THE SKELETON OF THE PLANT 45 
In cell-walls which have undergone considerable thick- 
ening the membrane shows a marked differentiation. The 
centre of the wall is found to possess a chemical composi- 
tion very unlike that of the thickening layers. It marks 
s’ 
cs 
Wu 
Fic. 48,—Lonerrupinat SECTION oF VascuLaR BunpLE oF SUNFLOWER 
Srem. (After Prantl.) 
p’, pith; s, s’, spiral vessels ; w', w, wood-cells; p, p, pitted vessels ; 
c, cambium; st, st, sieve-tubes; ph, fibres; 1, bundle sheath ; 
c, cortex, 
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 
dissolves and the cells become sepa- 
rated 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 probably not confined to 
1G, 44.—THICKENED Woop- 
CELLS, SHOWING MippLE 
Lamecua, (After Sachs.) 
them, but exists in all cell-membranes, even when they are 
