HISTOLOGY. 17 



Tho study of Anatomy an<J i^cvolopmont has broiig'ht to 

 light certain generalizations of wi(Je applicability and great 

 iiii])ortance. 



1. 1l has been said that llic great majority of living }>eings 

 pres(;nt a vitry delinite structun\ L'nassistfMi vision and or- 

 dinary diss(;ction sullice to se[)araL(i tlie body of any of i\\(i 

 higlK.r animals, or [)Jants, into fabrics (jf different s<jrts, wiiich 

 aivvuys pjcsent tin; sarn(; gerKiral arrang(.'in('nt in th(i sarin; 

 organism, but are conibined in different ways in different 

 organisms. 'J'he discrimination of tliese comparativ(,']y few 

 fabrics, or tiasues, of wiiich organisms are composed, was the 

 first step toward that ultimate analysis of visible structure 

 which has become possible only by the recent jjerfection of 

 mi(;roscopes and of methods of [)reparation. 



Histology, which embodies the results of tliis analysis, 

 sIjows that (iVitry tissue of a plant is composed of mon; or less 

 modified structural elements, each of which is ternn-d a cr/// 

 which c(,'ll, in its simplest cojidilion, is merely a sphf^roidal 

 fnass of )>rotoplasm, surrounded by a coat or sac — the r{M- 

 vxill — which contains cellulose. Jn th(; various tissues, these 

 cells may undergo innum(;rabh; modifications of form — the 

 jnoloplasm may become diflV^rentiated into a nucleus with il8 

 nucleolus, a jjrimordial utricle, and a cavity filled with a wa- 

 ter v fluid, and the cell-wall may be variously altc^red in com- 

 position or in structure, or may coalesce with others. But, 

 however extensive these changes may b(i, the fact that the 

 tissufjs are made up of morphologically distinct units — the 

 cells — rt^mains patent. And, if any doubt could exist on the 

 subject, it would be removed by the study of developm(;nt, 

 which provf.'s that every plant c(jmmf;nces its (;xist(;nr(.- as a 

 simple cell, identical in its fundamental characters with the less 

 modified of thos<; cells of which the; whole body is comprised. 



]5ut it is nf>t necessary to the mor{>hological unit of the 

 plant that it should be always provided with a cf;ll-wall. (cer- 

 tain plants, such as /Vo((''xy>r'C</.s, spend longer or shorter peri- 

 ods of thfiir existence in the condition of a merr; sphc^rrjid of 

 protoplasm, devoid of any cellulose wall, while, at other times, 

 the f)rotoplasmic bofly becomes inclosed within a cell-wall, fab- 

 ricated by its superficial layer. 



'rhr;n!ff>re, just as the nuchtus, the primordial utricle, and 

 the (-(Mitral fluid, are no essential constituents of the morpho- 

 lo;.'ical unit of the plant, but n^present results of its meta- 

 irjorphosis, so th(! cell wall is eqiially un(;ssential ; and (hither 

 the term "cell" must acqiiire a merely t('chnical significance 



