THE SKELETON OF THE PLANT 51 



very unlike that of the thickening layers. It marks 

 off the limits of the cells, occupying the position of the 

 original thin membrane, and looking as if it were the basis 

 on which the thickening layers have 

 been deposited. When a piece of 

 tissue is warmed gently with a 

 mixture of potassic chlorate and 

 strong nitric acid, this layer dis- 

 solves and the cells become separated 

 from each other. It has by certain 

 writers been termed the intercellular 

 substance and by others the middle 

 lamella (fig. 44). Though it is most 

 easily seen in thickened cells, it is FIG. 44. THICKENED WOOD- 



., CELLS, SHOWING MIDDLE 



probably not connned to them, but LAMELLA. (After Sachs.) 

 exists in all cell - membranes, even 



when they are very young. Treatment with the reagent 

 mentioned will disintegrate the tissue of even the growing 

 points of stems and roots, and will cause their cells to 

 become isolated. A thin layer of this nature therefore 

 probably exists even in the primary cell-wall. It is added 

 to materially, however, during the growth in thickness of 

 the walls, and in many cases it can be seen easily under 

 a comparatively low magnification. 



This middle lamella is composed of a material which is 

 very unlike that of the rest of the cell-wall. Besides dis- 

 solving readily under the action of potassic chlorate and nitric 

 acid, which do not affect the inner layers of the membrane, it 

 resists completely the action of sulphuric and other mineral 

 acids, which cause the inner layers to swell and ultimately 

 to dissolve. Kecent investigations have led to the view 

 that it is composed of a calcium salt of pectic acid. 



Whether the primitive cell-wall is homogeneous or not is 

 uncertain. If it is, it must be regarded as being formed of an 

 intimate mixture or perhaps of a compound of cellulose and 

 pectose constituents. At a very early period in its develop- 

 ment the middle lamella becomes differentiated, owing 



4* 



