



171. Leaves usually extend horizontally, and have an tqyper and an twdc}- 

 surface, differing in anatomical structxu-e. In the cellular stratum of the 

 upper surface the cells are closely set and placed vertically, with their 

 smallest ends next the surface ; in the lower stratum the cells are more or 

 less horizontal, more loosely placed, and very generally have large empty 

 spaces between them. 



172. Leaves are functionally the most active of the vegetable organs. 

 In them the process of digestion or assimilatio)i is chiefly conducted. 



173. Assimilation is the name given to the process which accom- 

 plishes these following results : — 



1. The chemical decomposition of the oxygenated matter of the sap, 

 and of the carbonic acid which is absorbed by the leaves ; re- 

 sulting mainly in the liberation of pure oxygen, the fixation 

 of carbon and the elements of water in the tissues of the plant. 

 Thus, through the powers of life, and under the influence of solar light 

 and heat, oxygen^ hydrogen^ nitrogen, and carbon^ or some of these, uniting 

 in certain proportions, become gluten^ chlorophyll^ gum, sugar, or starch, etc. ; 

 and in like manner all other vegetable compounds pass from dead matter 

 into the condition of Uvi)ig substance. 



174. The oxygen liberated by plants during the process of assimilation 

 passes into the air ; and as assimilation is constantly going on duriag sun- 

 light, the amount of oxygen thus poured into the air by plants is enonnous, 

 and indeed, so far as we know, vegetation is the only great operation in 

 nature which restores to the air that fi'ee oxygen gas which is consumed 

 by animals, and in all processes of combustion is indispensable to animal 

 Hfe. 



III. Classification. 



175. It has been already said (2) that descriptions of plants should be 

 arranged, as nearly as possible, under natural divisions, so as to facilitate 

 the comparison of each plant mth those most nearly allied to it. The 

 descriptions here alluded to are descriptions of species ; the }iatural divisions 

 of the Flora refer to natural groups of species. 



176. A species comprises all the individual plants which resemble 

 each other sufficiently to make us conclude that they may all have descended 

 from a common parent stock. These individuals may often differ from 

 each other in striking particulars, such as colour of flower, size of leaf, 

 etc. ; but such dift\'r(.!nces, observation teaches us, may occiir in seedlings, 

 raised from one individual. 



177. ^\1lon a number of individuals of a species differ fi'om the majority 

 in any striking ])articular, they constitute a variety. If the variety 

 generally comes true fi-om seed, it is often called a race. 



178. A Variety can be propagated with certainty only by grafts, cut- 

 tings, bvilbs, or tubers, or other method which prodiices a new plant by 

 the growth of buds taken from the old one. A race may very frequently, 

 but not with certainty, be propagated by seed. 



179. The kno^vn species of plants (now nearly 100,000) are far too 

 numorous to be studied without classification. To facilitate their study, 

 an admirable system, invented by Liniueus, has been universally adopted, 

 viz. one comiuon, substantive name is given to a number of species which 



