46 VEGETABLE PHYSIOLOGY 
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. It dissolves 
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 cellu- 
lose and pectose. At a very early period in its development 
the middle lamella becomes differentiated, owing possibly to 
the conversion of the pectose into pectic acid and the inter- 
action of the latter with some salt of calcium derived from 
the cell-sap which infiltrates the wall. The calcium pectate 
becomes deposited in this way halfway between the con- 
tiguous cells which are separated by the particular mem- 
brane in which the change is taking place. 
If the cell-wall is not at first homogeneous, we must 
suppose that the original thin membrane is composed of 
three layers, a central one of calcium pectate, on each face 
of which is a layer composed of the mixture or compound 
of cellulose and pectose. We never find, even at the 
moment of cell division, that the membrane is formed of 
calcium pectate only. 
It is possible to explain the growth in thickness of the 
middle lamella on either hypothesis. It is clear that the 
