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 Fa. 44—Turcxenep Woon- 
probably not confined to them, but ie ee caer 
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 geen easily under 
a comparatively low magnification. 
This middle lamella is composed of a material which is 
very unlike tliat 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. Recent 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. Ata very early period in its develop- 
ment the middle lamella becomes differentiated, owing 
4* 
