CHARACXBES OF THE VBGETABLB CELL. 249 



cause it to contract by drawing forth part of its contents (Fig. 

 175). Its composition is indicated, by the eftects of reagents, to 

 be albuminous; tliat is, it agrees with the formative substance of 

 the Animal tissues, not only in the proportions of oxygen, hydro- 

 gen, carbon, and nitrogen which it contains, but also in the 

 nature of the compound formed by the union of these elements. 

 The external layer, on the other hand, though commonly re- 

 garded as the proper " cell-wall," is generated on the surface of 

 the primordial utricle, after the latter has completely inclosed 

 the cavity and its contents, so that it takes no essential part in 

 the formation of the cell. It is usually thick and strong in com- 

 parison with the other, and may often be shown to consist of 

 several layers. In its chemical nature it is altogether dissimilar 

 to the primordial utricle ; for it is essentially composed of cellu- 

 lose, a substance containing no nitrogen, and nearly identical 

 with starch. The relative offices of these two membranes are 

 very different; for whilst there are many indications that the 

 primordial utricle continues to participate actively in the vital 

 operations of the cell, it seems certain that the cellulose-wall 

 takes no concern in them, but is only their product ; its function 

 being simply protective. The contents of the vegetable-cell, 

 being usually more or less deeply colored, have received the col- 

 lective designation of endochrome (or internal coloring substance) ; 

 and they essentially consist of a layer of colorless " protoplasm" 

 (or organizable fluid, containing albuminous matter in combina- 

 tion with dextrine or starch-gum) in immediate contact with the 

 primordial utricle, within which is the more watery cell-sap, 

 particles of chlorophyll or coloring substance being diffused 

 through both, or through the former only. 



148. But although these component parts may be made out 

 without any difficulty in a large proportion of Vegetable cells, 

 yet they cannot be distinguished in some of those humble organ- 

 isms, which are nearest to the border ground between the two 

 kingdoms. For in them we find the " cell-wall" very imperfectly 

 differentiated from the "cell-contents;" the former not having by 

 any means the firmness of a perfect membrane, and the latter 

 not possessing the liquidity which elsewhere characterizes them. 

 And in some instances, the cell appears to be represented only 

 by a mass of endochrome, so viscid as to retain its external form 

 without any limitary membrane, though the superficial layer 

 seems to have a firmer consistence than the interior substance ; 

 and this may or may not be surrounded by a gelatinous-looking 

 envelope, which is equally far from possessing a membranous 

 firmness, and yet is the only representative of the cellulose- wall. 

 This viscid endochrome consists, as elsewhere, of a colorless 

 protoplasm, through which coloring particles are diffused, some- 

 times uniformly, sometimes in local aggregations, leaving parts 

 of the protoplasm uncolored. The superficial layer, in particular, 

 is frequently destitute of color; and the "primordial utricle" ap- 



