Igor] THE MIDDLE LAMELLA 5 
mode of formation. He found it to consist, not of pectose 
proper, but of a cement of insoluble pectates. To demonstrate 
the presence of pectic acid, tissues are macerated in a mixture 
of alcohol and hydrochloric acid, and then treated with a weakly 
alkaline solution. The tissues dissociate into their constituent 
cells and fibers, the intercellular substance passing into solution. 
The solution is shown by analysis to contain pectic acid, which 
appropriate staining now shows to be absent from the cell walls. 
In this process, the insoluble pectates are first changed into 
pectic acid, which, in the alkaline solution, forms a soluble alka- 
line salt. If sections first treated with acid alcohol are stained 
with a pectic stain, as phenosafranin or methylene blue, the 
pectic acid present stains more deeply than the pectic com- 
pounds associated with cellulose in the wall layers lying between 
the intercellular substance and the cell cavity. This indicates 
that it is not pectic acid and its derivatives, but some of the 
neutrgl pectic substances, as pectin or pectose, which are in 
combination with cellulose in the later deposited layers. 
The intercellular substance thus deeply stained forms a thin 
layer on the whole surface of contact of adult cells; where the 
cells draw apart, it produces a thick cushion; when the cells 
separate so as to form intercellular spaces, these spaces are 
bounded by a pectic layer. This frame of pectic acid is some- 
times thickened irregularly, so as to form knobs, points, and 
sculptures of various forms ornamenting the frame itself or pro- 
jecting into the intercellular spaces. Sometimes the spaces are 
partially or completely filled with a jelly-like mass, a soluble 
transformation product of pectic acid.» In the meristem the 
intercellular layer is not disclosed by staining, but the chemical 
teactions of cellulose and pectic compounds are given by the 
cellmembranes. That a thin layer of intercellular substance ts 
present is shown by the dissociation of the cells as in older tis- 
sues after treatment with acid alcohol and an alkaline solution. 
Mangia explains the variations in thickness and form of the 
intercellular cement by its partial transformation in the course 
of the development of the tissues into soluble pectates; this 
